Vox NT15 various problems

[g1]

Had a Vox before that ran the heater trace under one of the filter caps and close to it's high voltage terminal. Couldn't find the arcing until I removed the cap. A bit of electrolyte from the leaky cap led to the arcing.
Since C14 is already suspect, and you can test the amp without it, I highly recommend you remove it. Then proceed as per post #14. Replace any resistors that are bad and then proceed to test voltages with C14 removed.

[Helmholtz]

Without C14 the amp might oscillate/motorboat.

[g1]

It might. But with no signal I thought it would probably be ok.

[Helmholtz]

Oscillation can be avoided by pulling V3.
Oscillation doesn't necessarily require an input signal.
When there's positive feedback at some frequency a turn-on transient or even thermal noise may suffice to start oscillation.

[rockman627]

Thanks. This amp is my personal amp but 2 DRRI's came on my desk that needs attention. Will keep you posted.

[rockman627]

Damn, I finally got back to this. I found a pack of new 22uf, 450V caps so I replaced C14. I forgot to take measurements before putting in the new cap. R1 is no longer smoking but we are kind of back to square one. Should I remove the new cap and take measurements?

[rockman627]

Ok, I removed C14 and took new measurements.

120vac in, mode is pentode/bright.

Q1-R1 340vdc
R1-R18 321v
R18-R16 175V
R16-R13 106V
R13-R26 40v
V1p7 342 v
V1 p9 318v
V2p7 335 V vV
V2p9 321v
V3p1 28V
V3p6 39V
V4p1 143v
V4p6 82v

OT ct 342vv
tag 11 322v
tag 12 322v

I can see there is a little.more voltage, but it looks like there is still too much current draw. Other than that, I don't know how to interpret this.

[g1]

Disconnect C17, remove V3. Recheck voltage at both sides of R13.

[rockman627]

Will do, on Tuesday.

[rockman627]

Ok, I had to remove some hard goop (a ceramic type substance as opposed soft silicon gel). But I was careful and got it done. From r16 to r13, I read 103dcv; from r13 to r26 I get 39v. Pretty close to what I got Thursday. Here's the rest.

Q1-R1 331vdc
R1-R18 310V
R18-R16 169V
R16-R13 103V
R13-R26 39V
OT ct 331V
V1P7 327V
V1P9 310V
V2P7 327V
V2P9 311V
V4P1 138V
V4P6 78V

[g1]

R16-R13 103V
R13-R26 39V

Lift the end of R13 that goes to R26. Check that the other side of R13 now has proper voltage, or higher.
Then with R13 still lifted, measure resistance across C14 or C17.

[rockman627]

Ok those readings were with C14 and C17 and V3 disconnected. I hope that was alright. I will lift R13 on the R26 side, reconnect C14, leave C17 out and take the reading.

[rockman627]

I reconnected C14,C17 and V3. I broke C17 taking it out so I have a .015 uf chicklet cap instead. I lifted R13 and I get 279vdc on the other side. Turned the amp off and measured 5.7k ohms resistance on C17. Tell me what you think. I'm going to study the schematic and see what I come up with.

Just remembered something. Do you want me to measure esr of the cap? I don't have anything to do that.

[rockman627]

Hey I removed C17 and measured it. I get OL. There is 5.7k ohms resistance between the solder pads.
I also removed C14 and measured it. OL also. And also 5.7k ohms between the pads.
That can't be right. But the board looks clean.

[g1]

No signs of arcing anywhere?