GE A35825B No TX

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GE A35825B No TX

Post by Johnno »

Radio Make and Model:General Electric A35825B
PCB Number: PTRF003DOX and PTBM071COX
PLL Number: O2A
Test gear available: Digital multimeter,SWR/power meter, soldering iron and solder and de-soldering pump
Symptoms:No transmit AM or SSB.

What is working:Everything else fine.

What has been tried: Replaced Q208,207 and 206, tested D202 (one end lifted) checked ok

Did it suddenly happen ?:Radio was put away for a while, can't be sure.
Anything else that may be of help?: Its an 18 channel but has 2 switches for extras mounted through casing.
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Re: GE A35825B No TX

Post by 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 change, then you need to either replace the VCO (good luck getting one these days) or repair it (a job for an expert!).

If the VCO is OK, then, while feeding it mic audio on ssb tx, trace it out from the tx mixer IC thru to the RF output stage using a CRO.

That covers 89% of the GE "no tx" faults I've seen in the last 10 years...

Less common: Failed TX mixer IC / Failed DC switching transistors (both rare faults, about 1% total), and damage done by "screwdriver experts" (about 10%).

Hope this helps.

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Re: GE A35825B No TX

Post by Johnno »

Hi lbcomms, thanks very much for that, I will give it a try.

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John
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Re: GE A35825B No TX

Post by Johnno »

Hello again, I replaced Q210 before putting it back together, set up another HF radio using an UHF antenna, and was getting signal of 9+ all modes and sounding very nice, but, still no movement on power meter. With mic keyed it was drawing 1.3 amps and under modulation 1.5 amps (all modes). I don't have a CRO nor the technical skill to use one, but, would much appreciate a list of components most likely to fail, bearing in mind that I have replaced the RF pre amp, RF driver, RF power and mixer transistors and I have schematic, pcb layout and service manual. I really want to get the old girl going as it is quite a nice example of the model.

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John
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Re: GE A35825B No TX

Post by 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 analog one is all you need, these work fine at 27 megs.
If you can use a multimeter, you can use a CRO with a few minutes practice. A multimeter measures DC or low AC voltages, a CRO measures RF voltages easily. Not as accurate, but it dosen't need to be, you are only looking if the voltage is getting stronger or weaker. A CRO can also show your modulation waveform on AM and SSB, something you can't do with a meter.
It can also accurately measure PEP (ssb) wattage, again something that most meters can't measure.
Ditto for AM modulation percentage, this can't be measured with a meter, even it the meter says it can.

You should be able to get one, including probe cables, for under $100. For faultfinding and repairing RF faults, once you have used one, you'll wonder how you ever managed without it.

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Re: GE A35825B No TX

Post by Johnno »

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?

Cheers
John
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Re: GE A35825B No TX

Post by Classic CB »

Hi Johnno: A 60MhZ scope is ideal for radio servicing work, try to get a good make such as Hitachi, Gould, Hameg, Tektronix etc as these are made to good spec. Also make sure you get the test probes, power lead & if possible the user manual for it.
When looking at scopes on ebay look for scopes that are shown lit up & preferably with a trace on the screen, a couple of people have been caught out this way & bought dead scopes.

HTH: Andy@ Classic CB
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Re: GE A35825B No TX

Post by Johnno »

Hi Andy, thanks very much for the information, I will be keeping a look out for one in the next month or so, money is a little tight at the moment, unfortunately :(

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John
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Re: GE A35825B No TX

Post by 222 »

johnno...you've been given alot of good advice here. only thing i would add at the moment is 2 things: 1, look at your schematics for "source voltages" listed in the tx sections. great, easy method of tracking a problem. if your schematics don't provide much of that, then i recommend "sams". i don't know offhand how old a g.e. that is, but many of them are in the sams books, if you have any access to (library?). those include all the "source voltages", as well as where to find everything. invaluable tool.

2nd, is bad solder joints. such an underrated, underreported, but #1 cause of stuff not working. bad joints are due to diff. reasons, but often very hard to find. esp. ones that "look good", but are internally, not. i've become extremely good at "sniffing them out" & tracking them down & can say they they are extremely common. consequently, when you have an issue like you're having, always (besides visual close inspection of pcb) "flex & stress" the heck out of the pcb, in this case, while you are tx'ing. i have found "the problem" so many hundreds of times, by diligently doing this practice, that it's not funny.

and if you "get any action" out of this way of checking, but cannot pinpoint location, "those" are the toughest ones, because almost any pressure applied almost anywhere, shows an "intermittent, indicating a likely bad joint....so then, just start desoldering & resoldering. i can safely guarantee you that many "repairs" done by many techs, with all kinds of measuring equipment backing up "their findings" of bad componentry, in reality, when they replaced the component, they were inadvertently correcting the joint problem. that's fact!

good luck & while this may not be the fix on this one, i guarantee it will find & fix many radios.

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Re: GE A35825B No TX

Post by Classic CB »

Hi 222: Can't argue with that mate, seen it many times where Tech's have repaired a faulty radio by changing a non faulty component. As you said the problem was cold solder joints in the first place, a high power illuminated magnifier is a godsend on any bench for finding cold joints & even then you won't find them all that way, it comes down to experience & perseverence.


Andy @ Classic CB
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Re: GE A35825B No TX

Post by 222 »

yes...but what's important to note is that the "issue" with the joint, is not necessarily the classic "cold solder joint"...there are numerous causes & resulting "bad joints"....in fact, ironically, i just walked in a couple of minutes ago from figuring out about 6 "issues" a driver had with his system, that no one else was finding, one of which was indeed a bad joint, but in the case of this bad joint, it was not due to a "cold solder joint". in this case, it happened to be a "popped solder joint". the point is, that there are different types of bad joints & in some cases, in fact many....the magnifier will not show the problem, because in many cases (many for me, because i have learned to "root them out"), the problem is simply not visible. an example would be if due to poor technique, and/or contamination of the solder, that a bubble is created inside an otherwise perfectly normal looking joint. and the wire or component leg is sitting inside that bubble....well, just use your imagination, as to how "elusive" that might be to find, or for another different example....someone puts pressure where they should not be doing so, because they don't think of or realize, at the time, what problem it is likely to cause...such as "popping" a solder joint, which in the case of very small components, results in a "ring" of a break in the joint, which not only can be microscopically small, in some cases, but as such, now what results is a "make/break" condition, that is affected by hot/cold, vibration, mounting bracketry, case screw tightness, even atmospheric pressure changes.....and because of how easily that "make/break" condition is affected, then it can become an extremely elusive, difficult thing to find, because of something i call "deferred action". what that means is the bad connection is so easily made or broken, that not only you can't see it, but when you try to "hone in" on it, it stubbornly eludes detection, because as you "manipulate" the board, looking for the intermittent, everything you seem to touch, causes the reaction, because everthing is connected to each other, so a tad pressure here, causes an equal tad pressure there....and the actual site of the break is always reacting, sitting somewhere in between. here, in that case, is where the real skill comes in, to find & fix, in a "reasonable amt. of time".

i can safely guarantee this is quite common, based on how many times i have found it to be the case, and the fix.

i've taken the time to spell it out like this because there are many that don't realize and i bet that somebody will read this, and finally find "the problem" that couldn't be found before & then hopefully will post it here, to validate.

incidently, when they are "that" hard to find, only years of experience & patience will help to find, but when you know it's there, but can't find, for above reasons, then simply start desoldering/resoldering.
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Re: GE A35825B No TX

Post by Johnno »

Hi, just on CROs how bout a 40mHz Hitachi V422 anygood? Has no probes but I found a manual on the net.

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John
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Re: GE A35825B No TX

Post by Johnno »

Hi, these are the top and bottom pcbs of the GE B-model. Schematic too.

Image
Image
Image


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John
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Re: GE A35825B No TX

Post by 222 »

yeah..chassis looks pretty familiar..i think i might have 1 or 2 of them kickin around. how you fixed for parts? and why bothering with...learning project or sentimental or collectors items? just curious.

that schem. jpg. is useless in that format, to view, but maybe renders up in a better pic program? so they didn't have after all, on mkham?

on last i think i said 35-xxx, but i think i meant 3-58xxx.
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Re: GE A35825B No TX

Post by 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 high - you can if you like, it will just cost you more...
If using it for faultfinding and for showing waveforms, you only need at least half the operating frequency for good results - for "where has my signal gone" faultfinding, and for showing AM and SSB modulation at 27 megs, you can get away with a 15MHz CRO if you are doing it on a limited budget. A 15 MHz CRO is much better than no CRO at all!
Back when I was a poor struggling uni student, I had a single-trace 15Mhz Hitachi CRO that served me well for years (and I've still got it as a emergency spare).

A new set of probes for it, if you don't get any with the one you buy, will set you back $20 to $30 at dick smiths, jaycar, or the like.

Have fun...
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Re: GE A35825B No TX

Post by Johnno »

222 wrote:yeah..chassis looks pretty familiar..i think i might have 1 or 2 of them kickin around. how you fixed for parts? and why bothering with...learning project or sentimental or collectors items? just curious.

that schem. jpg. is useless in that format, to view, but maybe renders up in a better pic program? so they didn't have after all, on mkham?

on last i think i said 35-xxx, but i think i meant 3-58xxx.
Hi 222, they are my favourite radio of all time and for a learning project (prolly not ideal for learning with twin board design). I resized the image, it is much better at original resolution and you can zoom right in on it. VKHAM.com (see it here http://vkham.com/Info/manuals.html ), yes thats where I got it from.

Cheers
John
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Re: GE A35825B No TX

Post by Johnno »

Hi lbcomms, ahhh good to know, thanks for the useful information , I will keep that in mind for bidding purposes.

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John
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Re: GE A35825B No TX

Post by Johnno »

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). I have yet to put GE on the bench.

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John
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Re: GE A35825B No TX

Post by Johnno »

Ok, I was checking voltages on the DC switching transistors, one showed voltage on one leg (Q220) and the other (Q219) showed no voltage anywhere, this prompted me to look closer at nearby components and I happened to notice a resistor (R221) had broke away from the board at one end which forced me to look even harder and sure enough there was a diode (D205) in the same state, I have no idea how they got like this :shock:
but will replace both tomorrow, hopefully my first repair :).Will report back.

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John
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Re: GE A35825B No TX

Post by 222 »

hold the phone. i'd like to ask you something at this point. have you done anything to change the state of what you found? if not, i would ask you whether or not the broken connections are close enough the board to "make/break" connection with a flexing of the board? and if you have already done something, i would ask if you at least think the above was maybe possible?
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Re: GE A35825B No TX

Post by Johnno »

Hi 222, no chance, the breaks in both left quite a gap, the pieces are still sticking up through board, you would have needed to snap the board in half for them to make any contact at all, the diode had actually shifted sideways ( thats how I spotted it ) and the resistor was at about a 25 degree angle.

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Re: GE A35825B No TX

Post by 222 »

hmmm...well out of curiosity...it sounds like your speaking about the "component side" of the board, and saying the "gap" was too far, and you don't know how this happened....so i'm wondering...were they both "cut"....as in a hatchet job (mod) of some sort, or if not, are you saying the 1 leg from ea part broke it's solder joint & then pulled completely thru the solder side of pcb and wound up as you found? (in which case, it still could've been someones technique for modding.
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Re: GE A35825B No TX

Post by Johnno »

222 wrote:hmmm...well out of curiosity...it sounds like your speaking about the "component side" of the board, and saying the "gap" was too far, and you don't know how this happened....so i'm wondering...were they both "cut"....as in a hatchet job (mod) of some sort, or if not, are you saying the 1 leg from ea part broke it's solder joint & then pulled completely thru the solder side of pcb and wound up as you found? (in which case, it still could've been someones technique for modding.
Yes component side, upon further investigation and bearing in mind your input about a mod, yes it appears to be something to do with the external switches for the extra channels. One side of the diode leads to OSC1 section of schem and the resistor appears to lead to the noise blanker section which leads me to believe this radio has many more channels than I originally thought :banghead:

I guess I will just change 2 of the transistors in the DC switch and see what happens. Thanks for your help 222.

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John
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Re: GE A35825B No TX

Post by 222 »

sounds like it could be a clarif. mod, but you'd know better as i'm not looking at schem. if it's not a poor connect somewhere, then from what you've said so far, my guess is to look at coupling caps in final section. i mean, if i'm not mistaken, the radio does indeed switch into & does tx, and on correct freq (?), just lo pwr, and you've replc'd the necessary pwr components, right?...so i'm thinking stll....back to bad connect or perhaps open/shorted coupling device in back end. (better take a close look at pcb in area of so-239...)
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Re: GE A35825B No TX

Post by 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 the VCO tuning voltage, if it receives you will already know this is OK.

Remove the shield on the print side of the board, covering the finals. You'll see where the two transistors go thru small rectangular cutouts and solder onto the board.

One side of each is ground (emitter). The other side is the input (base). The centre pin is output (collector).
The one nearest the front is the driver, the one nearest the back is the final.

Put the rig in AM TX, the CRO in AC on a high voltage (2 or 5 volts) and if you have a X10 switch on the probe, turn it to X10.

Touch the probe tip on the input to the driver, and key the mike.

You should see a waveform (a carrier looks like a very fat line) on the screen. If not, turn up the CRO volts/Div until you do.

Move to the output of the driver. The line should be 3 to 10 times fatter.

Move to the input of the final. It should be roughly the same.

Move to the output of the final. The line should be 3 to 10 times fatter.

Move to the center pin of the antenna connector. It should be roughly the same.

At any stage, if the waveform fills the screen, reduce the CRO Volts/Div until its 2 to 3 cm thick.

This will tell you where to start looking for the loss of TX signal. If the current is going up markedly on TX, some RF must be at least reaching the driver, which would rule out DC switching transistors, mixer ICs, and so on.

One thing I have seen on these is dry solder joints on the adjustable (screw cores) inductors between the final and the antenna socket. More common is a failed final, almost always due to a bad antenna or coax. If this is the case,
comparing the input to the output pins on the final will show a decrease in the thickness of the waveform, instead of the expected increase.

Have fun...
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Re: GE A35825B No TX

Post by Johnno »

Hi lbcomms, wow, nicely explained mate, even I could understand that. I will give it a try and see what happens, and report back my findings. Once again thanks very much!

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Re: GE A35825B No TX

Post by Johnno »

Hi lbcomms, here are my findings: 0v to rf driver Q207 BC+E measured on ac setting on CRO.
Discontinued testing on RF section.

Started testing DC switching transistors DC setting on scope radio in SSB mode:
Q220
base:1v unkeyed 1v keyed
collector:0v uk 1v k
emitter:1v uk 1v k
Q219
b:1v uk 0v k
c:1v uk 1v k
e:1v uk 0v k
Q218
b:0v uk 10mv k
c:50mv uk 50mv k
e:0v uk 50mv k
Q217
b:5mv uk 40mv k
c:50mv uk 30mv k
e:50mv uk 25mv k

AM mode:
Q220
b:1v uk 10mv k
c:5mv uk 8 mv k
e:8mv uk 15mv k
Q219
b:0v uk 0v with slight disturbance to negative on keying
c:5mv uk 0v with slight disturbance to negative on keying
e:5mv uk 10mv k
Q218
b:1v uk 1v k
c:0v uk 1v k
e:1v uk 1v k
Q217
b:1v uk 1v k
c:1v uk 1v k
e:1v uk 50mv k

Do these voltages sound correct or am I barking up the wrong tree and was I correct to stop testing RF if there was nothing at driver?

Cheers
John
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Re: GE A35825B No TX

Post by lbcomms »

Image

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 code. What do they mean?
B = regulated voltage (9 volts). R = Receive, T = Transmit, A / U / L /S = Am / USB / LSB / SSB.

Therefore:
SB means 9V on SSB
AM means 9V on AM
BR means 9V on receive
BT means 9V on transmit
BB means 9V at all times in "cb" mode.

Two of the above terms can be combined, for example BAT means 9V on AM transmit, and BRS would mean 9V on SSB receive.

Try to trace of the missing voltage. you could have a bad mode switch, bad PA switch, etc.
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Re: GE A35825B No TX

Post by Johnno »

Put the rig in AM TX, the CRO in AC on a high voltage (2 or 5 volts) and if you have a X10 switch on the probe, turn it to X10.

Hi lbcomms, apparently I didn't understand that part (I did put CRO on DC to check DC switch), my mistake, all the readings I obtained were incorrect (something to do with x10 on the probes me thinks) :oops: will get back with correct voltages tomorrow, thanks once again for your fantastic input!

The RF driver is showing bugger all BTW.

Cheers
John
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Re: GE A35825B No TX

Post by 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. First of all, disconnect the bases and collectors of the driver and final, then measure the TX current draw again. If it goes back to half and amp, the biasing circuit is in trouble - R235 (47 ohms) and R238 (can be either 10 or 12 ohms) sometimes go high or open, and cause the TX current to rise a lot, sometimes taking out the driver or final at the same time.

If the current is still 1.3A on deadkey TX with the driver and final bases / collectors isolated, the trouble is not in the bias section - tn that case, it could be a shorted/leaky cap or diode connected to the BT line somewhere.

Good luck!
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