Blocking cap question

[Randall]

Doing a DRRI re-rebuild. First one had a conductive black fiber board, so i ditched it and re-wired up a G-10 turret board. Went thru the firing up process tonight with the dim bulb limiter. With just the recto tube, while watching the power supply load up, there was a tick, tick, tick sound once the V's got into the upper range, so I shut it down. Checked, double checked, same thing.

Eventually, not finding any bad wiring, I got brave and let 'er rip. Tick, tick, snap, and I actually saw a flash from the inside of one of the Mallory 150 0.1uF 630v blocking caps between the PI and outputs. Then, quiet.

Everything measured well. I've never experienced this. I replaced it, no more snapping. Was this a bad cap from the get go, or do you think maybe from soldering, unsoldering, re-soldering it went bad? I've also heard they can heal somehow, but this I don't know.

Heat damage from pulling all of these components to re-wire the turret board has been tugging at my mind right from the start. I did measure everything before installing, but not sure that is always enough.

[bluesky636]

When I'm swapping components to try out different values of caps or resistors, if I decided to go back to a value used previously, I'll use a new component. Mostly because the leads have been trimmed or bent up during installation and removal. However, on several occasions I have reused components without any problems. I use a 25 watt iron for circuit board building and a 40 watt iron for soldering to the back of pots or ground planes, etc. Never had a problem with heat damage.

[Tillydog]

Was this a bad cap from the get go, or do you think maybe from soldering, unsoldering, re-soldering it went bad? I've also heard they can heal somehow, but this I don't know.

If it was OK the first time, then it's likely something that has happened that has damaged it - either heat from soldering / de-soldering or mechanical damage (dings or burns in the casing).

Very likely it was a one-off.

I think the 150s are billed as 'self healing' which means that due to the combination of materials used, the cap will likely not self-distruct if the insulation breaks down (I think the metallised film burns away if there is an arc so it makes a second arc less likely, but don't quote me on that).

I think replacing it was the best thing to do in any case.

Is the amp working now?

[Zippy]

With just the recto tube, while watching the power supply load up, there was a tick, tick, tick sound once the V's got into the upper range, so I shut it down.

Eventually, not finding any bad wiring, I got brave and let 'er rip. Tick, tick, snap, and I actually saw a flash from the inside of one of the Mallory 150
0.1uF 630v blocking caps between the PI and outputs.

Now you know what arcing can sound like.

Was this a bad cap from the get go, or do you think maybe from soldering, unsoldering, re-soldering it went bad?

I'm guessing that it got cooked. I usually use a heat sink when removing parts that I want to use again.

[Randall]

Thanks guys. I replaced it, and the amp is working like a charm! Everything is right now, it sounds great, and it is quiet as a mouse. I couldn't be happier. Both channel vols up full, and I can barely tell that it's on.

HUGE improvement over the scratchy pots, bias ripple, moans and groans coming from that black fiber board.