MonAmour troubleshooting - no music [resolved]

[keithp]

I completed both amps last night. One plays music, the other does not, The voltage readings on the  quiet amp are the same are the same as the good one. I assume I have a problem in the signal path. I have resoldered the binding posts and RCA jack. I tested the RCA to make sure it is not shorted. Does anyone have some additional troubleshooting tips? Would any pictures help?

[kgoss]

Voltage readings and pictures are the best troubleshooting aids. You said all voltages are good but do you see the tube filaments glow?  A million to one longshot that I always do is swap tubes between amps or channels. Its never been tubes for me but that rules them out.

[Doc B.]

If the tube filaments weren't glowing the voltages would not be right since the tubes would not be conducting. But swapping tubes is a good idea as there could be a very slight chance that there is some other problem with a tube.

Most likely it's a miswire or bad solder joint somewhere. Double check all the connections of the output transformer next.

[keithp]

I swapped tubes and verified that all tubes are functional. I rechecked the output transformer wiring against the assembly manual and resoldered the connections. I also rechecked the wiring and connections on both tubes. I should also mention that when I performed the hum pot adjustment, the pot did affect the current reading on the multimeter. The voltage readings on the circuit board during the final voltage test were 297 VDC on the A side and 175 on the B side. Are there any checkpoints on the signal path that I can measure to narrow down the problem? I have built five kits previously but my troubleshooting skills are limited and I haven't seen this kind of problem before.

[Paul Birkeland]

I would suggest posting some build photos.  The kinds of issues you would be looking for would be miswires that should be pretty evident when looking at the bottom sides of each amp.  It could be something as simple as not wiring the center post of the RCA jack to the correct terminal, or taking the output from the 5670 driver from the wrong terminal. I suppose one end of the coupling cap could be installed in the wrong spot too. 

It's these kinds of errors that leave the DC voltages and resistances alone but cause no sound issues, especially since you are able to measure hum on the output transformer (and I'm guessing it looks to be roughly the same magnitude as the other amp?).

-PB

[keithp]

I compared the working amp to the non-working one and still can't find the problem. Here are some pictures.

[Paul Birkeland]

Does your meter have a mV scale for AC voltage or a 2V scale?

[keithp]

I have both 2V and 200mv scales.

[Paul Birkeland]

OK, that's good news.   Take your smartphone or digital music player and play a 60Hz tone into the amp at full volume.  With the black meter probe grounded, you should be able to see AC voltage at the center pin of the RCA jack.  It should be about 300mV or so (0.3V).  Once you have that setup, measure the AC voltages at:

Terminal 10
B3
Terminal 16

For those terminals, you will probably have to go up above the 2V scale, so if you get something weird be sure to try the 200V scale. 

[keithp]

OK. I will have to use my MacBook. I have to get a cable first. I will get back when I have it set up and tested. Thanks.

[keithp]

I found a cable! The readings are low. 10mv at the center pin, less at B3. Less than 1V at Terminal 16. Does that mean the RCA is the problem, or just that my MacBook has low output?

[Paul Birkeland]

Try turning all of the level controls on the computer to maximum.  (Your meter is set to AC volts not DC right?)

If you can resolve 10mV reliably with your meter, then you can just report the AC voltages at those terminals with the low signal level and we can get an idea of what's going on.

[keithp]

This time I got 315mv at the RCA center pin. My meter showed 1mv or less (unreliable)at Terminals 10 and 16.

[Paul Birkeland]

Well hey, that's helpful news. Can you unscrew the C4S board, lift it up a bit, then post a photo of your socket wiring?

[keithp]

I took one picture with the C4S board pushed aside, then loosened the board to get better views.