700B blew one Right channel 5A supply fuse

laatsch55

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I've got to study on that. Check both resistors on Q11, Q12....and the one between Q12,Q14...Compare the other Q6 readings with the right, see if they are similiar....

The left channel is looking good Gary,, with fullpower it will come around. The right I think Q6 is wounded, maybe too much heat soldering on the leads, we'll see though. I'm beat, had a long one and got to get up early. Maybe Nav, or Joe can help ya some more tonight but I'm toast...
 
NavLinear

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laatsch55 said:
I've got to study on that. Check both resistors on Q11, Q12....and the one between Q12,Q14...Compare the other Q6 readings with the right, see if they are similiar....

The left channel is looking good Gary,, with fullpower it will come around. The right I think Q6 is wounded, maybe too much heat soldering on the leads, we'll see though. I'm beat, had a long one and got to get up early. Maybe Nav, or Joe can help ya some more tonight but I'm toast...
Click to expand...
Sorry but I'm toasted. It's cool to have grandkids here but when their 6" 2" plus and teenagers to boot it makes life more interesting. We love them though.
 
62vauxhall

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laatsch55 said:
I've got to study on that. Check both resistors on Q11, Q12....and the one between Q12,Q14...Compare the other Q6 readings with the right, see if they are similiar....

The left channel is looking good Gary,, with fullpower it will come around. The right I think Q6 is wounded, maybe too much heat soldering on the leads, we'll see though. I'm beat, had a long one and got to get up early. Maybe Nav, or Joe can help ya some more tonight but I'm toast...
Click to expand...
Right channel resistors
on Q11: 190.3 Ohms
on Q12: 186.3 Ohms
between Q12 and Q13: point 6 (.6) Ohms

Reading from left bias transistor
B to C: .669
B to C: .665
C to E: OL

B to E: OL
B to C: OL
C to E: .894

The resistor between Q12 and Q13 should be .33 Ohms and it measured at .6 Ohms. Since that is beyond 10% tolerance, I'll get at least one of those - .33 Ohm - 5W - carbon composite if available. Let me know your thought on if I should replace this.

I removed Right channel Q6 this morning and found that the legs fell away without much encouragement. When I first wired them, I used heat shrink along the transistor's legs to insulate them from each other and thought it would aid in strain relief. That may have had the opposite effect since they detached from the transistor so easily. When I put in the new bias transistors, I shall use fine silicone "spaghetti" that does not need heat. To be on the safe side, I will do same with another new bias transistor for the Left side.

This is the wires after the transistor "fell off". It was enclosed in the "Nando Bracket" and secured to that with a zap strap.

IMG_2099.jpg

There are 4 stores in the within an hour's drive radius that may have 2N5088's. The two most likely are just yards away from each other. If neither have 2N5088's, I will obtain a couple of what they recommend as a substitute but will wait for your opinion before I use them. If I come up empty at those two, I will try the others. Hopefully I come home with something.
 
laatsch55

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That was a NavLinear a bracket. DO NOT SUB the 2n5088. It chosen for its unique abilities. R41 is very critical to bias and negative current drive so good call on swapping it.
 
62vauxhall

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Apologies to NavLinear for misdirecting credit where credit is due.

I got 2N5088's and 2 watt .33 Ohm resistors, not carbon but they should do I think.

Will post again once I've done the re & re.
 
62vauxhall

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So far, my day's been a case study of Murphy's Law and it's continuing


I put in just the Right channel 2N5088. Only the one because I didn't want to tempt fate more than I have been plus the left one ain't broke. Also put in new .33 Ohm resitors Q12 to Q13 on both sides.

Despite a new Right channel bias transistor, the readings still don't seem right compared to the Left channel.

Right channel Q6
B to E: .394
B to C: .583
C to E: OL

B to E: .392
B to C: OL
B to C: .629
 
62vauxhall

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Not surw what I should do here. The RCA's are back in and as I was increasing voltage on the variac, the DBT started to glow at 29 volts whereas before replacing the bias transistor and .33 Ohm resistors, this level of glow took 50 volts. Variac is now backed off to zero.

Should I proceed with measuring bias and offset voltage and if so at what voltage on the variac?

EDIT: With variac at 29 volts bias is 1.84 volts and unaffected by adjusting bias pot as is intensity of DBT. Offset is minus 1.58 volts.
 
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