100 watter blowing fuses

[Big Jim]

Hello folks,
I've been away from this forum for a while, venturing arouind in Trainwreck land. My son has a 69 spec 100 watter that we built that is blowing fuses. The Mains stay lit while in stby, but as soon as you drop the stby, the other fuse blows and no sound. I inspected the inside and did not see anything cooked. When I was draining the caps, I was measuring the drain, and the #5 cap would arc and spark whenever I was trying to measure it. That does not seem normal. Is the cap bad? I almost hope so as that would be a relatively easy fix. Any ideas?
Jim

[elronhoover]

Hello folks,
I've been away from this forum for a while, venturing arouind in Trainwreck land. My son has a 69 spec 100 watter that we built that is blowing fuses. The Mains stay lit while in stby, but as soon as you drop the stby, the other fuse blows and no sound. I inspected the inside and did not see anything cooked. When I was draining the caps, I was measuring the drain, and the #5 cap would arc and spark whenever I was trying to measure it. That does not seem normal. Is the cap bad? I almost hope so as that would be a relatively easy fix. Any ideas?
Jim

The other fuse being the HT yes?

Often HT fuse blowing can indicate a short; a tube, filter cap, or possibly a coupling cap.

If all your filter caps are OK (they will measure very high resistance across if not shorted, like in the megohm range), and your coupling caps are also OK and you see no other obvious shorts in measurement elsewhere, pull the power tubes and see if the HT blows. If it does not, install another set of power tubes and try it, and then so on thru the pre tubes.

Dave

[Big Jim]

elronhoover,
Yes, the HT fuse. I was measuring voltage off of the Filter caps, and yes I was getting large measurements until I tried to measure the #5 filter can cap. It would spark when I tried to measure with the DMM. I'm thinking that cap may be bad. The pilot light stays on. It just immediately blows the fuse and no output. I'm not used to having to fix amps. I've built three, and they have all been pretty trouble free. Not only that, it may be a little tricky discharging that cap!
Jim

[elronhoover]

elronhoover,
Yes, the HT fuse. I was measuring voltage off of the Filter caps, and yes I was getting large measurements until I tried to measure the #5 filter can cap. It would spark when I tried to measure with the DMM. I'm thinking that cap may be bad. The pilot light stays on. It just immediately blows the fuse and no output. I'm not used to having to fix amps. I've built three, and they have all been pretty trouble free. Not only that, it may be a little tricky discharging that cap!
Jim

Jim, quickest and easiest way to drain all the filters is to place a wire jumper from between one of the v1 100k plate R's and its v1 socket, and then to ground. This will drain the whole B+, and keep the voltage down while attached. Make sure you always remove it before you power up though!!

Make sure you have your meter set properly when taking measurments too, you dont want to be set for resistance and then try to measure voltage (and vice versa). This can cause sparks and can damage your meter, FWIW..

Once you get all the filter caps drained, and verifed with a voltage measurement on them all, switch to resistance and measure across all the filters. You should see maybe for a sec a lower resistance, then it should go up to mohm range if it's OK. If it stays low, or just is low or shorted, there you go..

Also, are you using a slo blow fuse on the HT? Fast blo can pop from the inrush when you go off standby..

Keep at it, you'll get it!

Dave