Right channel hum and no signal

[Sp33ls]

So, after replacing all of the transistors on the offending channel, I pulled the LEDs on the A side of the high current board and noticed that they no longer light up when I use my multimeter in diode mode. I pulled the ones of the B side, and they still light up.

It appears I blew the LEDs on the A side of the high current board.

And it appears that in the circuit, they are required to work properly in order for the transistors to function properly, correct?

[Paul Birkeland]

Yes, but if they blow, they generally explode into little pieces and are missing off the board.  I haven't ever had one fail that was previously working without another related major failure. 

[Sp33ls]

Interesting... I wonder why those two were behaving differently than the other LEDs -- I pulled them from the circuit to test.

Well, either way I feel I might as well replace them now. Once I get some new ones in, I'll post back.

[Sp33ls]

So, after replacing the LEDs that weren't lighting up, 3 out the 4 now light up on the offending board, previously none were lighting up.

The voltages seemed to drop closer to normal, but the 'X' closest to the center on the offending channel still drops after just a little bit to next to nothing. I noticed that it also shows continuity to ground, whereas the 'X' on the B side does not. Is this normal?

[Paul Birkeland]

Nope, not normal.

[Sp33ls]

It appears it was shorted due to the LM431 shorting out again. After replacing that, I noticed that the 'X' on both sides closest to the center are about 2.5VDC.

Terminal     Voltage (DC)
Low Current C4S
IA        150V   (182)
OA 60-90V   (72)
KregA  3-6V   (1.93)
bRegA 150V    (182)
IB        150V   (149)
OB   60-90V   (65)
KregB   3-6V   (5 )
bRegB   150V   (150)

High Current C4S (offending channel)
IA         190V   (185)
OA        150V   (182)
bA         0V   (0)
IB          0V   (0)
OB  90-110V   (105)
bB        150V   (182)

High Current C4S (working channel)
IA         190V   (196)
OA        150V   (150)
bA         0V   (0)
IB          0V   (0)
OB  90-110V   (91)
bB        150V   (150)

I noticed  that the heatsink of the MJE5731AG wasn't really warming up at all.

[Paul Birkeland]

Have you tried resoldering the joints on your high current C4S boards?  I still believe you have a bad solder joint on the 6AQ5 socket or the high current C4S board associated with that side.

[Sp33ls]

Oh, man. It looks like I finally might have got it! 

I ended up replacing the 2907A and an LED (that wasn't lighting up) on the high current board, as well as resoldering the red connections to the 6AQ5 socket.

After I use it for a bit, I'll report back. But, so far it sounds amazing.

Thanks, PB! Really appreciate all of your patience with me here. I definitely became a lot more intimate with how my kit works through this experience.

I'm curious... with new components, do you notice a break-in period for things like new caps & transistors? I know it's getting into more of the voodoo, but curious if in your experience you've noticed differences between recently built kits vs broke-in kits?

[Paul Birkeland]

I'm glad things worked out.  It can be a bit of a challenge to find those pesky loose connections. 

[Paul Joppa]

capacitors have a fairly clear break-in effect. It varies widely depending on the cap's place in the circuit and the particular cap (brand, dielectric, and rating). Audio transformers have, for me, the second most significant predictable break-in effect. In most cases, both of these can get worse before the start to get better (around 10-20 hours) with close to full performance around the 100 hour point - but teflon caps seem to take a few hundred hours.

Tubes have some break-in effects, but since many are either used or old stock this can vary all over the place, so it's hard to generalize.

Also loudspeakers have some big effects. In this case, it's probably the mechanical breakdown/softening of the cone and suspension. Incidentally, this same effect occurs in string instruments such as guitars and the violin family and is well known in that context.

I can't say that I've heard anything I can reliably attribute to other components.

[Sp33ls]

Thanks for the response Paul Joppa. I swapped out 10,000uF caps on the high current board with their "upgraded" Nichicon FG counterparts.

I also swapped out the caps on the low current board. I decided to try a Kemet paper film cap in the 0.1uF location just for fun. Not expecting to hear a noticeable difference the such a small capacitance, just thought it'd be fun

[Paul Birkeland]

The 10,000uf caps on the big board are well out of the signal path.

[Sp33ls]

Okay, so yeah, likely no discernable difference with those either. I mostly just bought replacements just in case I somehow damaged those with whatever connection issue occurred initially.

So far so good, though! There was a moment where I heard a faint, higher-pitched noise (mostly in the right channel again), show up, disappear for a minute, and show up again for several seconds. But, I haven't heard it since. Thought it might've been my phone, but it was on the opposite corner of my desk. I live in downtown Chicago, so I wouldn't be surprised if there were some interference, or noise on my AC circuit.