Quad 303 67v rail problem

[PJL]

There are some odd readings there, TR200 seems to be cutoff but TR201 is showing some base voltage.

I suggest you take fresh readings with the -ve on point 14.

[johndoe]

Originally Posted by PJL

There are some odd readings there, TR200 seems to be cutoff but TR201 is showing some base voltage.

I suggest you take fresh readings with the -ve on point 14.

TR200
E 26.1
B 28.5
C 0.7

TR201
E 0
B 0.6
C 13.3

Terminals
12 85v
13 13.2v
15 -0.6v

[frsimen]

This is showing all the signs of TR3 not being controlled properly due to thermal runaway. Check the PCB tracks around R205 and R207 carefully for hairline cracks, which could be opening as the PCB warms up. If all looks well there, consider replacing TR3. When you do, make sure that you use some heat sink compound to give the best thermal contact with the heatsink.

Your measured voltages show that TR200 is cut off, and can’t reduce the current from TR3 any further. TR201 won’t be conducting much in that state either, so TR3 is probably faulty. The resistors, R205 and R207 allow TR201 and TR3 to be cut off. If the connections to them are bad, it could give the impression that TR3 has failed, when it is actually not faulty.

Paula

[johndoe]

I've checked the traces, given them a flex/resistance check and used a magnifying glass, I can't see anything wrong. I've resoldered R205/207 for good measure. I've replaced TR3 with a new MJ802, paying attention so as not to short to the les to the heatsink. Still no difference to the 67v rail.

[PJL]

The circuit I am looking at has 15 badly printed so it should have read with the -ve on 15 but no matter as it's just 0.6V difference.

It's TR200 that looks faulty to me. It's cut-off so should not be conducting so TR201 base should be 0 but it has sufficient voltage to make it conduct. Try replacing TR201 first. Did you check the value of R205?

The PCB is paxolin, does it look 'cooked'? It is possible there is leakage in the PCB material.

[PJL]

I meant TR200 first!

[johndoe]

Originally Posted by PJL

The circuit I am looking at has 15 badly printed so it should have read with the -ve on 15 but no matter as it's just 0.6V difference.

It's TR200 that looks faulty to me. It's cut-off so should not be conducting so TR201 base should be 0 but it has sufficient voltage to make it conduct. Try replacing TR201 first. Did you check the value of R205?

The PCB is paxolin, does it look 'cooked'? It is possible there is leakage in the PCB material.

I have already tried substituting TR200 with a BC556, that made no difference to the operation of the unit so I've put the original transistor back in for now as it tested fine out of circuit.

The only parts I haven't changed are the TR201, C200, C201. I'm waiting on the postman for the TR201, C200/C201 test fine out of circuit.

The PCB is in relatively good shape with no heat damage at all, there were some signs of moisture ingress/tarnish, probably condensation from loft storage, but nothing major. The dust was 40 years thick and has been completely removed, maybe that was holding it together?

[PJL]

Something is providing current to bias TR201.
Measure R205 in circuit with your DMM to confirm it is not faulty (your black probe should go to TR201 base to avoid forward conduction of the be junction).
Remove TR200 and measure the voltages around TR201 again but this time from 15 and not 14. Also measure 13 and 14, 13 should now be much higher meaning the output to the amp will be far too low.

[johndoe]

Originally Posted by PJL

Something is providing current to bias TR201.
Measure R205 in circuit with your DMM to confirm it is not faulty (your black probe should go to TR201 base to avoid forward conduction of the be junction).
Remove TR200 and measure the voltages around TR201 again but this time from 15 and not 14. Also measure 13 and 14, 13 should now be much higher meaning the output to the amp will be far too low.

I've checked R205 as suggested, it's reading 4.68k. After removing TR200;

TR201

E 0.6v
B 1.3v
C 14.5v

Terminal

12 86v
13 13.5v
14 0.6v


Thanks

[PJL]

Very odd fault as the voltages don't show up any obvious signs of transistor failure with the be voltages looking reasonable despite their being no source for TR201 base current.
You could remove TR201 and measure the voltages at the points where the TR201 was soldered just to confirm that there is no PCB leakage. Without TR201 the output voltage on 13 should be high.

[johndoe]

Originally Posted by PJL

Very odd fault as the voltages don't show up any obvious signs of transistor failure with the be voltages looking reasonable despite their being no source for TR201 base current.
You could remove TR201 and measure the voltages at the points where the TR201 was soldered just to confirm that there is no PCB leakage. Without TR201 the output voltage on 13 should be high.

With TR201 removed I get 72 volts on the collector and nothing on the other pads. Pin 13 is also 72 volts.

[PJL]

TR201 is faulty then...

[johndoe]

Originally Posted by PJL

TR201 is faulty then...

Thanks for your input, as and when the BC441 arrives I'll change it out and see how I get on and report back

[johndoe]

BC441 arrived, all is now stable and happy, I'll leave the new TR3 and TR200 in there seeing as I've changed everything else, should be good for another 40 years

Thank you for the input everybody.

[PJL]

Pleased to hear that sorted it.