Completed Stereomour—left channel crackles/pops/hisses/distorts [solved]

[funandfunny20]

Man….turns out my multimeter was terrible.

Got a decent one from radioshack and everything looks good…except OB and terminal 2 are only at about 70. 

Also, R2 (the one closest to the transformer), flashed a couple of times.  And my LEDs on that side do not light up.

What should i try next?

btw, Thank you Doc, Paul, and Paul! 

[Paul Birkeland]

Flashing could be a sign of a funky solder joint, I'd start there. 

Does the ~70V reading persist after the funky flashing?  Is the flashing initiated by you poking in there with the meter?

[funandfunny20]

Okay, re-soldered the R2 and now no flashing. 

The voltage on T2 and OB still settles at 70v.

Is it bad to try it with speakers?

[Paul Joppa]

OB at 70v means the 431 in the driver cathode on that side is shorted. You can test voltage at kreg; they should beoth be 2.5v but I suspect one of them is at 0 volts. Could be a short at the tube socket or on the PC board.

[funandfunny20]

Both Kregs are actually 2.5.

I'll inspect the socket and the 431 right now.  Could the carbon from the flex (between OB and IB) cause the short?  I cleaned it off pretty good and it no longer sparks. 

[funandfunny20]

Here is a shot of the PSB. 

[funandfunny20]

I've removed the PC board.  A simple resistance test between OB and IB yield 20.0.  Whereas, a resistance test on IA and IB yields 0.0. 

Does that help with anything?

[funandfunny20]

All my soldering looks good.  Just in case, I re-heated all three joints of the troubled 431. 

Should I re-install the PC board and test the voltages again, or are there any other tests I should try before?

[funandfunny20]

I checked the resistance of R1 (on the side where the LEDs are out) and it's only showing 4.3, whereas the other one is reading 237.0.

Does that mean it's fried?  This happens to be the same resistor that was flashing…

[Paul Joppa]

Yeah, that's it. I can't imagine how it could have flashed, the current is small, but if it doesn't read 237 (or very close) it's toast and could easily explain the voltage reading.

There's no way to say whether replacing it will solve the problem, or whether some other bits may have been fried. It could be interesting to try - you can probably find a 240 ohm at RatShack or something locally - but I'd say the best bet it to get a replacement board and stuffin's.

[funandfunny20]

Thanks Paul.  I think I'd rather just get new stuffin' and the board…

Should I just order the S.E.X. C4S upgrade kit?

[funandfunny20]

Got a new PC board (and parts), soldered, and installed.

The only resistance readings I have that are out of spec are: Terminal 20 is 1.4 instead of 1.2; C3 settles at 395; and terminal 12 settles at 387.


Is this a major problem?


Suggestions?

Thanks in advance!

[funandfunny20]

Did the voltage tests:

Here are the issues.  1) Kreg on the A side is only at .9v.  2) the LED closest to IB/OB is not lighting up.  I re-soldered it, but to no avail…still not glowing. 

What should I check now?

[funandfunny20]

I again, tried soldering the LED closest to OB/IB, and it worked!  Did my voltage tests and everything sounded good..for five minutes before the RIGHT channel made this terrible loud popping/crackling noise. 

I turned over the amp and started taking measurements again.  After the amped is turned on, all four LEDs glow as they should.  After about 10 seconds, I noticed that the LED closest to OB/IB is starts glowing way brighter than the other two on the side closest to the volume knob.  And, also, the LED closest to the transformer and on the A side goes completely. 

I performed all voltage measurements again this morning and everything is within spec, except for OB is at 374 instead of 165-220.

Both kregs are at 2.5. 

   

 


Terminal 4 22.;

[funandfunny20]

Does anybody have any suggestions?