C2a issues and readings

[gary]

Helping a friend diagnose and repair his C2a
See test point results

Looking for help diagnosing and troubleshooting and repairing

LowCurrent c4s

IA 177

OA 77

Krega 1.064

Brega 177.5


IB 149.1

OB 76.3

Kregb 5.2

Bregb 149


Right HighCurrent c4s

IA 178

OA 176

Ba 0

IB 0

OB 103.9

bB 177

(2) leds are out- top of board”A”/ front side
Low level hum right channel


Left HighCurrent c4s

IA 190.7

OA 149

Ba 0

IB 0

OB 93.6

bB 149


The terminals all check out with the exception of 16 and 6 reading 35Kohm. There are (2) quite large Mundorf capacitors in this build 100uf 350vdc see photos below

 

[Paul Birkeland]

Did this amp previously work? Seeing the giant caps in there and the attenuator upgrade makes me wonder if this was a working circuit that now needs repair, or if the amp never worked but got the upgrades anyway.

Half the amp is working properly, the right channel high current C4S board likely has an error on it, like a solder bridge between pins that is causing too much current to be sent through the regulator in order for it to work properly. Also double check that the 431 regulators are in their proper places.

 

[gary]

To add context. I recently previously posted on this aml and you had replied. I believe that was posted under General Discussion Forum.
The amp has been previously working. Overall in evaluating the build quality, it could be better. For example I previously removed the left high current board, and in doing so a few of the wire leads broke at connection points.
It seems “fragile” in that way.
There was a 220ohm resistor on pin 5 of the 12au7 that was cooked. I replaced this.
The current posted readings were taken directly after this.
One thought that came to mind is perhaps when checking the voltage specs on right high current board, a wire connection
( fragile ) came loose ? Causing a short?

I will follow you reply and lead as a next step.

 

[Paul Birkeland]

Ah, this is that one. I'd set your meter to beep for continuity and check that there aren't any shorted transistors on the A side of the right channel high current C4S. A shorted transistor will make the meter beep between a pair of legs.

You could also try replacing the 431 regulator, as they are not particularly robust when there are faults present.

 

[gary]

I would say you are spot on.

MJE5731A at RIGHT HIGH CURRENT BOARD
TESTS BAD / SHORT

LM431A at RIGHT LOW CURRENT BOARD
TESTS BAD / SHORT

Did not test uf4007 diode

Could not test PN2907A on RIGHT HIGH CURRENT BOARD- not enough room without further disassembly

The other PN2907A’s seemed OK

Did not test 2N2222a

TIP-50 RIGHT HIGH CURRENT BOARD OK

MJE350 LOW CURRENT BOARD OK - both

Are these parts available from Bottlehead ?
Is this the best source or somewhere like Digikey/etc ?

Does it seem likely the 2907 on right high current board could have failure ?

I am near Eugene, Oregon

 

[Paul Birkeland]

The 2N2222A is OK by your voltage readings. I would replace the PN2907 on that portion of the board just to be safe.

Yes, you can e-mail the mothership to request some replacement parts from us. These can also be bought from Mouser/Digikey if you prefer.

-PB

 

[gary]

Thanks for your Assistance