Ecc81 red plating. [resolved]

[Mr. Self-destruct]

Hey what's going on BH Friends? okay I recently purchased a pair
of Telefunken ECC 81 they've been working fine, they still work but
one I noticed is red plating on the plate that's on pin 9 side. Imoved
the red plating tube to the second amp and it did not follow so it
seems like the amp is red plating the tube What could be possibly
causing this? Thank you,

 

[Paul Birkeland]

Can you post your voltages?  Mostly you would want the I and O terminals on the A and B side of each board, as well as the Kreg pad on the half of the board that's populated.

 

[Mr. Self-destruct]

Thanks, Paul. Ok i seenthe I and O i will measure.  What is the kreg. Would you specify for me please? I'm assuming it's marked k?

 

[Paul Birkeland]

Can you send a picture of the version you have?  There were a few different ones...

Also if just this tube red plates, but no other 12AT7 tubes red plate, then just don't use this tube as it may have an issue.

 

[Mr. Self-destruct]

I moved the red plating tube to amp 2 and it didn't redplate anymore. The tube tht ws in amp 2 that didn't red plate, red plated in amp 1

 

[Mr. Self-destruct]

The board pic

 

[Mr. Self-destruct]

Right side
456 I
385 O
Left
385 I
227 O
1.95 K

Terminal
16-455
17- 1.8
19- 224
20- 1.7

 

[Paul Birkeland]

I would replace the MJE5731A and 2N2907 transistor on the B side and the 431 regulator, and that will likely pull things back into proper operation.

 

[Mr. Self-destruct]

I have done them but not the lm431 because I did it on the other amp.
9 pin readings-
1.223v
2-0
3-2.5
4-3.1
5-3.1
6-386
7-0
8-1.9
9-3.1

 

[Mr. Self-destruct]

New mje on b side

 

[Mr. Self-destruct]

There are a lot of lm431 models to choose from looking at mouser or digikey. Which should I choose?

 

[Paul Birkeland]

I'd use these:
https://www.mouser.com/ProductDetail/onsemi-Fairchild/LM431ACZ?qs=fAPocZeZcjirQBCPnqK0uQ%3D%3D

Be extra careful not to reheat the solder joints on the red LEDs as they don't do well with being soldered after they've been exposed to humidity for a long period of time.  Also note that the MJE5731A will sink a lot of your soldering iron heat on the center leg, so leave your iron there much longer than you believe to be necessary. 

When you have that board out, set your meter to beep for continuity and check each of the 75V zener diodes to be sure they aren't shorted.  Based on the voltages you are reporting, that doesn't seem to be an issue, but it's so common that it's worth double checking. 

 

[Mr. Self-destruct]

I also found the 220ohm resistor b7 to 18L is 293 ohm. B2 to 20L is 235ohm. Assuming I should replace, what resistor should I use?  Just 1/4 watt carbon film, metal ox? 220?. Ordering from mouser. Thanks. 

 

[Paul Birkeland]

Those values aren't critical and can be left in the circuit.

 

[Mr. Self-destruct]

Thank you.  I already have spare mje5731AG, 2n2907a, HLMP-6000, and zener diodes, is there any other recommended components I should order for future?

 

[Paul Birkeland]

If the zener diodes fail, I think the 90.9 ohm resistor can pop, so having some spares of that wouldn't hurt either.

 

[Mr. Self-destruct]

Yes back in November the led and mje popped, so I replaced thoes along with the zener string. I didn't know the 90.9 would suffer.  I will check mine and order some. Thank you.

 

[Mr. Self-destruct]

The 90.9 is reading 32.7 and looks Blown underneath. 

 

[Paul Birkeland]

Yeah, so definitely pull that, probably best to replace the zener diodes under that board.  With the transistors out, you can set your meter to diode mode and check that the LEDs read about 1.5V in the correct direction and nothing in the other.

 

[Mr. Self-destruct]

Back in November I replaced those zener diodes already I just didn't notice that 90.9 was burnt I will check the zener diodes again also should I replace the 431 on B3 as well?