left channel dies after 5 minutes

[denti alligator]

Done... and for the first time ever I have smoke !!

Turned it off instantly. What to do now?

(Looks like the wire connecting 14 to 20 was melting... if that tells you anything)

 

[Paul Birkeland]

Measure the resistance between the "O" pad on each side of the big C4S board and ground.  We are getting somewhere I think.

-PB

 

[denti alligator]

I'm not getting a reading at all on either side.

 

[Paul Birkeland]

Redo the voltage test with the 6080 removed.

-PB

 

[denti alligator]

1  -0.23
5  -.156
7  -.091
9  1.392  (and steadily decreasing)
15  244.5
18  96.9
19  96.9
21  245.1

A1  -.187
A3  .859
A6  -.079
A8  .909

B1  -.034
B3  .087
B4  -.113
B6  1.281 and steadily decreasing

all the rest are 0

 

[Paul Birkeland]

What do you have at terminal 2?  (Your 12AU7 is still plugged in, right?

 

[denti alligator]

12AU7 is still in.
Terminal 2 is getting no reading. The +/- is flickering and it shows 0.

 

[Paul Birkeland]

How about terminal 13?  How about B2 and B5?

 

[denti alligator]

I could check them again, but everything I didn't list was 0 or no reading.

 

[denti alligator]

B5 and 13 are 0
B2 is 0.002

 

[Paul Birkeland]

T13 and T15 connect together with a 270 Ohm resistor.  To get full voltage at 15 and no voltage at 13 indicates that either the 270 Ohm resistor between them isn't well connected, or you have a short somewhere down the line of terminal 13 that is pulling all the B+ down across said 270 Ohm resistor.  Based on your relatively high available voltage, I can determine that the resistor between 13 and 15 is either open or poorly connected

-PB

 

[denti alligator]

Yeah, that resistor is looking discolored. Wiring going to the power switch from the transformer (under the resistor) and from 20 to 14 (as well as part of the wire going from 12 to 14) are looking partly singed/melted. This is clearly where the smoke was coming from.

What to do? Can I request extra parts from you? I don't understand how this area got hit when I was trying to solve an issue on the PC boards.

Can use 250ohm or 300ohm 5W resistors? Parts Express only has those, not 270ohm.

 

[Paul Birkeland]

250 Ohms is OK.

Generally this resistor cooks when there is a short wired into the amp.  The most common cause of this is when the TIP50 transistors aren't properly insulated from the heatsinks (the mounting kits aren't installed properly, almost always the shoulder washer is upside down or not seated well).  Once or twice we have seen people replace the 270K/1W resistor with a 270/1W resistor, and that will also cause similar issues.

-PB

 

[denti alligator]

Do you suggest replacing the transistor, too? Wasn't that the source of the problem to begin with--probably?

 

[Paul Birkeland]

Do you suggest replacing the transistor, too? Wasn't that the source of the problem to begin with--probably?

I've never seen a failed transistor cause that issue.

 

[denti alligator]

I'm going back to the original problem of the channel going out after 5 minutes. If I fix this problem at terminals 13-15 I'm back to where I started with the PC boards...

 

[denti alligator]

Also, remember that the amp worked fine for over a year. Unlikely the transistors aren't well insulated, or else they would have failed sooner. Or not?

I'm afraid of replacing the wirewound resistors and then getting them fried again. How do I prevent that?

 

[Paul Birkeland]

I'm going back to the original problem of the channel going out after 5 minutes. If I fix this problem at terminals 13-15 I'm back to where I started with the PC boards...

Your problem is that the high voltage isn't making it to the tubes.  You can focus on the issue of the amp going out after 5 minutes, but your voltage measurements indicate that the amp isn't working at all.

 

[Paul Birkeland]

Also, remember that the amp worked fine for over a year. Unlikely the transistors aren't well insulated, or else they would have failed sooner. Or not?

I'm afraid of replacing the wirewound resistors and then getting them fried again. How do I prevent that?

I've seen the insulation issue come up both randomly and at the time of construction.  Do keep in mind that one of the primary reasons that we came out with Speedball 1.1 was because so many builders were improperly mounting the TIP50's. 

Replace the 270 Ohm resistor, then run the amp without the 6080.  Use a pair of clip leads to have your meter reading the voltage at terminal 2 while you turn the amp on.  If this voltage drops below 130V, then turn the amp off and report back.

 

[denti alligator]

OK, thanks.

Any chance these resistors could work? I can't tell the wattage.