Disaster struck! [resolved]

[Skipperrik]

I was listening to a combination of new and old music on my Crackatwoa last night. After about 30 minutes a familiar song came up I realized that I had connected the interconnects backwards, right to left and left to right. I turned the amp off, swapped the interconnects on the amp end and turned the amp back on. After a few seconds I heard incredibly loud pops coming from my headphones. I was lucky I wasn't wearing them! I immediately turned the amp off. I examined the interconnects to see if they had somehow been connected wrong and nothing was amiss. The connections were rock solid. I decided to wait until morning to explore what might have happened.

In the morning I took the amp to my workbench, plugged it in and turned it on. All of the tubes glowed. I turned it off and inverted the board so I could do voltage checks. Upon turning it back on I noticed that the LEDs were burning on all three boards as well as those at the A tube socket. The voltage checks told a startling story though. None of the checkpoints registered voltage on any of the three boards.

I decided to check upstream at the transformer. There I found that all of the voltage at each of the terminals was spot on.

Moving over to the two terminal strips next to the transformer I got the following readings:

22  123.3
23  0
24  0
25  0
26  81.4
27  90.3
30  0
31  0
32  0
33  0

Could it be that the wirewound resistors blew? What would have caused it?

Thanks for any help you could shed on this.

[Paul Birkeland]

The Crack-a-two-a voltage checks do not list these terminals, could you provide a list of the voltages that come at the end of the manual? 

If you see 122V at terminal 22, your meter is set to AC volts.

If the 270 ohm resistors blew, they would turn into dust in the process.

For future reference, if you have your interconnects hooked up backwards, just turn the volume all the way down on the amp or switch to a different input, then swap the cables while the amp is running.  If you turn off a piece of tube gear, then turn it on again suddenly, there can be substantial current demands that wouldn't otherwise be there with a cold piece of tube gear.  Additionally we have a soft-start circuit on the 6080 C4S that needs several minutes to reset after power cycling, so turning the amp off and then back on a very short time later won't allow that circuit to function properly.

[Skipperrik]

This is kind of bizarre. This morning I measured the voltages on the high current and low current boards and the values were all zero. I know the meter was working because I measured the values at the terminal strips and the transformer. Tonight I measured the boards again and things were working. These are the values:

Low Current
IA 150 / 162.4
OA 60-90 / 63.7
KregA 3-6 / 7.6
bRegA 150 / 111
IB 150 / 150.1
OB 60-90 / 63.2
KregB 3-6 / 5.18
bRegB 150 / 150.1

High Current (left)
IA 190 /198
OA 150 /162.5
bA 0 / 0.3
IB 0 / 0
OB 90-110 / 94.7
bB 150 / 162.4

High Current (right)
IA 190 / 195.6
OA 150 / 150
bA 0 / 0
IB 0 / 0
OB 90-110 / 86.7
bB 150 / 150

I can't explain why the values were zero when I measured them this morning. Perhaps the reset got hung up and didn't release until later today.

Hold on. Something else is going on. It just blew the fuse.

I'll have to do a deep dive tomorrow and see if I can find the cause. I suspect that was the problem all along.

[Paul Birkeland]

There is definitely an operational issue there.

Blowing the fuse narrows things down to the terminal strips back next to the power transformer, as everything else is current regulated and would have a tough time popping the fuse.

The appropriate Kreg voltage but lack of regulated B+ to go with it would have me looking closely at the 6AQ5 socket on the offending side.

[Skipperrik]

Thanks, Paul. I ordered some fuses last night so it will be several days before I receive them. I'll let you know what I find in the meantime.

[Skipperrik]

The fuses arrived so I took the amp back to the workbench. I looked carefully at all of the connections at the terminal strips like you suggested and couldn't find anything that would cause a short. I replaced the blown fuse, plugged it in and it immediately blew the fuse. It was really puzzling me until I noticed that the base of the capacitors were actually touching the terminal strips. I had cut the leads a little short and they weren't standing clear of the strips. I bent them at an angle to the strips, replaced the fuse and powered on the amp again. That was apparently the problem. No blown fuse this time. See the before and after photos.

I also looked at the connections around the left 6AQ5 socket that might cause the low bRegA value. I found that the resistor connection at 18U had a tight connection but I had missed soldering it. I remedied that issue and re-flowed several other connections.

Below are the new voltage test results:

Low Current
IA 150 / 163.3
OA 60-90 / 63.6
KregA 3-6 / 7.5
bRegA 150 / 163.1 (111 previously)
IB 150 / 150
OB 60-90 / 63.1
KregB 3-6 / 5.39
bRegB 150 / 150

High Current (left)
IA 190 /198.3
OA 150 /162.6
bA 0 / 0
IB 0 / 0
OB 90-110 / 94.3
bB 150 / 162.5

High Current (right)
IA 190 / 196.3
OA 150 / 150
bA 0 / 0
IB 0 / 0
OB 90-110 / 86.7
bB 150 / 150

Other than bRegA, the rest of the values are almost identical to the previous test.

The problems seem to have been solved but I have one additional question. There are several values that are higher than spec. Are any of those a problem?

Thanks.

[Paul Birkeland]

I would swap the 6AQ5 tubes to see if the high regulated voltage switches sides.  If it doesn't, I would suspect the 431 regulator to be damaged on the side that's not pulling down properly. If the problematic voltage swaps sides, then a replacement 6AQ5 would be in order.

[Skipperrik]

That's interesting. I swapped the tubes and now both OA values are 150V.

Are the other high values ok?

[Paul Birkeland]

Which other value would be high?

I would triple check your 7 pin sockets, as you may still have a loose connection.

[Skipperrik]

bB was 162.5 but it has also come down to 150.

I'll give everything another round of checks. Thanks for your help!

[Skipperrik]

I finally gpt around to doing another thorough check. Sorry it took so long. A few of the numbers are a little high so please let me know if they need more investigation.

Low Current
IA  150 / 153.9
OA  60-90 / 63.7
KregA  3-6 / 7.77
bRegA  150 / 153.8
IB  150 / 150.2
OB  60-90 / 63.3
KrebB  3-6 / 4.08
bRegB  150 / 150.1

High Current Right
IA  190 / 199.9
OA  150 / 153.6
bA  0 / 0.3
IB  0 / 0.2
OB  90-110 / 90.7
bB  150 / 153.4

High Current Left
IA  190 / 197.2
OA  150 / 150.2
bA  0 / 0.6
IB  o / o.6
OB  90-110 / 86.9
bB  150 / 150.2

Thanks!

[Paul Birkeland]

153 is plenty close to 150, those all look good.

[Skipperrik]

Great! Thanks for your help with this.

Here's wishing you and all the Bottlehead staff a very merry Christmas and a hapy new year!