C4S Install for S.E.X. 3.0

[wdavis009]

Having issues with C4S board. Amp was working fine before install. After install I have the following readings:

(1) IA - 408vDC
(2) IB - 407vDC
(3) OA - 39.24vDC
(4) OB - 18.59vDC
(5) Only 1 of the 4 LEDs light up

Wade

[Paul Birkeland]

Can you post a photo of the bottom of the PCB?

[wdavis009]

Here is the bottom of the board.

[Paul Birkeland]

The solder joints on the MJE5731A on each side need to be reflowed.

[wdavis009]

Did that already and checked continuity, but just did it again to make sure. No change when powered up other than the 680R resistor on B channel now burned out.

[Paul Birkeland]

It's very visible that the MJE5731A is not properly soldered in the photo you posted.

If the 680 ohm resistor burned up but the 8.2K resistor did not, then you have a different issue with your build that is unlikely to be related to the C4S installation.

[wdavis009]

The 8K2 resistor and 22uf capacitor has been removed based on your discussion on this thread: https://bottlehead.com/smf/index.php?topic=11101.0

I did reflow the solder joints based on your feedback (pic below). After reflowing the joints the 680R resistor blew (pic below). I replaced the 680R resistor, discounted the IB wire and powered up the amp. The 680R resistor was fine with the board disconnected. However, the A side of the board still showed the same voltages with no LEDs lit (input was still 408vDC). Since I am running out of 680R resistors, I then removed the board and reinstalled the 8K2 resistors and 22uf caps. The amp now works fine and all voltages check out. Based on this, I don't see any evidence of another issue with the amp besides the C4S board (but I am not an electrical engineer so anything is possible).

Anyway, thanks for you quick responses.

Wade

[Paul Birkeland]

So the C4S boards work with the 8.2K/22uF parts intalled but don't work when your wire the boards right to the 100uF caps? 

Did you remove the 8.2K resistors and 22uF capacitors while installing the C4S boards? 

[wdavis009]

I removed the board when I reinstalled the 8K2 and 22uf. Amp works fine. I did not try the board again with those components reinstalled because I am out of 680R resistors. That said, I tried the board with those components installed originally and the voltages were those posted above. I then removed those components and the voltages stayed the same.

[Paul Birkeland]

I removed the board when I reinstalled the 8K2 and 22uf.

So the board exhibited the low voltages, then you put back in the 8.2K resistors, 150K resistors, and 22uF capacitors?

[wdavis009]

Order of events:

(1) C4S board installed with 8K2 and 22uf installed. Low voltages as posted above.
(2) 8K2 and 22uf removed. Low voltages as posted above.
(3) Reflowed solder joints. 680R resistor burned out.
(4) Reflowed solder joints again. 680R resistor burned out.
(5) Discounted IB wire and 680R resistor was fine. Low voltages as posted above on A side of board.
(6) Completely removed board, reinstalled 8K2 and 22uf. Amp works fine.

[Paul Birkeland]

These joints also look like they aren't all the way soldered.

You can check the resistance between each pair of legs of each transistors to be sure they are still OK.  If any two legs read as a short, that transistor needs to be replaced.

Having the 8.2K resistor in place will protect the 680 ohm resistor.  The 8.2K resistor can be replaced by a 10K resistor in a pinch if you need to.

-PB

[wdavis009]

Thanks Paul. That photo was from before reflowing everything but today I removed all the components on the board, cleaned it, and started reinstalling. With the exception of the resistors I ordered all new parts even though the transistors test fine.

Bigger issue today is that I replaced the output cables/wiring to the headphone jack and now both channels are dead (no sound whatsoever, not even the slight noise floor I hear on my headphones). Spent the last hour checking voltages, continuity, etc. and everything leading up to the output transformers checks out. Not sure where to go from there but I haven't searched the forum yet for possible clues.

[Paul Birkeland]

My strongest recommendation would be to take out all of the aftermarket parts and mods to this amp and take it back to stock, then debug from there. 

If you have no noise floor at the headphone jack and all your DC voltages look good, then there's an issue with the wiring to the output jack (output jack meaning headphone jack, not binding posts).

As I mentioned in another thread, we have never had an OT-2 fail and it would take enormous wiring mistakes to destroy one and it would end up smelling horrible. 

[wdavis009]

Hi Paul,

I get it from your perspective. Output jacks have never been connected as this is used only as a headphone amp (I did run a ground wire from the headphone jack to the output RCAs and installed the 0R jumpers. Other than that, there are no "mods" other than upgraded parts (resistors and caps). As I mentioned, before changing the cables to the headphone jack, it was working perfectly. So I am sure something got knocked loose when I was fiddling with the wiring. Unfortunately, all the original caps and resistors were trashed a while ago. So if that is the only option and there is no other advise on how to troubleshoot it will just end up in the closet for now.

Wade