High Voltages and Odd Behaviour from LEDs

[Steve468]

I've just gotten my Crackatwo back from repair, and unfortunately it still is not right and I was hoping I could get yet more help with it. It booted up just fine the first time, so it was at least fine when it got to me. I changed the tubes and when I turned it on later, I got a loud humming in the left channel. Thinking something had shook loose in shipping I reheated all the joints in the amp, but when I checked voltages this is what I got:

Low Current C4S
IA 149.4
OA 80.5
KregA 3.755
bRegA 149.5
IB 189.7
OB 80.1
KregB 17.46
bRegB 189.9

High Current C4S
B side
IA 186
OA 149.4
bA 0
IB 0
OB 90.2
bB 149.4

D side
IA 192.5
OA 190
bA 0
IB 0
OB 117.2
bB 189.9

I noticed when I was checking voltages that when I start it up, all the LEDs light, aside from the 2 at the rear of the D side - they light up for a brief second, then die down. The one on the right stays lit very dimly, the other goes completely out.

Does anyone have any ideas as to how this amp got repaired, went literally to every corner of North America shipping back to me, turned on fine, only for me to do nothing but change the tubes and completely kill it? Any help is much appreciated!

[Paul Birkeland]

It booted up just fine the first time, so it was at least fine when it got to me. I changed the tubes and when I turned it on later

Is it possible that you may have a defective tube in your collection?

KregA 3.755
KregB 17.46

Do these change if you put the tubes back in the amp that were with it when I repaired it?

[Steve468]

Thanks again for the quick reply as always! I had already put the stock tubes back in when I checked the voltages.

[Paul Birkeland]

If I had to guess I would say you have a loose or broken connection on the 7 pin socket under the D side high current C4S board.

[Steve468]

I've taken your suggestion and tried redoing all the connections on the D side of the 6aq5, but no luck so far. You mentioned the possibility of a bad tube, so I threw in yet another set, this time a 6005 instead of a 6aq5. When I did this, none of the LEDs on that side lit up, and the voltages that were already high went a bit higher still. With the stock tubes back in, behaviour returned to what was noted above. Does this point to anything, or is it just because of the different properties of the 2 tube types?

I've attached a picture - maybe someone with better and smarter eyes can see something I can't.

[Paul Birkeland]

If you want to mail it back to me, I can look at it again.  It would be my preference that you'd go easy on the tube rolling once it comes back.


The 17V at the offending Kreg terminal means the 6AQ5 on that side doesn't have adequate operating current.  This could come from a shorted 6080 or possibly damage to one of the active loads from an unknown source.

[Steve468]

Thanks for the tips. I thought the green wires going to the 6080 might be a little too close, so I resoldered those to give a bit more space between the insulation and the joint. But that wasn't it. I then redid every connection on the 6080, but still no luck. I've attached a picture - does anything look wonky?

Shipping damage is not out of the question - a lot of screws had rattled loose, but nothing appeared to have moved too much. How would I go about checking to see if a part has died?

I'm trying not to send the amp back again, so any tips are appreciated. I'm still at the determined to fix it myself stage, but Mr. Birkeland, I guess you know how that turned out the last time.

[Paul Birkeland]

You would swap the large high current C4S boards between channels.

[Steve468]

Well, I finally found the time and wherewithal to switch the two C4S boards, and the results are...exactly the same. The problem remained on the D side. Voltages were basically identical to what I posted earlier. I guess that rules out the C4S as the cause of the issue? What if anything is the next thing I can check?   

One more data point: I've noticed that the 6AQ5 on the D-side has lost it's silver getter flashing (so that's it's just clear glass on top), and now has a scorch mark just below the exhaust tip. Just to rule out a tube problem, I switched the two 6AQ5 and nothing about the voltages changed. I assume the tube is getting that way because of the wonky voltages.

[Paul Birkeland]

Uninstall the center C4S board and rotate it 180 degrees, then reinstall it.

[Steve468]

Aha! That did something! Now the wonky voltages are on the B side, and KregB remained at 17.71 even after it had been rotated. In fact, all the terminals on the low current board remained as above even after being rotated. Does that point to anything?

[Paul Birkeland]

You could rebuild/replace the center board. There may also be a broken jumper wire on the board or a solder joint that requires some attention.

[Steve468]

Okay, thanks so much for your help today. After reheating all the wire joints and inspecting for stray solder blobs, I think I’ll just go about replacing the whole board.

[Steve468]

It looks like it's all good! On a(n educated?) whim, I started by replaced the LM431 on the B side, and that was it! All voltages are perfectly in spec. I'm listening to it right now, and it is dead (DEAD!) silent and sounding great.

I guess the last remaining question is why these LM431s keep dying - I believe one had to be replaced when it was in for repair, but on the other side?

Thanks again so much Mr. Birkeland for all your help - I really feel like I've learned some valuable troubleshooting skills in this process. Though I'm hoping I won't need them again!

[Paul Birkeland]

The 431s are generally the first thing that fails from a faulty tube.  My suggestion would be to go easy on tube rolling and just enjoy the amp.