The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

lbcomms

Eprom technology was hot stuff in the 70's and 80's, most of us have moved on since them... Use a flash based PIC or AVR instead - much easier once you know your way around them... for example, it you want channels 41 to 80 with an EPROM, you'll need the codes for input (switch) and output (PLL) - t...

lbcomms

i found an alignment page for checking bias amp draw.... i made some adjustment Did you check the circuit was working correctly first? i have approximately those volts... 6.8's and 13.7ish and 0.1's When replying with multiple answers, tell us where you measured each voltage. The numbers above are ...

lbcomms

audio sounds off on SSB,but on am it sounds clear By "sounds off" I'm assuming that you mean distorted, not off-frequency. First thing to check then is the biasing. On the driver and final in SSB TX mode ( with the mic gain turned all the way down! ) you should have zero volts on the emit...

lbcomms

Temporarily disconnect the base of the 2SC2166 driver. Dead key on AM (the radio will not actually transmit a signal). Measure the voltage on the collector (metal tabs) or the driver / final. Are you getting about 6V? I've only ever seen this on radios where noobs have fitted so called "swing k...

lbcomms

Are you sure it's a 244, and not a 122 (i.e. PC122) or 144 (as in AX144, AR144)? I only get about 1-2 watts when modulating on SSB That's all a cheap (RMS) meter will show, unless you shout really loud into the mike. Are people receiving you OK, or are they saying you are scratcy/fuzzy/distorted? In...

lbcomms

Detailed instructions can be found in the following book.

It's no longer sold, but can be downloaded for free from the original publisher:

http://www.cbcintl.com/docs/EpromBook.htm

Go there, and click "Get it now" at the bottom of the page to download it.

lbcomms

These things always drift like crazy - they certainly needed the factory fitted slider :) They can be made better (not fixed completely) by adding a PTC heater to the main reference crystal. Crystal heaters can be had from: http://www.minikits.com.au/kits3.html#eme177 They also sell the component al...

lbcomms

so I totally understood the complicated maths I think your sarcasm meter just went right off the scale :) Seriously though, it's fairly straightforward for those of us that work with this stuff for a living. That was only with 2 or 3 unmodulated carriers - if you want complicated , try digital info...

lbcomms

Changing C204 brings the radio under frequency......so the problem does not happen Then it's a low drive problem. This will be on the crystal side of the transformer. A slightly technical, highly simplified explanation: A capacitor's impedance (AC resistance) reduces with increasing frequency - in ...

lbcomms

There must be some instability in the circuit somewhere... No, instability causes unwanted signals (such as audible squealing or loud hissing) to be created. What you are looking at is low gain, somewhere in the oscillator circuit. Guess i'll just work through it testing each component in turn... M...

lbcomms

From memory, I just removed / disconnected the tripler components and fed it 14.725 from an crystal oscillator made up on a tiny PCB. Quick and dirty, oscillator just made it to the USB and LSB offsets, and had to wire a varactor from the clarifier to get a decent range out of it. That was my last c...

lbcomms

Just looked back thru the records and we did an AX144 about 18 month ago - converted to NZ40 only. It involved one crystal and I had to add a second varactor to the circuit to get a decent range out of the clarifier - other than that, it looked straightforward enough. The AX144 is a decent enough se...

lbcomms

First I just want to convert a Uniden 40 ch am set model "Pro 520e" ... also consider converting my fabulous Uniden AX 144 The first one would not be economic to convert (would be in the order of $300 Aus plus). Cheaper to buy a secondhand or even a new radio, and I'd say it would be a lo...

lbcomms

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 t...

lbcomms

You'll get more chance of a reply if: 1) You don't try to put lots of sentences into one. Punctuation, paragraphs, and correct capitalisation make things much easier to read for someone to help you. 2) Do a bit of basic research first. Try Google. 3) Learn a bit about what you are trying to archive....

lbcomms

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. p...

lbcomms

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 h...

lbcomms

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 c...

lbcomms

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 oscillat...

lbcomms

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...

lbcomms

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 var...

lbcomms

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...

lbcomms

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 :shock: Putting the sco...

lbcomms

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 sh...

lbcomms

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 stil...

lbcomms

First thing to do is know what you are doing - a search of this site will give you all the details: 1) You are going to have to increase it by a factor of 10 (to 30 watts) to make noticeable difference. If you increase it to 4 or 6 watts, it will make NO difference to your signal and range. 2) How a...

lbcomms

Look on the main board at the electrolytics. If they are rated at 10V, mark its top with a red marker pen. Check all of them. When you are done, replace ALL of the marked caps. And make sure you get all of them... Doing this will cure 95% of these types of fault from this chassis (Uniden MB8719/34) ...

lbcomms

Ive seen this with the 858 and other old chassis that have been "broadbanded" by changing the taps on the 2-in-1 transformer can following the mixer. This "mod" makes it rather sensitive to stray RF - antenna too close to radio or poor grounding on the radio/amp/antenna and the o...

lbcomms

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) = 1...

lbcomms

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 f...

lbcomms

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 an...

lbcomms

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 a...

lbcomms

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 solv...

lbcomms

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 a...

lbcomms

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). http://i47.tinypic.com/f6stx.jpg Note the "PEP moni" pull switch on the CAL control. This will show the peak AM pow...

lbcomms

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 "swi...

lbcomms

Hi, You can replace the 8719 with a '151, although you will have to make a new filter circuit for the PD output: That's easy though... - Connect the PD pin to the tuning voltage on the VCO via a 3k3 resistor - Connect the IC side of the resistor to ground via a 0.1uF tantalum capacitor - Connect the...

lbcomms

Mixer mods won't do you any good with that PLL - it does not use mixing on the PLL, instead it divides the VCO frequency directly to synthesise the frequencies. If there's nothing to mix, you can't change the ingredients :-( You can't change the 10.240 crystal either, as the 455Khz shift (used to sw...

lbcomms

A simple pi-filter (also known as a band pass filter) is unlikely to help you - unless your own gear is so "dirty" that you shouldn't be putting it on air in the first place! In 99% of cases of interference to TV, radio, audio systems, telephones, etc it needs to be tackled at the device b...

lbcomms

These radios (and almost any other SSB radio) can have the drifting fixed by keeping the two (reference and carrier oscillator) crystals at a constant temperature. Minikits in south australia sell a component designed just to do this: 50 degree C PTC thermistor suitable for using as a Crystal Heater...

lbcomms

For measuring DC voltages, stick to your multimeter. Use the CRO for audio, RF, and waveform measurements. With mic keyed it was drawing 1.3 amps If no RF is getting to the base of the driver, that is not a reading to expect. Should be less than half an amp. Something is drawing way too much current...

lbcomms

http://i40.tinypic.com/fn9umv.jpg No, the radio will not work with those voltages! The voltages marked with the triangle is a TX voltage, i.e. Q219 emitter = 8.5V on transmit only. Looks like you lost the 9V rail somewhere. Look at the markings on the drawing, you'll find some with a 2 or 3 letter ...

lbcomms

Hi, I got my self a CRO (25MHz), delivered today, I have been learning how to use it from the manual, I checked pin 5 on another O2A radio (Phantom Communicator) and the line was very gradually indicating voltage changes (setting 1v/cm) as i went through the channels (1-18). Hi Johnno, Thats just t...

lbcomms

Hi lbcomms, thanks so much for your help, you are a champion. I am seriously thinking of getting a CRO now, I had a look at them on ebay. Is a 60 megahertz ok too or does it need to be near 27mhz? The MHz rating is the maximum frequency you can measure accurately with it. You don't need to go that ...

lbcomms

If the current is going up by an amp when you transmit, but there is no power coming out, then it's one of the RF components (a coil or a capacitor). A CRO would tell you which one in seconds. I don't have a CRO nor the technical skill to use one There are some on eBay for under $50. A basic 20 Mhz ...

lbcomms

The two most common causes of this are the RF output transistors and the VCO block Can you hear it when transmitting to another radio alongside it on SSB? If you can, that clears the VCO. If not, check the voltage on pin 5 of the PLL IC, it should change as you go through the channels. If it dosent ...

lbcomms

Not sure what you mean by "usb/lsb channel frequencies" - the lsb, usb, am, and fm are all the same "frequency" when you are on a specific channel. Only the operating mode is different. Or do you mean the list of frequencies that each channel is on? The list can be found here: ht...

lbcomms

Hi, I found the circuit, and time has not been good to it - very faded and lots of old repairs with stick tape, the gum from the tape has soaked into the paper leaving inch-wide dark brown lines all across it. I'll have a go at scanning it on the weekend, but I don't think it will be pretty... Befor...

lbcomms

It's actually fairly easy to find the info, you just need to know what to look for... From the web link of the amp: - 200 Watt PEP* - 1.5db. Rf Compression Limit That means that it's 1.5db past its limit at 200 watts. OK, math time again. 200W - 1.5dB = 141.59 watts, but that's under ideal condition...

lbcomms

If you need to "lower your dead key" then you are overdriving your amp, causing distortion (at RF, this means unwanted nasties such as harmonics and spurious mixing products) that will interfere with other services such as TV reception. A "linear" amplifier (most backyard amps ar...

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