Dirty A-720 with no lights/sound, seems to get power though

[avionic]

hamish called it.

 

[zaibatsu]

Only niggling issue left, the Class-A bias is jumping fine on the right channel (with switch), but not changing at all on the left.

In case I've got my L/R mixed up, it's the side with even-numbered parts.

Are there some parts I can compare L/R?

 

[Hamish119]

...Are there some parts I can compare L/R?

This'll take quite a while, but there are no shortcuts!

Whilst connected to the dim-bulb tester (a slip of the probe can be very expensive) and with 'AUTO CLASS A' OFF/disabled, measure, compare and tabulate the BASE, EMITTER & COLLECTOR voltages (with reference to 0V) as follows;

TR223 & TR224

TR225 & TR226

TR227 & TR228

If there are no obvious discrepancies between the L & R channel results, perform the same measurements with 'AUTO CLASS A' ON/enabled.

Please post both sets of results.

Good Luck!

 

[zaibatsu]

So before I do what you said, I ran my own flavour of testing which is with everything off and comparing B-E resistances (bit safer), and I've spotted that the B-E resistance of TR228 (bad channel) is 101kΩ when it's 1.5-2.1MΩ on TR227 (good channel).

Something tells me TR228 or something near it (likely D218) is bad.

Judging by the in-circuit continuity test on D218 vs D217 I'm betting the diode is the culprit!

 

[zaibatsu]

Well, that fixed it!
Nice to spend 5 mins instead of 5 hours on a problem.

10mV stable on both channels in AB, jumps to ~50mV both channels in Class A.

I replaced both D217 and D218 in case the other channel decided to do the same thing in future.

HZ6C1L -> 771-NZX6V2B-133 (sub) in case anyone has the same problem in future.

Thanks again @Hamish119 and @avionic for taking time to help.

I've ordered the remaining caps to finish her off, the owner will be thrilled.
If anyone knows where to buy a rec-out knob (grubscrew type) let me know too.

 

[Bratwurst7s]

If anyone knows where to buy a rec-out knob (grubscrew type) let me know too.

If the knob is the same type as the AX-700, with 18mm OD and 6mm hole for the shaft, I haven't been able to find an exact sub. But I was able to find and adapt a reasonable temp solution until I find a better one...

http://www.audiokarma.org/forums/index.php?threads/ax-700.645067/page-9#post-10338768

Cheers,
James