Had a vintage radio blow a capacitor and take out a trace last weekend

I was working on a 1960s tube radio for a client, a Zenith model. I had just replaced the filter caps and was doing a slow power up with my variac. At about 70 volts, I heard a pop and saw a little puff of smoke. Turns out, a paper capacitor I missed had shorted, and the surge actually vaporized a section of the thin foil trace on the old phenolic board. It was a main B+ line, about an inch long. I had to stop, carefully scrape back the laminate, and solder in a jumper wire to bridge the gap. It got me thinking, do you guys always do a full recap on old gear before even testing, or do you prefer a slow power-up first to see what else might be wrong? I'm torn between saving time and risking more damage.

3 comments

3 Comments

xena6421mo ago

I used to test first but @oscarwright is right, a full recap saves bigger headaches later.

oscarwright1mo ago

You mentioned doing a slow power up with your variac. That's good, but you gotta check all the paper caps first, not just the filters. Those old caps are basically short circuits waiting to happen. I always do a full recap on anything with paper or wax caps before I even plug it in. The slow power up is for checking my work, not the old parts. Saves you from exactly this kind of trace repair.

umamoore1mo ago

Oscar's totally right about paper caps being time bombs. I read a repair log where a guy powered up a radio without replacing them first, and it took out a hard to find transformer. The slow variac ramp just gives the old caps enough time to fail spectacularly instead of instantly. It's not a real test. Full recap is the only safe starting point for anything that old. Then you use the variac to make sure your new solder joints are solid and nothing else is fried.