The Smoked 400

[laatsch55]

Good luck with the repair. I can see quite a few things need to be checked. Good thing there is some protection in there. Were you clipping the 400 when it blew?

I think just before it blew....yeah...

 

[FredR]

10 minutes. Clean heatsinks.

Heatsink porn.

 

[laatsch55]

Heatsink porn.

Yeah Fred....they take a lot of time and tedioiusnessessess...

 

[laatsch55]

So, I have a high current draw still. Similiare to what Dr. Lucitt had. Checked with joe and ther is a small difference in the isolation of a QC board and a full comp board. Mark's was a QC, mine i9s an FC.

Detach and tape off from the control board wires; 9L,R 6L,R PNPL,R.
Attach a 22 ohm resistor between control board pads 9L and 6L, and between 6L and PNPL...
Do the same on the right channel.

 

[laatsch55]

Powereed up with DBT and variac, have a dim bulb with +/- 68 volts rail voltage and -62.55 volts DC on each speaker out.

 

[WOPL Sniffer]

Yeah Fred....they take a lot of time and tedioiusnessessess...

Yep, that is probably the worst part of getting an amp ready for it's WOPness... But, if you gots the liquid dynamite, no sweat...

 

[Gepetto]

Powereed up with DBT and variac, have a dim bulb with +/- 68 volts rail voltage and -62.55 volts DC on each speaker out.

What is the reading on the control board pin 6L/R Lee under the conditions that you describe?

 

[laatsch55]

Minus 66 volts DC on both 6L, 6R.

 

[Gepetto]

That indicates you have control board work to do.

 

[laatsch55]

Yes it does...

 

[laatsch55]

Q5, Q10 test good in circuit, is there a voltage test I could before pulling them?

 

[laatsch55]

Should pull one end of d10 and check the protection circuit.

 

[laatsch55]

Pulling one end of D9 created a higher current draw...

 

[Gepetto]

Yes isolate both protection circuits. Lift R76 and see what voltage you are reading on pin6 and on Q5 collector and Q4 base.

Q4 base should be approaching 20V if the front end is working.

 

[laatsch55]

One end oif D9 lifted, One end off R76 lifted, Rail voltage 30 volts,

Pin 6 voltage 20.23
Q5 collector 19.8v
Q4 base 12.70
Q10 got finger BURNING hot in seconds./

 

[laatsch55]

That was on both channels Joe, done very quickly and a cool down period between.

 

[Gepetto]

Did you lift one end of D10 as well Lee?

 

[laatsch55]

No.

 

[Gepetto]

No.

To disable both protection circuits you should lift both D9 and D10

 

[laatsch55]

Ok, Lifted, measure again??