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Power transformer troubleshooting - would it short out intermittently?

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Posted on April 30, 2020 at 16:11:50
danjf
Audiophile

Posts: 83
Joined: July 27, 2002
 I have an Eico AF-4 amp (single ended stereo) that I restored 10+ years ago. It's been running no problem, until it recently started blowing fuses. I narrowed it down to the power transformer (I think) - after I replaced the filter caps, and pulled out all the tubes, it still blew a fuse (with a flash of the current limiter lightbulb.) But it took a minute or two before the bulb flashed and blew the fuse.

Last week I removed the power transformer from the chassis. The resistances are good, as are the voltages on the filament and B+ wires when I run 120VAC into it. I only powered it up for minute or two, enough time to take voltage measurements, and of course, with no current flowing since it was on the bench.

Here's my question (before I power it up again on the bench for an extended time,) I'm wondering if it would be typical (or possible) for a power transformer to short out intermittently? Or maybe I should suspect something other than the power transformer?

This is (luckily) my first ever power transformer problem. I'll appreciate any thoughts.

 

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RE: Power transformer troubleshooting - would it short out intermittently?, posted on April 30, 2020 at 17:47:53
Tom Bavis
Audiophile

Posts: 961
Location: Upstate NY
Joined: May 25, 2007
 Seems unlikely. I'd swap the rectifer tube. Could short plate-to-plate or plate-to-cathode. Or arc over plate-to cathode, which is more likely if it's gotten weak.

 

RE: Power transformer troubleshooting - would it short out intermittently?, posted on May 1, 2020 at 07:36:39
danjf
Audiophile

Posts: 83
Joined: July 27, 2002
 Thanks Tom. I was suspecting the power transformer originally, but by your reply, now thinking it may in fact be the EZ81/6CA4 rectifier tube - and suspect that the connection on the socket to pin 7 (one of the plates) may have gone bad. I appreciate your reply, I'll be looking at it again this weekend.

 

RE: Power transformer troubleshooting - would it short out intermittently?, posted on May 1, 2020 at 10:00:54
caffeinator
Audiophile

Posts: 1729
Location: Pacific Northwest
Joined: August 22, 2003
 The fact that it shorts out with all the tubes removed suggests it may be the power transformer, and perhaps the primary. Since it doesn't do this with the transformer removed, it seems to me the problem might not be the transformer alone, but perhaps the transformer as mounted in the chassis. My thought is a short from a transformer lead to the chassis or ground.

Carefully check the transformer leads, looking for any cracks or brittleness. If they are cloth covered, look for fraying, fuzzing or thinning. Consider wrapping the leads in heat shrink to be sure the insulation is sound. You may want to remove the end bells to trace the leadouts (presuming here that there are leadouts) inside and perform the same inspection and insulation upgrade.

Hope this helps - good luck!

 

RE: Power transformer troubleshooting - would it short out intermittently?, posted on May 3, 2020 at 14:06:49
danjf
Audiophile

Posts: 83
Joined: July 27, 2002
 Thanks - to check everything all over again I re-installed the original transformer and I'll be carefully checking all connections and possibilities for a short, bringing the voltage up slowly. (I'll replace the EZ81/6CA4 rectifier tube, the old one is now a prime suspect.) Good idea about the cloth insulation, I wrapped the leads in shrink wrap.

 

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