GE A35825B No TX

[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!

Cheers
John

[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

[lbcomms]

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.

[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) will get back with correct voltages tomorrow, thanks once again for your fantastic input!

The RF driver is showing bugger all BTW.

Cheers
John

[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!