A buzz in both channels

[Skipperrik]

Out of the blue my Crackatwoa has developed a buzzing sound in both channels. I looked at this post, https://forum.bottlehead.com/index.php?topic=14712.0 and saw where his problem was a dirty 12AU7 pin. I have changed all of the tubes multiple times with no resolution so it must be something else.

The voltage tests results are:

Low Voltage C4S
IA  193
OA  72.9
KregA  8.29
bRegA  163.7
IB  150.2
OB  72.8
KregB  5.42
bRegB  150.2

High Voltage C4S (A/B)
IA  192.9 / 190.5
OA  163.2 / 150.2
bA  0.3 / 0.6
IB  0.3 / 0.6
OB  100.2 / 93.1
bB  163.9 / 150.2

I also tested it with only the 12AU7 tube in place:
IA  216
OA  64.3
KregA  9.0
bRegA  217
IB  217
OB  64
KregB  1.4
bRegB  217

These values seem high with the exception of OA and OB which are normal and KregB which is low.

Do any of these values give a clue as to where the problem could be?

 

[Paul Birkeland]

Low Voltage C4S
IB  150.2
bRegB  150.2
High Voltage C4S (A/B)
OA 150.2

This is one channel's worth of regulated high voltage connections.  They are all wired together and they all have the same voltage. 

Low Voltage C4S
IA  193
bRegA  163.7
High Voltage C4S
OA  163.2

This is the other side.  You have 163.2V at the output of the high voltage C4S, but somehow 193V at the input of the center C4S board.  This is a bit of a contradiction, as these connections are wired together.  On the A side of the center C4S board (low voltage), +reg, Breg, and IA are all wired together, so they can't have different voltages unless one of those connections is no longer present or you have a very flaky ground connection in the amp disturbing the reliability of your voltage measurements.

My guess is that you have a broken wire that isn't permitting the regulator to work properly.  A buzz in both channels would make me wonder if there's a broken or loose ground wire in the amp, and this is both causing the buzz you're experiencing as well as the regulator malfunction.

 

[Skipperrik]

I believe I found an assembly error that could account for the voltag imbalance. On one of the high current boards I swapped the locations of the 22KΩ 5W and the 47KΩ 2W resistors. I don't think that would cause the buzzing though since it has been dead quiet up until yesterday. I've run a continuity check on every wire in the amp and have found no breaks. I'll reverse the positions of the two resistors and follow up when I get it completed.

 

[Skipperrik]

I found that one leg of a TIP-50 resistor at position B (see photo) has ground continuity. The corresponding leg on the TIP-50 resistor on the opposite board does not. Which is correct?

 

[Skipperrik]

Sorry, the photo didn't get uploaded. Here it is.

 

[Paul Birkeland]

The TIP50 portion of the circuit would not be very likely to cause the problem you're experiencing.  Is it the base, emitter, or collector that's showing a ground reference?  What is the DC resistance of this measurement?  Is it bidirectional? 

Since your OB voltages on the high current C4S boards are correct, I'd be inclined to ignore this for now and focus on how three terminals that are wired together are able to have different voltages on them.

 

[Paul Birkeland]

Sorry, the photo didn't get uploaded. Here it is.

It looks like your probe might also be touching IB, which is grounded.

 

[Skipperrik]

That was just for the photo. I was careful not to touch any surrounding surfaces and still got a ground signal. I'll go over all of the resistors, capacitors, etc. and make sure they are all correct. I don't believe there is a faulty ground wire since I have checked and re-checked every wire and connection. I'll keep looking.

 

[Paul Birkeland]

I'll go over all of the resistors, capacitors, etc.

You have three terminals that are supposed to be wired together that are showing different voltages.  This is not a bad capacitor, resistor, or transistor.  This is a flaky solder joint or broken wire in the amplifier.

 

[Skipperrik]

I found an fixed an errant wire on one of the regulators but I'm still getting high output voltages.

Low Voltage C4S
IA  192.4
bRegA  192.1
High Voltage C4S
OA  192.3

At least they are all pretty close but what could be causing the high output? I don't think it's a ground issue. I've checked the wiring and continuity of every wire multiple times and re-flowed every connection.

 

[Paul Birkeland]

What are the Kreg voltages now?

 

[Skipperrik]

KregA 8.87
KregB 5.8

 

[Paul Birkeland]

There's an X pad on each side of the center board where you measured those Kreg voltages, what DC voltages do you see at each X pad?

 

[Skipperrik]

A side
x=125.7
B side
x=2.51

 

[Paul Birkeland]

A side
x=125.7

Wow that is crazy, I'd be looking at the 2.49K resistor and the 431 regulator where that voltage is present.

 

[Skipperrik]

I checked the resistors. The B side measures 2.44KΩ but the A side measures 7.4KΩ. That's bizarre!

How do you check a regulator?

 

[Paul Birkeland]

I think I'd start by replacing the regulator resistor on that funky side. 

 

[Skipperrik]

Okay. I’ll order one and let you know the results.

Thanks for your help, Paul. I really appreciate it.

 

[Skipperrik]

I received a couple of new 2.49KΩ 1/4w resistors. I measured one of the new ones prior to installation and it measured 2.48KΩ. After removing the original one and installing the new one it measured 7.5KΩ, just like the original one did.

What could be causing this?

 

[Paul Birkeland]

Are you measuring the resistor with your meter across the resistor itself while it's soldered into the circuit?  Did you measure the ones you took out?