700B blew one Right channel 5A supply fuse

[laatsch55]

Gary the transistors are good....lets get that ground reference measurement....

 

[laatsch55]

Hello???

 

[62vauxhall]

Gary, we want to check continuity to ground. Black lead at the star ground point, red lead on each transistor case. In resistance mode, see what we got there...

There is resistance on the rows with Q12. Both other rows are OL.

 

[laatsch55]

Get the amp back on the dbt and variac, bring it up to 20%....let s see where were at..

 

[62vauxhall]

Get the amp back on the dbt and variac, bring it up to 20%....let s see where were at..

Variac is at 23 volts, DBT is just barely lit/ Any less and it would be off. Bias is 4.34 volts Right channel - 57mv Left channel.

 

[laatsch55]

bring it on up....

 

[laatsch55]

Fuses in?

 

[62vauxhall]

Fuses in?

Yes, fuses in but how far up should I dial it?

Variac at 60 volts now, DBT brighter, probably the same as last time with 60 volts. Bias voltage at 4.5.

 

[laatsch55]

At that voltage on the bias it will stay bright. Pull the fuses on the right channel and bring it up again..

 

[62vauxhall]

At that voltage on the bias it will stay bright. Pull the fuses on the right channel and bring it up again..

Right fuses are out, DBT is off at 60 volts. Bias voltage is fluctuating rapidly approximately between 5 and 50 millivolts.

EDIT: make the 5 and 70 millivolts.

 

[laatsch55]

OK, we have a problem in the right channel bias circuit yet....I'm toast, let's try again tomorrow, BUT!!! Now you have a channel to compare too...The left looks good. Check the diodes on the right channel for short/open, and check Q8,Q9 as they were under some stress with a dead short on the collector bus bar...if they don't look right, hop over to the left channel and check those...they should read as the left. Be sure to pull rail fuses when checking semiconductors on the board....

 

[62vauxhall]

OK, we have a problem in the right channel bias circuit yet....I'm toast, let's try again tomorrow, BUT!!! Now you have a channel to compare too...The left looks good. Check the diodes on the right channel for short/open, and check Q8,Q9 as they were under some stress with a dead short on the collector bus bar...if they don't look right, hop over to the left channel and check those...they should read as the left. Be sure to pull rail fuses when checking semiconductors on the board....

I will but not now, first thing in the AM. Too much has been going on lately and I need more sleep tonight than the couple of hours per that I've been getting.

Will post those findings tomorrow.

 

[laatsch55]

Good job Gary, we'll get er!

 

[62vauxhall]

Rail fuses are out.

Regarding the diode check, I wanted to clarify if those are to be the ones on the board, those on the back wall or both. And, doesn't checking a diode mean taking it out of circuit?

Out of curiosity, I randomly checked two on the board in circuit and got voltage in both directions. I thought that a functioning diode would only provide a reading in one direction and there fore must be disconnected at one end to test it.

I also in circuit checked D15 and D14 on the back wall in both directions and got these results:

D15 Left
.482 and .299

D14 Left
OL and .442

D15 Right
.440 and .303

D14 Right
OL and .439

I checked Q8 and Q9 and think I'm going to be off to the parts store again for 2N1304's and 2N1305's. If you think there may be difficulty getting these, is there a substitute?

It's too early for me to call the stores to ask if they have stock and I have to leave for a while but going in the opposite direction from where transistors are. When I get back in a couple of hours, I'll look for your sage advice.

Q8 Left
B to E: .053
B to C: .106
C to E: OL
B to E: .054
B to C: OL
C to E: .110

Q9 Left
B to E: .481
B to C: OL
C to B: .448
B to E: .171
B to C: .196
C to B: OL

Q8 Right
B to E: .138
B to C: .053
C to E: .149
B to E: .053
B to C: OL
C to E: .153

Q9 Right
B to E: .490
B to C: OL
C to E: .482
B to E: .179
B to C: .175
C to E: OL

 

[jbeckva]

Q8 and Q9 are germanium versus silicon, with a lower voltage drop. Readings are fine for those.

 

[62vauxhall]

Q8 and Q9 are germanium versus silicon, with a lower voltage drop. Readings are fine for those.

Despite two OL readings from Left Q8 and single OL reading from Right Q8?

 

[laatsch55]

Ok, lets pull the right channel rail fuses again, power up and see if the left bias will adjust..

 

[laatsch55]

Put ther left rail fuses in...

 

[62vauxhall]

Put ther left rail fuses in...

Left fuses in, right fuses out - variac at 60 volts - DBT off - bias is .400 (point 400) volts.

Adjusting bias pot causes voltage to range from .399 volts to .404 volts.

Voltage at left channel speaker terminal is minus 37 volts.

 

[laatsch55]

minus 37 volts DC on the speaker outs is a bad thing, Gary, I am wayyyyy behind on my own work here, and I've got to bow out for a few days, I HAVE to get some stuff done..perhaps Jer and Nav will help some. It seems we are taking one step forward and 2 back...