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#106 Re: 45 SET

Posted: Wed Oct 05, 2022 6:51 pm
by Paul Barker
Id be worried about having 240v into low voltage side. But I suppose if its survived its rest of life shouldn’t stress it. However up to the fuse blowing it was exposed to rediculously high voltage and get very hot, insulation damaged very likely, the small number of turns mean a dead short near enough, which usually fries the winding. But youre fuse might have taken it all before the transformer cooked. If you’ve got away with one IEC socket and a pile of blown fuses its possible and not sertain death, but it is possible death where we are with it now.

Ive made loads of other mistakes, but my strange mind would never let me wire a transformer backwards as I studied and wound power transformers so inside my head when Im hooking up I visualise the mechanics pictorally. So my strange visual mind wont facilitate that mistake as Im not just hooking up something I dont understand to every level.

My biggest fo pa on a newly hooked up amp was to fit a microwave emi filter on the input which Id got too close to the pain of the steel chassis, soon after it arked through and did that take me quite a time for the penny to drop as it was unseen damage, behind where I mounted it, and I never actually worked it out. I accidentally found it when rewiring other things chasing the current draw, and had to move the emi filter, where upon I saw the problem. In the event I cut it out.

Just history now.

#107 Re: 45 SET

Posted: Wed Oct 05, 2022 6:59 pm
by simon
Yeah that does nag at me a little, hopefully the secondaries are okay. I'll find out eventually I guess.

#108 Re: 45 SET

Posted: Wed Oct 05, 2022 8:19 pm
by Nick
Given you have blown up several fuses, it would be unlikely that the very last time you tried it would be the time the transformer broke. Possible, but seems unlikely.

Why would a (at least) 1.5A 2.5v winding get worried about passing current that would blow a 2.5A fuse?

Again, heat is a product of power over time, reduce the time and you get little or no heat, with little or no heat why would the transformer care.

Colin's 300b Amp is built around a pair of transformers that came out of a amp I built for someone, that due to a voltage spike on his mains, caused the bridge rectifier that was after the 300b heater winding to turn into a short. Lots of current was passed, the primary fuse didn't blow as the heater winding was a small fraction of the total power (good reason to fuse secondaries of transformers), the internal wiring was scorched and it was all a mess. I replaced the transformers of course, but Colin used them (with knowledge of the previous) to get the amps going with the plan to replace them in the future. That was probably at least 5 years ago. They are still both working.

YMMV.

The only lesson I would take from this is use ceramic body, high current break fuses.

#109 Re: 45 SET

Posted: Wed Oct 05, 2022 8:30 pm
by simon
Good point well made.

I thought they were ceramic when I ordered them TBH.

#110 Re: 45 SET

Posted: Fri Oct 07, 2022 3:47 pm
by simon
I made the changes this morning and with some trepidation plugged it in after checking voltages. Good news, sound! Bad news, only on the left channel.

I've switched input phonos, 5751 drivers, and 45s one at a time and it doesn't follow, so it's something to do with the right channel.

The voltages check out at anodes and cathodes of all valves so I think they must be conducting okay. So I broke the scope out, which was a bit of an adventure as it's a long time since I last used it. I've traced signal as far as the 45 grid but before I stick the probe on the 45 anode at 340V I thought it worth checking that my 300V x10 probe will be okay. Too many bangs this week already. And it's a bit of a pig to get to the anode tag of the socket.

My suspicion/fear at the moment is the OPT secondary has gone. I can switch the OPTs over to prove/disprove.

#111 Re: 45 SET

Posted: Sat Oct 08, 2022 7:13 am
by Paul Barker
Oh pooh

#112 Re: 45 SET

Posted: Sat Oct 08, 2022 1:11 pm
by Mike H
But heater TX's are OK?

#113 Re: 45 SET

Posted: Sat Oct 08, 2022 1:36 pm
by simon
Looks to be yes, both illuminated filaments visible

#114 Re: 45 SET

Posted: Mon Oct 10, 2022 4:28 pm
by simon
Phew! It's not the OPT. It took some finding but there was a single strand from the earth screen attached to the signal input to the driver.

Now I've just remembered I didn't reconnect the driver grid leak resistors...

#115 Re: 45 SET

Posted: Mon Oct 10, 2022 4:45 pm
by Nick
Yay!

#116 Re: 45 SET

Posted: Mon Oct 10, 2022 5:21 pm
by IslandPink
:wav:

#117 Re: 45 SET

Posted: Mon Oct 10, 2022 7:02 pm
by simon
Yeah a pretty good afternoon...

#118 Re: 45 SET

Posted: Mon Oct 10, 2022 7:04 pm
by Mike H
Image


This is why I use a magnifying glass...

#119 Re: 45 SET

Posted: Mon Oct 10, 2022 8:56 pm
by simon
I needed glasses and an illuminated magnifying light. If it had been on the other side of the wire tucked away I might still be looking now.

Actually the next job was to unsolder rhe signal and screen to inspect as it was looking like the wire was knacked when I checked it for continuity - that's how I found it.

Anyway, clearly I need to brush up on my scope skills as I was way off.

#120 Re: 45 SET

Posted: Mon Oct 10, 2022 10:43 pm
by pre65
Simon, I'm glad to see you got to the root of the problem, and without having to spend lots of beer tokens. :D