Audio issue a couple of weeks after Moreplay Upgrade [resolved]

[Galluch57]

Will do, thank you.

[Galluch57]

I took the board off and reflowed everything except the LED’s.
6U - 173.4v
16U - 173.1v
Essentially the same as before 🤷🏽

[Paul Birkeland]

You will need to take the board out and show us a picture of the entire bottom of it.

You have excessive current flow where current should be regulated.  If there were soldering issues present, it's possible that a transistor or two have shorted out from that fault, and you can check this by setting your meter to beep when you touch the leads together, then touch each pair of leads on each transistor (Q1/Q2-B &C) to see if there are shorts.

It's also possible that your main C4S board is working properly, but that there's a soldering issue related to the balance pot and the input/output wiring there, and the 6V6 grids are losing their ground reference and this is causing extremely screwy DC voltages.  I would evaluate this by temporarily soldering one bare wire across the L-Out, R-Out, and ground lugs next to the balance pot.  That will provide a DC short to ground, and if your voltages move dramatically, then you know the issues is in the input/balance pot wiring.

[Galluch57]

Thank you. I’ll start with the transistors. They each have 3 legs correct? So I set my multimeter for continuity and try each combination of lead pairs on each transistor?
If there is a short I will get a beep?
I also have to remove the transistors to test them don’t I?

[Paul Birkeland]

That is all correct.

[Galluch57]

One last question before I proceed. I labeled and desoldered all the leads running to the PC board to remove it so I could reflow the board on  my work station. How many times can I remove the board without damaging the pads etc.? Seems to me I now have to remove the board again so I can desolder the transistors for testing

[Galluch57]

Also, are the smaller PN2907 transistors more likely to have been damaged than the larger MJE5731A transistors?  Should I test those also?

[Paul Birkeland]

The board can come on and off 5-10 times most likely.  A lot of it depends on how gentle you are pulling the wires out. 

Definitely test all the transistors, I've seen both the PN2907 and the MJE5731A transistors short previously.

[Galluch57]

Can you give me the specs on both those transistor types and a source where I can order some spares? If I'm going to pull the board out again, I may as well just replace 5 transistors.

[Galluch57]

I ordered replacement transistors - once they arrive and I replace them, I’ll give you an update. TY

[Paul Birkeland]

No, please don't randomly pull out all the transistors and attempt to replace them.  The odds that you destroy the board in the process of doing this go up with every transistor you pull.

Please do the shorts test I asked you to do and report the results, then also do the second test I posted. 

The diagnosis of the problem has to be completed before a remedy is attempted. 

[hmbscott]

Paul, just for clarity.

Can the shorts test be done with the transistor installed on the board?

[Paul Birkeland]

Yes, you should be doing that test with the transistors on the board.

[Doc B.]

From a FAQ PB and I posted some time ago -

It takes some getting used to using meter probes in tight spots. Did you accidentally touched your meter probe to two terminals or leads at once? This can take out transistors. You can test transistors by unplugging the amp and measuring resistance across each of the three possible pairings of the three transistor leads. If any reading is very low, like under 100 ohms, the transistor is toast and will need to be replaced. It's usually the smaller 2N series transistors that fail.

[Galluch57]

Sorry that I missed the last several entries everyone. I did already remove all 6 transistors and replaced them. Same voltages at the test points.
6U - 173.3v
16U - 173v

B+ Reg TP - 177v