Not working after TwoQuiet upgrade - but it was working! [resolved]

[Steve468]

So, it is time once again for me to ask for assistance from all of you fine people. I had built a Crackatwoa, and it worked flawlessly for two weeks before I decided to put in the TwoQuiet upgrade. Install of the TwoQuiet went well, I thought, and everything was fine aside from a little hum and lower volume on the left side. So I went and redid some of the soldering on the TwoQuiet, and the hum was gone - but I thought there was a very slight imbalance now with the right side being lower (honestly, it was so close it could have been my ears). Anyway, I went and fidgeted with it again, but this time there was no sound in the left side at all. I looked under the hood and the 2 LEDs on the side closest to the capacitor of both high current C4S boards are not lit. I've gone and rechecked and resoldered almost all the connections in the amp - honestly, my untrained eyes cannot see anything too problematic. But still, the same problem persists.

All resistance checks passed, but the voltages are definitely wonky. The voltages are off at:

Low Current C4S
KregA: 2.3
bRegA: 0
KregB: 0.462

High Current C4S
OB B side: 2.12 / D side: 1.389

All other voltages are fine.

Since it was working I must have just knocked a poor connection loose when I was moving it and poking it, right? But I've re-flowed the solder a few times, so I'm just wondering if anyone has any other suggestions.  Thanks for your help!

[Paul Birkeland]

I would suspect that you have a broken wire. What's IA/OA on the high current side?  If OA is 0V, pull all the tubes out of the amp and recheck.

[Steve468]

Thanks for the absolutely lightning fast response!

IA B side: 188.7 / D side 189.2
OA B side: 149.5 / D side 153.2

So I believe those are in spec.

[Steve468]

Should I still pull the tubes and check voltages again?

[Paul Birkeland]

You must have had the 6AQ5s in to get those readings.  This is good though, that suggests that a lot more is working than isn't.

What are OA/OB on the center PC board?

[Steve468]

I did some more poking around and managed to get all LEDs to light, but as I was getting ready to take measurements I very stupidly blew the fuse by lightly touching one part of the meter probe to the IEC inlet while it was hooked to the ground lug (at least, I think that's what did it). So, I have to wait to get in some more fuses. All I have are 1 amp fuses from when I was screwing up my original Crack build - I imagine those wouldn't be safe to use, right?

Anyway, thanks for checking in on this thread, and on a weekend, really appreciate it.

[Paul Birkeland]

Yes, a 1A fuse should hold in the C2A.

[Steve468]

Okay, thanks for confirming that. So I threw in a 1a fuse, checked the voltages, and it looks like some voltages have gone back to spec, but some have annoyingly fallen out of spec. Here were my latest readings:

Low Current C4S:
IA 149.2
OA 79.6
KregA 6.69
bRegA 149.2 (up from 0, so that's good!)
IB 109.2 (this went down from spec, so that's bad)
OB 86
KregB 0.266
bRegB 108.4 (went down from spec)

High Current C4S
IA: B 186 / D 186.5
OA B 149.2 / D 108 (this went down from spec)
bA B and D 0
IB B and D 0
OB B 97.7 / D 88.3 (back in spec)
bB B 149.2 / D 107.9 (fell out of spec)

It's worrying to me that some voltages fell just as I fixed some. But I guess to me it still looks like my blindly poking around is breaking some things - it doesn't look like anything more serious like a part failure, right? KregB was the only terminal that was out of spec both of my last readings, so I'll focus on that now.

[Paul Birkeland]

Can you try again with no 6080 but the other three tubes?

An intermittent problem lie you are experiencing is often a loose connection or a broken wire.

[Steve468]

Without the 6080, the readings are:

Low Current C4S:
IA 150.7
OA 77.6
KregA 1.979
bRegA 151.3
IB 166.5
OB 84.3
KregB 0.462
bRegB 165

High Current C4S
IA: B 190.3 / D 190.5
OA B 152.4 / D 165.2
bA B and D 0
IB B and D 0
OB B 1.4 / D 1.3
bB B 152.8 / D 166.6

So it looks like many of the voltages that had dropped popped back up without the 6080, except for those stubborn Kregs. Does that give any information?

Anyway, I'll keep plugging away at finding that bad connection. I'm often surprised when I do find a bad connection at just how subtly it was off - and that makes finding them pretty hard, at least to me.

[Paul Birkeland]

The really low Kreg would tend to indicate a lot more current flowing through the regulator circuit than the C4S board should allow, though I have seen weird stuff like this happen too when the little resistor that goes from grid to ground on the 6AQ5 socket isn't well connected. (That's pin 1 on the 7 pin sockets)

[Steve468]

Could this have anything to do with the resistor between D5 and D6? I noticed it got a bit damaged, even “cooked” looking when I was resoldering those pins.

Other than that, no change yet.

[Paul Birkeland]

You can measure that resistor in circuit with the amp off to see.  If that resistor is open, then that could definitely be a problem.

[Paul Birkeland]

Hello Steve,

I believe I have your C2A here on the bench.  The black wire going from the left channel high current C4S board to ground looks like it moved a bit when you soldered it, and the teflon insulation slid into the joint just a bit and disrupted the joint from doing what it needs to do.  This was not at all obvious and took me a long, long time to find. 

This flaky connection also took out the 431 regulator chip on that side of the amp.  With those two repairs things are working properly again.

-PB