A buzz in both channels

[Skipperrik]

Thanks for chiming in, Doc. I'm sorry you have experienced the wrath of nature. My heart goes out to you!

I've been having an ongoing problem with wires breaking in my Crackatwoa. Apparently the wire stripper I used put a ding in them and over time and movement from tracking down a ground loop hum, it causes them to break. I'd like to order more wire so at your convenience, please have someone give me a call so I can give you a credit card number.

Thanks again and I hope things smooth out for you!

 

[Skipperrik]

I've been over every solder joint in the amp and checked each wire's continuity and haven't found the source of the hum. This morning I noticed a burned spot on one of the 270Ω wirewound resistors. Could this be the cause, and what would cause it to burn like this?

 

[Paul Birkeland]

See page 39 in the manual, those are not properly installed.

 

[Thermioniclife]

PB beats me to the punch once again!

 

[Skipperrik]

New resistors are installed correctly along with new wires from 25L to 33U and from 25L to 31L. I’m afraid the hum is still there. I’ve reflowed every joint in the amp again and  replaced several wires. I’ve even run a direct wire to ground at every ground termination point to check for broken wires but the hum is still there. It’s a little more melodious now (a slow pulsating sound) but it’s very much present. I’ve run out of options with the solder joints so I have a couple of questions:

1.) Is it possible that a component on one of the boards is bad?  I’ve prodded all of them with a chopstick but there’s no indication of trouble.

2.) Could it be the transformer? I’ve reflowed all of the taps.

 

[Paul Birkeland]

If you had a whacky voltage, then maybe there could be a dead part on one of the PC boards.  Likewise, a failing transformer is going to throw out some odd voltages. 

Loose hardware can also create the problem you're experiencing.

 

[Skipperrik]

I just checked the resistors on the low voltage board and the 2.47Kohm at the RC position measures 28.9 ohms. This is the opposite side from the one that had the bad regulator. That side measures fine.

What could cause the regulators to go bad?

Here are the voltage figures from the low current board:

IA  150
OA. 64.0
KregA. 4.3
bRegA. 150.1
IB. 169
OB. 63.4
KregB. 1.54
bRegB 168.2

 

[Paul Birkeland]

Those are not working voltages.  The 1.54V at KregB is no good, and bregB is also not correct.

This can happen if the A side high current C4S feeding that side is shorted and dumping excessive current into the regulator, from a miswire, or potentially from a failed 431 regulator.

The 2.49K resistor does not go in the RC position.

 

[Skipperrik]

Sorry, I mistyped. It’s the 2.49Kohm resister at R3.

I’ll replace the regulator and see if the voltages straighten out.

 

[Skipperrik]

I swapped sides with the low current board to see if the hum followed (it didn't) and now a new problem has cropped up. The two LEDs next to the "A" on one of the high current boards do not light. The following are the voltages on that board:

IA  187.1
OA  172.5
bA  0
OB  0
OB  98.8
bB  172.5

Opposite high current board:

IA  195.4
OA  148.8
bA  0
OB  0
OB  85.4
bB  148.8

Low current board:

IA 172.7
OA  67.1
KregA  1.943
bRegA  172.5
IB  148.9
OB  62.6
KregB  5.56
bRegB  148.9

Do you have any suggestions about how to solve the LED issue?

Originally I thought the ground hum was in both channels but the right channel is faint so I think it's probably bleed over and the real problem is with the left channel. I've gone through every step in the manual to make sure the wiring is correct, I've checked the continuity of every wire, examined every connection, checked the value of every resistor, and swapped all of the tubes in an effort to find the ground fault. Everything checks out. I'm about to throw in the towel. Do you have any other suggestions?

 

[Paul Birkeland]

The LED issue is the regulator not working, and this could be for a very, very wide variety of reasons.  The most important thing to do is to not assume that the LEDs are the problem and go about resoldering them, as that will damage them and make more problems. 

Issues that would cause the problem you're seeing would be as follows:
1. A 6AQ5 might no longer be glowing or isn't present in the socket, causing the shunt regulator not to pull the B+ down to 150V.
2.  The high current C4S board is improperly installed on the faulty side.  The regulator might be having issues from excessive available current which can happen from mixing up the wires connected to that board.
3.  There's an actual short on the high current C4S board causing the current to not be regulated.
4.  There's a broken jumper on the center C4S board where the 431 regulators live, which is causing the regulator not to work appropriately.

 

[Skipperrik]

Thanks. This is helpful. I’ll check each item and get back to you.

 

[Skipperrik]

I replaced the regulators and several other components. It wasn't until I replaced the LED's that the problem was solved. The voltages are back within spec.

I still have the hum problem. I have no idea what to do at this point. Is it something you are willing to track down if I send the amp to you, and if so, how much would it cost?

Thanks.

 

[Paul Birkeland]

My next repair opening is late summer (August), and I can have a look at it then.

-PB