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 Post subject: Before I put the covers back on...
PostPosted: Nov Fri 09, 2018 2:07 am 
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Joined: Dec Tue 06, 2011 3:31 am
Posts: 74
Location: Hamilton Ontario
My Grommes 24pg amp (daily driver) blew it's fuse last night. I never even saw the pilot light light. The fuse was blown violently. So, of course I tried another fuse. As my mother would have said "Sam Tink"... the fuse blew real good.
Today I put it on the bench, and approached it with the full arsenal of variac, dim bulb, and amprobe (with the fuse holder jumpered). It came up fine. I checked it with the output tubes out, then added them channel by channel, swapped them around. Checked bias / voltage, and ran music threw it. Wiggled and twisted the line cord, and the wires coming out of the transformer. Dropped it an inch onto the bench. It's now running fine. I had a good look in case the upper and/or lower covers could be shorting something. Can't see a thing.
So, before I put the covers back on, can anyone think of something I missed? The blown fuses were 2.5 amp. I see that it wants a 3 amp fuse which I will get tomorrow. I'm wondering whether an intermittent filament short could have been the trouble, or something else weird that I might have missed.

All ideas welcome.

David

PS it was re-capped about five years ago, and got new tube sockets at the time. It's been running on that 2.5 amp fuse since then, almost daily.


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 Post subject: Re: Before I put the covers back on...
PostPosted: Nov Fri 09, 2018 2:53 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 13829
Location: Fernandina Beach, FL
David, SloBlo fuse or just a regular fast-acting fuse?

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Don


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 Post subject: Re: Before I put the covers back on...
PostPosted: Nov Fri 09, 2018 2:57 am 
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Joined: Sep Tue 15, 2015 1:16 am
Posts: 263
Location: 18424 PA
See how many watts it draws from the outlet. Monitor that while using, if it creeps up, filter caps are going bad.


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 Post subject: Re: Before I put the covers back on...
PostPosted: Nov Fri 09, 2018 3:06 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 13829
Location: Fernandina Beach, FL
Kill-A-Watt is great for monitoring currend at the outlet.

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Don


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 Post subject: Re: Before I put the covers back on...
PostPosted: Nov Fri 09, 2018 2:13 pm 
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Joined: Dec Tue 06, 2011 3:31 am
Posts: 74
Location: Hamilton Ontario
Fast acting fuses. It blew two in a row, then on the bench was fine. It draws about an amp, spikes to almost two at turn on, cold filaments I presume. It uses a pair of indirectly heated rectifiers (ez80), so the b+ takes several seconds to build up. The fuses blew instantly.

I thought maybe a short in the line cord at he strain-relief or something with the two convenience outlets, but that's all before the fuse. Still stumped.

David


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 Post subject: Re: Before I put the covers back on...
PostPosted: Nov Fri 09, 2018 3:26 pm 
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Joined: Aug Fri 12, 2016 1:49 am
Posts: 313
Location: Houston, TX
Since they blew instantly when cold, we know it's not heat related.

Since they didn't blow on the bench with it opened up, we know it's not a [permanently] internally shorted component.

Definitely sounds like a shorting condition, mechanical in nature.

You might try gently tapping on the tubes to check for intermittent internal trouble.

If it blows again with covers back on, and you haven't moved anything inside, you'll know for sure whether the covers are forcing a short.
Maybe recheck your work on those new sockets.


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 Post subject: Re: Before I put the covers back on...
PostPosted: Nov Fri 09, 2018 4:36 pm 
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Joined: Dec Tue 06, 2011 3:31 am
Posts: 74
Location: Hamilton Ontario
I tapped and wiggled everything including the tubes, but it just occurred to me that the first thing I did at the bench was to turn it upside down to remove the bottom cover. Has anyone ever had a tube where debris inside caused a short? That said, I have cured one or two crt shorts by smacking the tube facing down on the table, but those were inter-electrode shorts that weren't fuse-blowers.


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 Post subject: Re: Before I put the covers back on...
PostPosted: Nov Sat 10, 2018 2:00 am 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4170
Location: Sunnyvale CA
David Stringer wrote:
Has anyone ever had a tube where debris inside caused a short?


Yes, I have seen a few tubes that had loose parts inside that shorted out when put in to the right orientation. I never figured out whether or not it was because debris was built into it, or part of it came apart after aassembly. In one case, it was the former, because there was an entire loose mount inside it.

Brett


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