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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 28th Nov 2018, 1:48 pm   #21
mhennessy
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Default Re: Hot mains transformer

Yes, that's exactly why I suggested testing with a variac in post #16.
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Old 28th Nov 2018, 2:18 pm   #22
mhennessy
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Default Re: Hot mains transformer

Quote:
Originally Posted by mhennessy View Post
I can't find any data on-line about the Fidelity UA6, but I really doubt it would have been fitted with a 100VA transformer. Any idea what the power output would have been?
Just doing a bit more searching - information on the UA6 is definitely thin on the ground - but a post here suggests that it's only 8-10 watts per channel:

https://www.vintage-radio.net/forum/...15&postcount=2

So, definitely not a 100VA transformer. More likely 30VA or thereabouts. Possibly 50VA at a push, but I seriously doubt that would have got past the bean counters at Fidelity

We don't know the rated load impedance, but as this was the early '70s let's assume 15 ohms. Let us also assume that the transformer is supposed to be 24-0-24, as I can't think of a good design reason to want 0-24-36. Finally, let's assume a pair of diodes is used to produce a single rail of around 35V unloaded. All that fits pretty well with the 8-10 watt power output, and actually isn't a bad fit for 8 ohms given the rail will sag quite a bit under the increased load.

Either way, in lieu of more information from Fidelity, that's my best estimation of what this transformer is (or rather, "was once" ) capable of.
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Old 28th Nov 2018, 2:20 pm   #23
Skywave
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Default Re: Hot mains transformer

Lots of ideas above to try to discover the cause of the excess heat: thanks.
There are two things I can now do: use my Variac; measure the temperature of the core when it is hot.
Lots of other things demanding my attention right now - but I will investigate this xfmr. further . . . ASAP.

Al.
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Old 28th Nov 2018, 2:27 pm   #24
Skywave
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Arrow Re: Hot mains transformer

So where did this xfrmr. come from? I have to say that I really don't know; the Fidelity was just a guess. And yes, it might be designed for 220 v.a.c.: there are no labels, etc., stating what the pri. voltage should be.

Al.
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Old 28th Nov 2018, 2:40 pm   #25
Argus25
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Default Re: Hot mains transformer

Quote:
Originally Posted by kalee20 View Post
Good advice from Hugo!
Thanks Kalee20. Your confidence has inspired me to scan & attach the following page of information that demonstrates how the B-H curve results in the form of the off load current wave. I hope it is readable
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Old 1st Dec 2018, 10:03 pm   #26
Skywave
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Arrow Re: Hot mains transformer

New test: I ran this xfmr. from 230-v.a.c. and a load drawing 2 amps. After 2 hours, load voltage was 35 v., (so a drop of 1 v.); core temp was 87°C ! (Ambient temp. was 21°C). So I reduced the load to 1 amp. to see what temp. the core would drop to. When I looked again, 2 hours later, the core temp. was 28°C. Upon inspection, the xfmr. primary was open-circuit.

The fleeting thought did cross my mind if, perhaps, this xfmr might have been intended for 115-v.a.c., but I haven't 'scrapped for spares' any USA kit for several decades, plus it is labelled 'DE Ltd' (i.e. not 'DE Inc'), so in retrospect, I doubt it.

Anyway, it's all a bit academic now. At least it wasn't embodied in a piece of test gear, or W-H-Y.
You win some; you lose some.

My thanks to everyone for the helpful advice.

Al.
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