Ham Jumbo MK1 - No Swing AM

Warf135 · Post by **Warf135** » Monday 7th Jun 2010, 8:40

Radio Make = Ham International
Radio Model = Jumbo Mk1
PCB number = PTBM059COX - Main board
PTOS006AOX - VCO\Crystal board
PTPW007COX - Power Supply board
PLL number = PLL02A

Test gear available: TRIO FC 756 frequency counter, basic scope, swr/power meters, dummyloads

Thanks to Dave and Scott in my other topic "jumbo alignment problem" the radio is now alligned and working ok except there is no swing on AM.

If I turn RV-8 on the chassis down so that the AM carrier is at minimum (2 watts) the radio will swing to about 4 watts, however if RV-8 is turned so the carrier is at or above 4 watts, the radio shows no swing (and even slight inverse swing) on external meter.

AM modulation is showing as good on modulation meter, so i know the mic/audio stage in the radio is ok
Anyone seen this problem before?

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Monday 7th Jun 2010, 11:06

The Cybernet chassis (which the Jumbo is) is known for having poor AM power, I have come across many radios that could not do 4W on AM because of the audio problem, in reality your probably best to just set the AM power at 2.5-3W so that you have some swing on AM, if you need more power just drop a linear amp after it.

I wish I could get to the bottom of this issue, I know others have tried and failed.

Hopefully someone else has found a fix for this, so that we can all learn how to get the AM power better.

Warf135 · Post by **Warf135** » Monday 7th Jun 2010, 11:43

Hi Scott, Thanks once again for your prompt reply.

I will probably never use AM - I just wanted to make sure that the radio was right whist I had it on the bench.
I'm sure my old lafayette 1200 would do 5 watt carrier and swing to about 10 watts (that too was cybernet if i remember right, but probably a different board) so I expected this jumbo to do the same... I'm sure theres a way to improve the swing, I'll have to get my brain into gear and study the schematic.

The only problem with reducing the AM carrier power to 2.5-3 watts is that it will also reduce the FM power to the same and I use FM a lot.

UPDATE: Ok, after studying the schematic, it appears that swing is obtained by feeding some of the TX audio into the collector of Q9 (2SC2166). If the radio has very little swing then it would appear that not enough of the audio is getting to the collector of Q9. My radio has had some of the coils changed and i wouldn't mind betting that L6 or L22 have been replaced with the wrong values... I'll update this post again when i actually find the fault

UPDATE 2: Checks with scope proved that good audio is getting to collector of Q9... so I swapped out Q9 and still no swing. Why wont this thing swing???

lbcomms · Post by **lbcomms** » Monday 14th Jun 2010, 2:47

AM modulation is showing as good on modulation meter

How does it look when using the scope to see your modulation? Use this to see if you are getting close to your 100%.
Many radios, but especially the later Cybernet SSB ones that use a 2SD8xx or similar as a modulator, will not show much "swing" on a meter that is not designed to show the increase in carrier level when audio is applied.

It sounds to me like the radio is functioning correctly, as it's makers intended.

You can disable limiters / use "swing kits" to get more "swing", but all you are really doing there in increasing the distortion in the signal you are transmitting!

For more info on "swing" , have a look here: http://www.radiomods.co.nz/forum/viewto ... f=2&t=2684

Hope this helps,
Sue

Warf135 · Post by **Warf135** » Monday 14th Jun 2010, 4:11

Hi Sue, thanks for the interesting reply.

I have just tested for swing on my known good President Lincoln and that is showing no swing either, so it would appear that you are correct in that the Jumbo is probably working ok and that neither of my SWR/Power meters are upto the job...

Any suggestions for a decent meter I can buy are welcomed!

lbcomms · Post by **lbcomms** » Monday 14th Jun 2010, 16:24

Any suggestions for a decent meter I can buy are welcomed!

A reasonable "guess meter" is the Revex 502. (Same as the one in the picture, but covers 1.8 to 60 MHz).

Note the "PEP moni" pull switch on the CAL control. This will show the peak AM power - i.e. it will show 4W on a carrier, and 6 to 14 watts when you modulate it.
This is only a rough figure though, the only accurate way to measure modulation is with an oscilloscope.

Measuring modulation accurately using an oscilloscope

Connect a 1K resistor (a carbon or metal film one only, NOT a wirewound type) in series with a 33pF capacitor - one side to the centre of the coax connector, the other to the CRO input. Don't do this without an antenna or dummy load connected - either make up a test box with 2 x SO239 connectors, or connect it to the antenna connector inside the radio.
Key the transmitter and adjust the CRO for the picture "1":

Transmit a tone and adjust the CRO so it looks like picture "2". Increase the tone to a high level until making it louder doesn't change the pattern on the CRO screen.
Now adjust your modulation limiter.

Picture #3: Correct adjustment. There should be no gaps between the shapes.

Picture #4: Overmodulation. There are gaps between the shapes. This causes distortion and splatter. Turn the modulation limiter down.

Picture #5: Non-linear operation. Note that the tops and bottoms of the shapes are not a nice smooth curve. Can be caused by faulty electrolytic capacitors in the radio (dried out with age), but is usually caused by having your radio power set too high when using a linear amplifier.

A meter cannot be used for accurate modulation measurements, because no meter can respond quickly enough for the changes in carrier level (300 to 3000 times per second), and cannot take into account the different positive shifts that different modulator circuits produce.

Hope this helps,
Sue

Warf135 · Post by **Warf135** » Friday 18th Jun 2010, 2:29

Thanks Sue, but the modulation of the radio was not the problem here, the problem was that the radio was showing no RF power swing. I understand that a meter will never be completely accurate to measure modulation, but it can give a good idea as to wether the radio is modulating or not. and with a constant tone source can give reasonable results as the constant tone does not alter therefor the audio signal will not be constantly changing...

Anyway, I've now ordered a Revex W560 meter (1.4 - 525 Mhz), so lets hope the Jumbo will show some RF swing on it

lbcomms · Post by **lbcomms** » Saturday 19th Jun 2010, 2:16

reasonable results as the constant tone does not alter therefor the audio signal will not be constantly changing

If there is any "tone" the signal will be constantly changing - a 1Khz tone at 100% modulation will mean that the carrier power level will be rising to a peak and reducing to almost nothing, 2000 times per second. It will only "not be constantly changing" on FM mode.

A meter cannot see that the low end is too low (cutting off the carrier) or is not linear (distorted / overdriven).

If you want to do a "quick check" there are a couple of workshop tricks that you can do:

1) Get an old CB or car stereo speaker. Connect a wire from one end to a 3.5mm mono plug, soldered to the tip. Connect the other side of the speaker to a 100 ohm resistor. Connect the other side of the resistor to the negative of your power supply.

Plug this into the EXT or PA speaker jack and transmit on AM. You'll hear your modulation coming through the speaker.
Note: This only works for Japanese radios like the Cybernet chassis, and only on AM mode.

2) Get a small metal project box (from RS, Farnell, or any other electronic components shop) and mount 3 sockets in it - 2 x SO239 (CB antenna connector types), and one BNC socket.

Inside the box, connect the two centres of the SO239s together. From the join, connect a 1K resistor. From the centre of the BNC, connect a 33pF capacitor. Finally, connect the other end of the resistor to the other end of the capacitor. Make sure nothing can short out, and put the box together.

Test gear available: TRIO FC 756 frequency counter, basic scope, swr/power meters, dummyloads

You have made a modulation monitor (total cost under $20 if you had to buy all the parts new) that will beat any meter.
Connect the box in place of your meter, and use a BNC to BNC patch lead to connect it to your "basic scope". Total setup time: under 1 minute.

If you build a two tone generator http://www.cbtricks.com/members/rick_eu ... ne_gen.pdf you will also be able to check how well the SSB side of the radio is transmitting.

Note: You don't need a 40Mhz or even a 20Mhz scope to do this - a basic 5Mhz one designed for servicing audio equipment will work fine.

Enjoy your modulation measurements,
Sue

lbcomms · Post by **lbcomms** » Saturday 19th Jun 2010, 3:35

The Cybernet chassis (which the Jumbo is) is known for having poor AM power...
I wish I could get to the bottom of this issue, I know others have tried and failed.
Hopefully someone else has found a fix for this, so that we can all learn how to get the AM power better.

Hi Scott,
This issue was solved here in 'oz in the early 90's sometime, although the cost of this mod meant it was only for the really keen AM'ers

It could easily add $100 to the price of the radio, by the time a PCB had been made and the extra metalwork has been done (PCB mounting, heatsink fitted to rear of radio, etc).

Sorry about the small scan, it was done under Windows 3.11 and had to fit onto a floppy disk...

This works great with any NPN modulated AM/SSB radio, like the later '02As and the President Grant / Cobra 148 Uniden chassis - especially when driving a linear.

Sue

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Saturday 19th Jun 2010, 11:38

Thank you very much Sue, that will definitely come in handy !

Warf135 · Post by **Warf135** » Sunday 20th Jun 2010, 10:52

Yay!

Tested jumbo with new Revex meter and found that it is working perfectly. It does 10 watt carrier and swings to about 18 watts, That'll do for me!

I've just ordered the parts to make the scope "breakout box" that Sue mentioned, so then I can check the modulation properly... I had previously used a two tone sine wave MP3 that i made on the computer, but now i think i'll build Rick's 2 tone generator aswell, as that will be better.

lbcomms · Post by **lbcomms** » Monday 21st Jun 2010, 2:23

It does 10 watt carrier and swings to about 18 watts

That's too high for undistorted AM power. You should see that when you get the 'scope adaptor built

Your output transistor is a 2SC1969 - that's rated at 20 watts. Divide this by 2.83 to get the maximum carrier level... If you want the maths and theory behind the numbers, search the forum for my posts, I've explained this in detail a couple of times already on the radiomods site.

This gives you 7 watts before the quality of your signal will quickly deteriorate.

And that's under ideal conditions - no power supply / line/component losses and no reflected power (i.e. a 1:1 perfect SWR).
Under real world conditions, this has to be derated by 15%... now you are down to 6 watts carrier. When the modulation limiter is correctly adjusted, a Revex meter will show 6W of carrier and 16 to 18 watts in peak mode with modulation applied.

The difference between 6W and 10W is about a third of an "S" point (less than the width of the meters needle).
Nobody on the receiving end will be able to tell the difference in signal level, but you'll sound clearer and be less likely to interfere with your neighbors TV reception or other communication services.

Two pointers for the scope adaptor:

1) Don't measure power or modulation with an antenna connected, a dummy load must be used, as any reflected power will affect the reading.
Before taking the power / modulation measurements, check it's SWR - it will give a perfect 1:1 reading if it's working correctly.

2) My scope has a low power switchable 50 ohm load in it - a basic model will NOT have this inbuilt.
To overcome this and prevent "ghosts" on the screen and/or false triggering, add one more component to the adaptor.
From the centre of the BNC connector (where the 33pF cap connects to it), connect a 47 or 51 ohm 1/4 watt resistor.
Connect the other side of the resistor to ground (i.e. the metal case of the adaptor).

Let us know how you get on with the scope measurements!

Sue

Warf135 · Post by **Warf135** » Tuesday 22nd Jun 2010, 10:02

Ok, Here is the finnished "Breakout Box"

: BreakoutBox2.JPG (22.24 KiB) Viewed 45077 times

It has a second BNC connector in parralel with the first, that fits a 50ohm 1/2 watt network terminator from my old computer junk box to give 50 ohm load to scope as required.

The only problem is that I dont see any audio on scope... if I remove the 50 ohm terminator I can see tiny movements on the scope.

Here is the circuit I built:

: BreakoutBox.JPG (21.67 KiB) Viewed 45063 times

I dont think I made any errors in the build... I fear that my basic scope may be just too basic for this task

However, if i connect scope directly to collector of Q9 (ptbm059) I get good audio on scope

lbcomms · Post by **lbcomms** » Wednesday 23rd Jun 2010, 1:15

First of all, if you require multiple BNC outputs, they should each have their own network (the 1K resistor, the 33p capacitor, and the 47 ohm resistor).
Otherwise, all sorts of loading and RF effects can happen.

If you have some BNC terminators, then use a "T" adaptor at the 'scope end and not inside the box:

2 x SO239 centres --- 1K resistor --- 33p capacitor ---BNC:BNC---------"T"--SCOPE INPUT--"T"---50 ohm load

i.e. "SCOPE INPUT is the centre of the T adaptor.

You should definitely have something on the scope:

10W of RF, at 50 ohms = 22 volts of RF.
The 33p cap has a "resistance" of about 150 ohms at 27 megs.
Therefore we have a voltage divider consisting of 1150 ohms / 47 ohms - this will divide our 20 volts to about 1 volt RMS, or nearly 3 volts as seen by the scope (the "peak to peak" voltage). Set the scope for 1V or 0.5V per division vertical scale, and you should see a signal. Any loading effects might mean you have to crank up the sensitivity a bit, but you should see something.

I dont see any audio on scope

Can you see carrier (like the fat line in picture 1)?

If YES, then you need to modulate it on AM and adjust the Time/Div on the scope for something like the picture number 2.

If NO, then you have a cable, component, loading or scope issue.
First of all, substitute the scope for a counter. On a carrier, does the counter show your frequency?

If no go there either, check your cables, connectors, and components with a multimeter.
Mine work with the values given - Ive made a 4 BNC port unit (signal generator / counter / scope / spectrum analyser).
Each BNC has a separate 1K resistor / 33p capacitor, but I didn't use the 47 ohm as the test gear has either got an inbuilt 50 ohm load (scope and counter) or has T pieces with 50 ohm BNC loads (sig gen and spectrum analyser).

If it shows a freq on the counter but not on the scope, then there is some sort of loading / reactance issue - this is unlikely. In my Uni days, I used an old hand-me-down scope (rated at only 10 or 15 megs) that worked fine for viewing modulation on 27 meg AM transmitters.

You should be able to find it easily enough with a multimeter and maybe some component substitution.

Sue

Warf135 · Post by **Warf135** » Wednesday 23rd Jun 2010, 7:11

Hi Sue, I had written a lengthy reply to you, but my internet connection went up the spout and it got erased

Here are the main points:
2nd BNC was only fitted to take terminator. Checks with FC confirm that box is working ok. Scope set to 0.05 volts/div shows no carrier. No audio on scope with 50 ohm terminator, very small audio on scope without terminator. (even using T junction at scope end)

My scope came from a science lab in a Uni, thats how i fear it may not me up to the job...its a UNILAB 032.602.

I hope my internet connection lets me post it this time

lbcomms · Post by **lbcomms** » Friday 25th Jun 2010, 4:25

Couldn't find any usable info on that one, except it was probably made in 1960. That's old...

The only thing I could think of is that it's a valve based, transformer coupled type, only suitable for audio. What's the highest Time/Div setting? That will give a clue as to how high it's meant to go to frequency wise. When you said "basic scope" I imagined something a little more recent than that

The other possibility is that it could do this, but has lost it's performance over the years - electronics from that era have a lot of component issues 50 years on.

My "basic scope" a while back was an old (1980s era) Hitachi rated at 10 or 15 megs that worked great for modulation adjustments, I've seen these go on eBay for under $50.
If yours is indeed a 50 year old model, you might want to thing about an update!

On the eBay UK site there are lots of them going cheap (search for "oscilloscope", set it to "Used items only", and sort by Price: Lowest first.
Avoid anything USB / PC based (too slow for what you need it for).
Avoid anything Valve based (Unreliable, hard to maintain, transport too expensive due to very high weight)

There were pages of them in the 40 to 70 pound range.

Get one that can do the measurements though - once you've correctly set one modulation limiter, you won't want to go back to a meter.

Sue

Warf135 · Post by **Warf135** » Friday 25th Jun 2010, 5:37

Hi Sue, thanks for taking the time to help me.

I'm pretty sure that my scope is not as old as 60's, It's transistorised and there are a few 8pin DIL chips in it:

: Scope internals; 24062010.jpg (39.49 KiB) Viewed 45022 times

: Scope; 24062010(002).jpg (69.33 KiB) Viewed 45027 times

It may be 70's-80's I dont know. I could find no info on it myself when I looked online. I think its one thats specially designed for use in schools/universitys, and probably not designed for "serious" work. It is pretty good at finding audio faults in stereo equipment, which is what I mainly use it for.

The maximum Timebase is 100ms/div
The minimum Timebase is 0.1ms/div
it also has a Timebase variable from x10 to x1
The maximum Volts/div is 10
The minimum Volts/div is 0.05

And yes, you guessed it in one! I got it cheap off ebay!!! Maybe it is time for an upgrade

UPDATE just found this info about my scope, it seems that this scope is still a current model, but mine is a few years old...

Student Oscilloscope
• Inexpensive, single channel oscilloscope with simplified control layout for student operation and permanent triggering feature • Introductory unit to teach the basics of oscilloscope use with a useful set of features e.g. 8 switched gains from 50mV to 10V per division and a timebase with switched ranges of 1, 10, 100 or 1000ms across screen with variable x 1 to x 10 control This extremely popular educational oscilloscope is robust and easy-to-use. The flat screen is 70mm across a diagonal with graticule marked in 10 x 10 0.5cm squares. Focus, Brilliance, X shift and Y shift controls determine the position and nature of the trace. A Y input can be passed to the Y amplifier directly or through an internal capacitor to block a d.c. component. The steel case is fitted with 1.8m of mains cable, a carrying handle which can be used as a tilt stand, an on/off switch with indicator LED, and cable wraps at the rear. Y amplifier: FET input, 1M d.c. to 100kHz X amplifier: 1M d.c. to 200kHz Sync. Input: 1M input, triggering at 20mV Mass: 3.6kg Size: 245 x 310 x 100mm

So it appears that my scope is just a 1 meg, that would explane a lot!

lbcomms · Post by **lbcomms** » Friday 25th Jun 2010, 11:57

The minimum Timebase is 0.1ms/div
...
So it appears that my scope is just a 1 meg

That's pretty "basic"

With a minimum like that, its going to go nowhere near 1 meg. Your "just 1 meg" is the input impedance, not the top frequency.
The inverse of 0.1mS (1 / 0.0001 seconds) = 10Khz, and with 10 marked divisions you can show 5 of these (2 div per cycle) your limit is 50Khz (0.05 Mhz)...
It barely makes it into audio

It looks like mid 80's vintage to me, after looking at the pic of the internals.

There are two ways around it - you could either upgrade to something more suitable (quicker and easier, but costs $) or make up a "downconverter" to convert a 27 meg or whatever frequency signal to the 50Khz the scope can handle (very little $, requires knowledge of low level linear amps, mixers, and PCB constriction amongst others, and lots of patience / time).

Personally, I'd spend the fifty quid or so a used 20 meg unit.

Sue

Warf135 · Post by **Warf135** » Friday 25th Jun 2010, 12:17

No wonder i'm not getting carrier on the scope!

Currently watching a couple of likely candidates on e-bay... Should know in a day or two if I have a new scope or not

Warf135 · Post by **Warf135** » Monday 28th Jun 2010, 11:50

Yes! Just won a 25 meg dual trace scope off ebay:-

: Harris 3133 25mhz Dual Trace 'scope; new_scope.jpg (31.74 KiB) Viewed 44978 times

First, lets hope it actually works when it gets here, and second, lets hope its up to the job of these here modulation mesurements

It turns out that my "bargain Jumbo" is ending up costing me quite some cash!

first i had to upgrade my power meter and now I had to buy another scope ...

This blooming Jumbo better be worth it...lol

lbcomms · Post by **lbcomms** » Monday 28th Jun 2010, 23:28

That scope should be more than up to the job of showing modulation.
Once you get used to using a tool like that for faultfinding RF/LO/IF type faults, you will never be without one again.

This blooming Jumbo better be worth it

That's your call. There were not bad in their day, and will probably still be going long after the current crop of Chinese junk has been consigned to landfill

My current radio of choice:

0.5 to 30 Mhz continuous (1 Hz steps) with self-SWRing antenna. Does me for CB, amateur, and the commercial HF channels, all in one neat (albeit rather expensive) radio...

Let us know how you get on with the new scope.
Cheers

Warf135 · Post by **Warf135** » Tuesday 29th Jun 2010, 9:51

Whoa thats one hell of a radio!!! thats the radio I'll have if I win the lottery!!!

I've been checking and I've found another possible issue with my Jumbo:- The voltage at pin 6 of IC2 (TA7310P on PTOS006) is reading 11.4v when it should be 8.3v according to the schematic I have. All surrounding voltages check ok far as I can see and the radio appears to be working ok. is this a mis-print in the schematic or a fault in my radio?

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Tuesday 29th Jun 2010, 10:03

There is a Zener diode on that PCB, marked on the diagram as MZ205 (next to C5), it would pay to check that it is OK.

Warf135 · Post by **Warf135** » Tuesday 29th Jun 2010, 11:37

Hi Scott,

Diode MZ205 (marked on board as D3) tests fine as does C4 and C5. The voltage at collector of Q2 (2SC710) is correct at 5.4 volts.

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Tuesday 29th Jun 2010, 11:54

What are the voltages of all the pins on that IC (mid band ch 19).

Warf135 · Post by **Warf135** » Tuesday 29th Jun 2010, 22:03

Voltages at IC2 (TA7310P)

Pin - Voltage
1 2.6v
2 1.9v
3 1.3v
4 2.3v
5 0v
6 11.4v
7 2.0v
8 4.3v
9 5.3v

As you can see its only pin 6 that is way off from what it should be.

lbcomms · Post by **lbcomms** » Tuesday 29th Jun 2010, 23:07

is reading 11.4v when it should be 8.3v according to the schematic

Your 9 volt regulator circuit is in trouble (a common fault with this chassis) This is labeled "AVR" (Automatic Voltage Regulator) on the cybernet schematic.

If the radio lights up but has no TX or RX the zener has gone short.

Too high a voltage on the 9V rail can be caused by three things.
Occasionally, the transistor itself goes short (this will also take out the inductor and zener).
A less common fault is a short at the mode switch (orange and pink or purple wires touching where they shouldn't).

The most common fault seen that makes the 9V rail go high is just an open circuit inductor (circled in red):

Not a critical value - anything close will do. A piece of link wire is a temporary fix, but this will degrade SSB transmit performance slightly.

If the inductor is open (a simple low ohms measurement will confirm this), check the rest of the components by lifting one side of the resistor that goes between collector and base. Power it up, there should be no voltage on the emitter - if there is, replace the transistor and zener as well as the inductor.

If you haven't got the parts, a 7808 or 7809 regulator can be used instead:
- Input pin to collector
- Output pin to emitter
- Common ground pin to base
- Remove R159
- Replace L19 and D50 with wire links
- Reattach the small heatsink
- Important! If you do this, add a 10uF 16V electrolytic or tantalum capacitor from input to ground, lying flat on the print side of the PCB.

Note: Until you clear this fault, most of the other sections of the radio will not align / work correctly.

Cheers

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Tuesday 29th Jun 2010, 23:14

I am trying to see how pin 6 can be fed with 11V, as it passes through the band switch and returns to the VCO via the VR1/VR2 trimmers, so I am suspecting that there may be a short on the band switch wiring.

(unless my diagram is a bit dodgy and is not showing a connection, which I suspect may be the case, it isn't great)

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Tuesday 29th Jun 2010, 23:17

lbcomms posted whilst I was...

Definitely worth checking the AVR to confirm it is operating correctly, I have replaced that circuit with a 7809 without any problems (I did add a 220uF 16V Electrolytic cap to it as well to ensure it had good smoothing, just a part I had a lot of, a smaller value would be fine as noted).

lbcomms · Post by **lbcomms** » Wednesday 30th Jun 2010, 0:18

The added cap is to stop the regulator from oscillating in the 40Khz region, which causes all sorts of strange things to happen.
Drove me nuts the first time I did this mod... eventually found it by looking at the 9V line with a scope - hmm, that line looks way too fat for DC

Putting the scope on AC and cranking the timebase up showed a beautiful triangular waveform, 2 volts or so p-p.

The value isn't critical - I just use a SMT tag cap on the print side, 4.7uF to 100uF, whatever value I've got handy at the time.
Even 100uF caps are getting tiny now - less than 5mm x 3.5mm:

Cheers

Warf135 · Post by **Warf135** » Wednesday 30th Jun 2010, 1:43

Thanks Scott and Sue, I'll be checking those things now...

UPDATE Good call Sue

just as you predicted, L19 was OC. replaced with same value from an old lafayette board. With one leg of R159 lifted I'm now getting 0v base, 13.8v collector and 1.8v emitter. The diode D50 is also blown and it looks like Q44 has gone south.

I want to try to get the correct parts to keep the radio as standard as possible but if i cant i'll fit a 9 volt regulator as you suggest... ebay here i come...

I have found a 2SC1398, but can only find the MZ310 diode in a pack of 200 which is overkill. Is this diode critical or would any 10v zenner do the job?

UPDATE 2 temporeroly replaced the MZ310 diode with a couple of 5v1 zenners in series and the voltages came back right (within 0.2v). So it seems that Q44 is ok, but as i've ordered a new one i'll replace it anyway just to be safe

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Wednesday 30th Jun 2010, 8:12

I would suggest upgrading the Zener, you can use any 10V Zener, but I suggest you get a 5W version, which is less likely to fail again.

Warf135 · Post by **Warf135** » Wednesday 30th Jun 2010, 9:45

Thanks for the advice about upgrading to a 5 watt diode Scott. I had assumed that virtually any 10v Zener would do but wasn't sure.

A good choice seems to be:- 1N5347B its a 10v 5W Zener designed for voltage regulation purposes and should be great in the Jumbo (plus its easy to get hold of!)

lbcomms · Post by **lbcomms** » Wednesday 30th Jun 2010, 22:30

With one leg of R159 lifted I'm now getting 0v base, 13.8v collector and 1.8v emitter

The transistor is stuffed (leaky). With nothing on the base, you shouldn't have anything on the emitter.

Is this diode critical or would any 10v zenner do the job

It's not critical at all. A common 1N4740 (10V, 1W) will work fine, or use two extremely common 1N4733A (5.1V, 1W) diodes in series.

The power rating isn't critical either. Both the 1 watt or the lower powered 400mW alternative will work OK.

Maths time:
Round the voltage to 14V and the resistor to 400 ohms to make the maths easier.
Voltage across resistor = 4V (14V - 10V). Current is 10mA (4 divided by 400, multiplied by 1000).
The zeners virtual resistance must therefore be 1K (10V divided by 10mA).
Power in zener is current multiplied by resistance (0.01A x 1000 ohms) = 0.1W, also known as 100 milliwatts.

Therefore, as long as your zener can handle more than 100mW, it will last for years. Even a puny 400mW one is only operating at 25% of its rated power limit.

The transistor isn't critical, either. I once used a 2SC2166 - a final out of an old AM only junk set - and as expected, it worked perfectly.
Any power NPN with a collector current of over one amp and a hFE (DC current gain) of more than 20 will do.

Cheers

Warf135 · Post by **Warf135** » Wednesday 30th Jun 2010, 23:14

So it seems that the 5 watt diode I just ordered is overkill, but hey, at least I know it will never blow again

New diode and new transistor should be here soon and i'm expecting the new scope to be delivered today too

lbcomms · Post by **lbcomms** » Thursday 1st Jul 2010, 1:49

The "problem" with 5 watt ones is temperature stability, they tend to drift a bit when they get hot.
The same applies to values under 3.3V or over 7V or so... though in a non-critical situation like this you probably won't notice any difference.
Here in Oz where in-car temperatures can vary wildly (below freezing at night to 50 degrees C plus in the daytime) it can affect things, so I normally use a pair of 400mW 5.1V types in series, to get it as stable as possible. A base set would not normally get these extremes, so you will be OK.

At worst, you might have to enlarge the PCB holes so that the thicker leads can go through them.

Cheers

Warf135 · Post by **Warf135** » Thursday 1st Jul 2010, 10:58

The pair of 5v1 diodes i have in there tempereoly are BZX55 rated at 0.5w 5.1v each, so i suppose i could just leave them in there... i'll see if the pins of the 10v diode will fit in the holes..if not i'll leave the BZX55's in there.

New scope arrived today, unfortunately ParcelForce have damaged it in transit which is quite annoying, but thankfully all of the main functions appear to be working fine (luckely it was the rear of the scope that got bashed and not the front)

Once i've put the new parts in and got the 9 volt regulator working properly i'll need to re-align the radio again, as i had previously alligned it with the blown regulator. it was a begger to align properly and now i know why!

Then, i can finally get round to mesuring the AM modulation with the scope and hopefully get good results

lbcomms · Post by **lbcomms** » Thursday 1st Jul 2010, 21:18

Leave the smaller ones in there for stability, and keep the 5W one as an emergency spare

Once you get the AVR working, you'll have to completely realign the thing, starting with the 10Mhz and 10.695 Mhz osc frequencies, right through to the antenna...
nearly everything relies on the 9 volt supply working correctly.

Let us know how you get on the the modulation measurements.

Warf135 · Post by **Warf135** » Friday 2nd Jul 2010, 9:05

I can't believe it.. I put the 9 volt regulator right (I left the 2 5v1 diodes in, the legs of the 5w diode were like tree trunks! lol) and now I found another problem with the Jumbo

Seems like the more I put right in this radio more goes wrong!

The tune control is acting like an rf gain in reverse, the further to the right I turn the tune control the weaker the signal gets. Its very bad on Hi band, not as bad on Mid band and barely noticable on Lo band. Guess I'll be checking all the wiring for shorts or bad connections. Could also possibly be a bad IC2 I suppose since it was getting 11v rather than 8.3v???

This jumbo is giving me a headache!

Thank heavens for RadioMods forum I say!

lbcomms · Post by **lbcomms** » Friday 2nd Jul 2010, 22:07

tune control is acting like an rf gain in reverse

This is a known issue with that chassis, usually seen when trying to get too much "slide" out of the regular 40 channel versions of it.
Unlike the Uniden chassis, the offset crystal (s) mix with the VCO to give you the first local oscillator, and not just the downmix signal that is divided down by the PLL.
Therefore, if the output level from that oscillator goes down, so does the local, and therefore so does your signal...

First of all, make sure the mixers and PLL are aligned correctly. There is a doubler circuit in there that's quite critical (T1 and associated components). T2 and T3 are the upmixer cans. For now, peak these on Low band channel 20. You can try adjusting these 3 for the most receiver noise, but it's best to adjust them with a scope (T1: adjust for maximum 20Mhz signal at TP4, T2 and T3 adjust for maximum 37.9Mhz at C21 - you'll need a X10 scope probe to adjust them accurately).

When Low band is correctly working, make sure the rest of the radio is OK (i.e. align it as a standard 40 channel version). If all is OK, align it as per the manual; if it's not OK then clear any other faults first.

Cheers

Warf135 · Post by **Warf135** » Saturday 3rd Jul 2010, 3:32

Thanks Sue, you must be getting fed up of me and my freaking Jumbo by now

(T1: adjust for maximum 20Mhz signal at TP4, T2 and T3 adjust for maximum 37.9Mhz at C21 - you'll need a X10 scope probe to adjust them accurately).

I have done this and Lo band is improved. Mid band is better than it was, but on high band the problem is still severe.
I have also re-alligned the 10.240, 10.695, the offset frequencys and the VCO according to the service manual and its no better.

Also i have removed the botched LoLo conversion, and double checked all wiring on the Tune and Clarifier controls

So if I read your last post correctly I should be focusing around T1 to look for the problem? ok, doing that now...

lbcomms · Post by **lbcomms** » Saturday 3rd Jul 2010, 4:16

i have removed the botched LoLo conversion

Good idea to remove the "golden screwdriver" type mods...

You've probably got a weak crystal - the only way to tell is with a scope on the testpoint - a weak one will drop in output, especially when you raise it above its centre frequency.
This chassis will drop in level slightly at the high extreme on the slider / tune control - that's normal.

I'd ignore it for now - get the rest of the radio (and your test gear) working correctly on the low (FCC 40 channels) band.
If it is the crystal and you can't get a replacement easily, there are ways around it - like EPROM conversion, binary adders, or even microcontrollers if you are feeling really adventurous.

If adjusting the T1 can shows a definite peak in level, it is performing correctly.

Is the AVR putting out its correct voltage (9V or so on the emitter of the AVR, on most chassis this is a pink wire that goes to the mode switch and the squelch control)?

Cheers

Warf135 · Post by **Warf135** » Saturday 3rd Jul 2010, 5:32

T1 showing definate peak on scope as are T2 and T3. AVR putting out 9.2v and pink wire at Squelch reading 9.5v

The dropout on the Hi band is severe, dropping out to nearly nothing. However, if i put the probe of my volt meter on the emitter of Q4 (2SC710) the dropout goes and the tune control seems to works as normal. All voltages at Q4 are normal as per schematic. All voltages at IC2 (TA7310P) are also correct
Could be weak Hi band crystal as you say...I'll have a look for one on the net...

Radio is pretty well aligned and *seems* to be working ok apart from this one last fault (hopefully last fault

). New scope now shows carrier and AM modulation and all is looking good there. revex meter now showing 5-6w carrier and about 15w pep on AM.

UPDATE Tests with FC connected to TP4 show that the frequency on midband and hi band drops off severly when tune control passes the 1 o clock position. on Lo band the frequency remains constant and slides as it should

lbcomms · Post by **lbcomms** » Saturday 3rd Jul 2010, 14:55

if i put the probe of my volt meter on the emitter of Q4 (2SC710) the dropout goes and the tune control seems to works as normal

That's because you are shorting out the tune control

Essentially, you are forcing the tune control to minimum.
To measure there, you need a X10 probe.

Have a look at http://www.syscompdesign.com/AppNotes/probes.pdf if you don't know what a X10 probe is or what "loading effects" are.

A couple of things to try:

1) Make sure the band switching (terminals 5, 6, and 7) is operating correctly. To do this, temporarily swap the positions of X2 (10.0505) and X4 (10.2775).
This will make the bands Low on the High position, and High on the Low position.
If the High position (which is now the Low channels) is still no good, you have a wiring or switch problem.
If the Low position (which is now the High channels) is still no good, you have a stuffed crystal.

2) You might be able to get things a bit better by aligning T1 on the worst performing band (in your case, the High band).
The peak will be at a different place on different bands, so if this does work, it will reduce your performance on the Low band slightly.

5-6w carrier and about 15w pep on AM

That's about right for this chassis.

frequency on midband and hi band drops off severly when tune control passes the 1 o clock position. on Lo band the frequency remains constant and slides as it should

Try the two suggestions above. That will at least confirm whether it is the crystal or something else.

Cheers

Warf135 · Post by **Warf135** » Sunday 4th Jul 2010, 4:29

I'm going to kick myself up the rear for not thinking of swapping the crystals

But I suppose I couldn't see the trees coz I was too deep in the forest...

Tests reveal that crystal is ok, and band switch wiring is ok. Tune and Claririer circuits must be ok as Lo band is working correctly.

Then, I backed off CT4 a little and it all came good, albeit now off frequency slightly so i'm guessing that CT4 is faulty... it looks like CT4 had already been replaced and for some reason they had put a green trimmer rather than a red one. soon as i find a red trimmer i'll replace it and post the results.

I have got a x1 x10 switch on my scope probe but not on my voltmeter... but even with negative probe disconnected probing emitter of Q4 was putting things back right - its probably that connecting the voltmeter probe to Q4 emmiter was just altering the capacitance of CT4 enough to bring things back... further evidence that CT4 is gone bad in that case.

lbcomms · Post by **lbcomms** » Sunday 4th Jul 2010, 5:25

The color of the trimcap is just a range indication - one is 1.5 to 12pF, the other is 2 to 20pF. Either will work here.

I backed off CT4 a little and it all came good

That's a definite sign of a weak crystal - if you increase the capacitance it will be driven a bit harder and start working again.

probing emitter of Q4 was putting things back right

Were you in X10 mode and with the earth and scope leads connected? If so, you shouldn't have any change in operation of the circuit.
If not, you were "bypassing" (shorting an RF signal to ground) the Tune control.

Anyway, the cure is easy in most cases for this one:

- Adjust the trimcap for correct operation on the most affected band. It will be slightly off frequency, ignore this for now.
- Move the Tune control to bring it back onto the right frequency. It will be off centre. Again, ignore this for now.
- Dont touch the tune control. Adjust the other 2 band trimcaps so it is showing the correct frequency on their respective bands.
- Move the tune control back exactly to the centre position.
- Add a resistor as below to the circuit to bring it back onto frequency.

If it's too high, add a resistor from the centre terminal of the tune control to GROUND.
If it's too low, add a resistor from the centre terminal of the tune control to the 9V RAIL (i.e. the emitter of the AVR 9V regulator transistor).

A resistor substitution wheel is a handy gadget here, but an old pot with a wires soldered onto it is a good enough substitute.
Turn the pot / wheel until it is showing the correct frequency, then lookup (if you use a wheel) or measure the resistance (if you use a pot) to figure out the value.

This will have the tune control working at a slightly reduced range and the radio perfectly on frequency.

Enjoy

Warf135 · Post by **Warf135** » Sunday 4th Jul 2010, 9:45

Hi Sue, Thanks for the workaround soloution, I'm sure that if i cant find the cause of the fault soon, i'll probably fall back on that, Though i really would like to correct the problem rather than work around it if possible.

I'm pretty sure that the crystal is ok, as i've swapped in a 9.8mhz crystal from another rig and the fault was still there... but I'm gonna try and order some new ones just to be sure.

Here's my findings so far. All component references refer to PTOS006AOX board unless stated:

1. Probing emitter of Q4 with the test lead flying loose (not plugged into the meter) corrected the problem and even if i touch a metal screwdriver on the emitter of Q4 the problem gets a little better which leads me to thinking there is a capacitance problem here somewhere...

2. Changing CT4 back to a red trimmer made the fault worse on mid band but made little effect on Hi band.

3. Putting finger near C23 improved problem slightly so it was replaced, now finger has no effect on problem and problem is still the same.

4. C24 is not present. I cannot find C24 on schematic so wonder if it was omitted for some reason (or could this missing component be the problem?).

5. Disconnecting C204 (39p) on Y3 riser makes the problem really bad. Replaced C204 and problem is still here.

Tommorrow I'm gonna swap out Q4 just to eliminate that as a problem. I know all the voltages on Q4 test fine which indicates its probably ok but strange things do happen sometimes

lbcomms · Post by **lbcomms** » Sunday 4th Jul 2010, 19:06

Probing emitter of Q4 with the test lead flying loose (not plugged into the meter) corrected the problem

There is probably 50pF of capacitance in that. Adding it across a 20pF cap = 70pF, and this means your tune control is not going to operate correctly

Changing CT4 back to a red trimmer made the fault worse on mid band

There is one trimmer for each band (CT2, CT3, and CT4) on one for LSB offset (CT5). It's not surprising that the change only affected one band...
The only difference in the colors is the amount of capacitance at the low freq end (maximum capacitance) of its adjustment.

C24 is not present. I cannot find C24

Missing components in electronic circuits is very common and intentional. They use the same board for different products and just omit what is not needed for that implementation. For example, C24 might be only used on a version that has 40 channels only (in this case, it would be missing lots of other parts, such as X3, X4, and so on).

Disconnecting C204 (39p) on Y3 riser makes the problem really bad. Replaced C204 and problem is still here.

Try increasing it slightly in value (say from 39p to 47p) to give it more drive. You might then have to do the resistor trick I mentioned earlier to centre the tune control.

There is a small (a very small!) chance that Q4 has gone low gain. I've seen this three or four times in 15 years, and I have repaired thousands of radios in that time

Good luck...

Warf135 · Post by **Warf135** » Monday 5th Jul 2010, 4:21

Hi again Sue, This Jumbo is fun eh?

It's not surprising that the change only affected one band...

CT4 is the trimmer for Hi band, Changing it affected the problem on Mid band which I think is strange...

This radio is my own personal project so i got plenty of time to figure out the problem / work on the radio thankfully and if i can, i would like to keep everything as original as possible.

I should know tommorrow if i can get some new crystals for the radio, which may or may not help. Not had time to swap out Q4 yet, but thats my next move.

UPDATE: Replacing Q4 has no effect on problem. I have also now removed a modification that had been done to the VCO block connections (some kinda FM audio mod from what i can tell) and that didn't improve things either

I'm not gonna do anything more to the Jumbo till i can get some new crystals... I'm pretty sure the crystals are ok but i'm running out of ideas...

UPDATE: Full set of new crystals ordered from Spectrum Comunications. Hi (10.165Mhz), Mid (10.0525Mhz), Lo (9.940Mhz), and a LoLo (9.8275Mhz) crystal for when I re-do that mod... 4 crystals and postage = less that £10, so I'm pretty happy about that!... just waiting for them to arrive now...

Meanwhile, I've built a diode based crystal switch board so I can now switch between the Lo and LoLo band crystals properly and neatly. Hopefully the new crystals will correct the fault and the radio will work good after - i'm not holding my breath...lol

Warf135 · Post by **Warf135** » Friday 16th Jul 2010, 9:34

Replacement Crystals fitted and problem is still there

There must be some instability in the circuit somewhere... Guess i'll just work through it testing each component in turn...

Summary of fault: Tune and clarifier acting as a severe RF gain...The more the tune and clarifier controls are tuned to the right the less the signal gets.

I have replaced IC2, Q4 and the three mixing crystals. tried swapping crystals (lo for hi) and that didn't help, so it would appear that the Band switching is ok. it seems that the frequency readings at TP4 drop off as the receive drops...so I guess I'll be working back from TP4 towards the mixing crystals...