700B blew one Right channel 5A supply fuse

[62vauxhall]

The bulb should have dimmed when you lowered the bias a wee bit...

Bulb goes off below 50V on the variac.

 

[laatsch55]

Nope, bring it back up to 50%, then ADJUST bias using the pots on the top side of the board. Use your fingers or a plastic screwdriver. I hope that right bias wasn't MINUS 10 volts DC....

 

[62vauxhall]

Yes it was minus 10 but I have not been using the bias adjust, just dialing down the variac.

So should I turn the variac to say 58 volts then use the bias pots to get under 200MV on the meter - both channels?

 

[laatsch55]

Yes....watch the bulb as you do it. The right chanel is a problem if we cant get it below 400 millivolts...

 

[62vauxhall]

Yes....watch the bulb as you do it. The right chanel is a problem if we cant get it below 400 millivolts...

That being the case, I checked the Right channel first. Lowest reading I could get was .912V, DBT dimmed somewhat while turning that bias adjustment all the way down.

Lowest I could get on the Left channel is .319V. Adjusting that bias pot did not affect the DBT at all. It barely glowed and did not change.

Variac set at 58 volts.

 

[laatsch55]

What's DC offset now?

 

[62vauxhall]

What's DC offset now?

Right channel 10.6MV
Left channel minus .8MV

 

[laatsch55]

Bring it up to 80%, heatsinks still cool?

 

[62vauxhall]

Bring it up to 80%, heatsinks still cool?

Variac at 100 volts. DBT brighter but still orange glow. Heat sinks not warm yet at this time.

Right channel offset minus 20MV
Left channel offset minus point 17MV

 

[laatsch55]

Bias?

 

[62vauxhall]

Bias?

Right bias 1.74 volts
Left bias 259MV

 

[laatsch55]

Try adjusting the right bias again..

 

[62vauxhall]

Try adjusting the right bias again..

Only way to adjust right bias is up, the control is hard over down. Adjusting it increases DBT brightness and I had it up to 2 volts before backing it all the way down again to 1.74 volts.

EDIT: Q7 & Q10 are slightly warm now and just noticeably more so on the right channel primarily Q7.

 

[laatsch55]

Okay, shut everything down, then after the big caps bleed down, do the semiconductor test to the right bias transistor. In circuit is fine.

 

[laatsch55]

then check both right channel drivers for solder splash, small wire strands touching, stray cutoff leads...rtc...

 

[laatsch55]

Only way to adjust right bias is up, the control is hard over down. Adjusting it increases DBT brightness and I had it up to 2 volts before backing it all the way down again to 1.74 volts.

EDIT: Q7 & Q10 are slightly warm now and just noticeably more so on the right channel primarily Q7.

Ok..

 

[62vauxhall]

Okay, shut everything down, then after the big caps bleed down, do the semiconductor test to the right bias transistor. In circuit is fine.

Had to find and consult notes on how to do that.

Right channel bias
B to E: .380
B to C: .675
C to E: 895

 

[laatsch55]

Reverse the leads and do it again..

 

[62vauxhall]

then check both right channel drivers for solder splash, small wire strands touching, stray cutoff leads...rtc...

Have to say that area look clear of debris.

 

[laatsch55]

Not looking good for Q6. The B-E should be up around .680...