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Heathkit TA-16 Tripping Breaker

Started by ilyaa, January 04, 2022, 08:13:22 PM

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ilyaa

hey happy new year everyone!

wouldnt be posting if i wasnt stumped....

got a Heathkit combo on the bench. kind of a weird one, but sounds rad! works alright, until you turn it up. after 3-4 seconds of full power, it trips the breaker (this amp has two 2 amp breakers rather than fuses).

i hooked an ammeter to it and its only drawing 0.5 amps or so right before it trips the breaker, so the high current draw must happen fairly quickly.

here is what ive tried so far:

replaced c104. thats the oldest electrolytic in there - the other ones are fairly new. replaced c34, c21, c17, c22. no diff.
outputs looked fine but i replaced em anyway (Q10 and Q9).

before i start swapping the rest of the transistors out, wanted to see if any of you brilliant minds had some thoughts as to what might be happening. the strangest thing is it works totally fine and if i reset the breaker it comes right back on, no problem. its only when it reaches near full power that something seemingly starts to cascade and draw too much current. the output stage is a bit tricky for my brain (mixed germanium and silicon, for starters...) - please enlighten me!!


g1

Is it the same breaker tripping every time?
Does your meter have a peak hold function or anything like that?  It may be that the meter is just too slow.
If it's not too involved, you could swap the breakers around and see if the problem follows one particular breaker.  It's not common, but they do get weak sometimes.

ilyaa

#2
i did try that by replacing the breaker with a 1 amp fuse. that blew, too

and its the second breaker, on the secondary side of the PT, that is being tripped.

Enzo

You replaced a 2 amp breaker with a 1 amp fuse?  Well that doesn't tell me much.  Please tell me it was at least not a fast blow 1A.

Actually weak breaker was my first thought too.  You are throwing parts at it based on possible current surges.  But how about if the breaker is weak, then it doesn't care about peaks.

A fuse isn't a bad idea, but at least use a 2A SLOW or even a 2.5A.

ilyaa

i tried a 1A slo blow - will try 2A slow blow and see what happens!

reason i thought 1A slow blow would be okay as a first go was that my ammeter was showing 0.5 amp when the breaker was tripping - was showing 0.5A when the 1A slow blow cooked.

i might try switching the breakers first and then try a 2A slow blow fuse depending on the results -

Enzo

SO when the fuse/breaker blew, you were playing through teh amp?  Did it just stop in an instant?  or was there any sort of momentary lean up to the blow?  Like a moment of distortion or something?

ilyaa

i was passing a 1khz sine through it into a dummy load, with a scope on the load.

no distortion - i crank it up slowly, it gets nice and square, achieves full power for about 3 to 4 seconds, and then *poof* turns off.

ilyaa

switched breakers - no dice

it trips the other breaker, as well -

mandu

May be no heatsink compound on the power transistors. Only mica washers?