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Post by peteb2 on Jan 9, 2024 18:08:44 GMT 12
I have been asked to 'go through the chassis' as the owner has already done a reasonable tidy up of the case having removed the white enamel paint job.
Everything is all there but i fear we will likely need new valves & a full capacitor replacement.
May i ask please whether anyone can assist with spec of the secondary center tap HT for B+ please?
Sadly, the transformer has completely failed not from a burn-up but the paper insulation is for all purposes just dust and the tail end primary & secondaries leads have lost their insulation completely! Doing a rewind is problematic in that it is not an E I core making it very awkward to disassemble.
The schematics are here:
I may then be able to match something else as a substitute but i have no idea (no voltages on the schematics) voltage & current requirement.
Thanks for any ideas.
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Post by Peter Walsham on Jan 9, 2024 19:00:54 GMT 12
Hi 'peteb2'
The Radio Corp. T57 HT winding is close to 350-0-350 Volts @ about 80mA, with a 5 Volts @ 2 Amps rectifier filament winding , and a 6.3 Volt winding @ about 3 Amps for the other valve heaters. Some Radio Corp. power transformers have the 6.3 Volt heater winding tapped at 5 Volts for the dial lamp(s). Unfortunately, Radio Corp. power transformers are literally 'shit', often falling apart exactly as you describe.
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Post by Philconut on Jan 9, 2024 20:50:03 GMT 12
Hi Peteb2
Unfortunately the old enamel varnish becomes very brittle over time & your description of the result is very familiar. You shouldn't have too much difficulty dismantling the core should you decide to go for a rewind. I am happy to help you with calculations of turns and the existing windings can be measured for wire diameter. It will help to measure the lamination tongue width & the stack depth as well as these dimensions will give a good lead to the transformer's total capacity in VA.
If you decide that a replacement is the way to go a Beacon R04 is 350V/350V @ 60mA, 5V @ 2A & 6.3V @ 3A would fit the bill nicely I think. I may have one in my collection but I am not back home until late Thursday so will check then.
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Post by peteb2 on Jan 10, 2024 6:24:10 GMT 12
Hi 'peteb2' The Radio Corp. T57 HT winding is close to 350-0-350 Volts @ about 80mA, with a 5 Volts @ 2 Amps rectifier filament winding , and a 6.3 Volt winding @ about 3 Amps for the other valve heaters. Some Radio Corp. power transformers have the 6.3 Volt heater winding tapped at 5 Volts for the dial lamp(s). Unfortunately, Radio Corp. power transformers are literally 'shit', often falling apart exactly as you describe. I managed to 'delam' the offensive thing & placed the core in the infront the previous post chassis photo. Apart from sneezing, alot, from all the paper dust this is the first time i have met a lamination that is effectively a dual split "8". I must say that whomever approached this design it certainly met the criteria for as cheap as possible by the look! Mind you the thing was made around 85yrs ago i assume which is understandable why it is now in such a condition.
I had set out on studying my RCA Radiotron Designers Handbook section on rectification in particular the 80 valve but finding it very heavy going if not for such fine print in search for an understanding on how to calculate the voltage & current requirements. Plainly i am over thinking it... but it is very interesting, & again, thanks for the advice.
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Post by Philconut on Jan 10, 2024 7:48:45 GMT 12
That style of lamination was quite common back then. The starting point for calcs is the cross sectional area of the central core limb. Next work out the wire sizes based on 2.5 amps/mm sq. It is quite an involved process as you have to take each winding voltage drop into account for the wire size & turns calcs. Then there are space checks to make sure the windings will fit within the core windows. I have a spreadsheet which takes all the grind out of the job and I'm happy to work it all out for you.
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Post by Richard on Jan 10, 2024 12:41:38 GMT 12
I remember someone telling me that Radio Corp Transformers weren't dipped in Wax or varnish, therefore contributing to their unreliability.
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Post by Philconut on Jan 10, 2024 14:18:27 GMT 12
Yes I think that is correct, hence the dust Peteb2 mentions. I don't know why they didn't do it as it seals the inter-layer paper and windings which prevents moisture ingress. At TWS we used a special varnish for the job. I use wax & have a home-made heated bath for the job. It works really well.
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Post by peteb2 on Jan 13, 2024 9:20:33 GMT 12
Hi Peteb2 Unfortunately the old enamel varnish becomes very brittle over time & your description of the result is very familiar. You shouldn't have too much difficulty dismantling the core should you decide to go for a rewind. I am happy to help you with calculations of turns and the existing windings can be measured for wire diameter. It will help to measure the lamination tongue width & the stack depth as well as these dimensions will give a good lead to the transformer's total capacity in VA. If you decide that a replacement is the way to go a Beacon R04 is 350V/350V @ 60mA, 5V @ 2A & 6.3V @ 3A would fit the bill nicely I think. I may have one in my collection but I am not back home until late Thursday so will check then. Well with these glorious summers days i should be outdoors getting sunburned (but i've learned that lesson too many times when much younger). I'm now completely committed to this chassis since the owner (a near relative) has already tried their hand at resurrecting the case & done quite a nice job. I've just realized it's the same as the one in image upon the Home page at nzvrs.com/ but with a black dial.
One variation to the info about the Model 84 is that this chassis has a ECH35 in place the 6A8, so i'll assume that Radio Corp were at the time either having a supply issue for the 6A8 or that the ECH35 was a cost advantage. Of course a radio repair tech may have substituted it but the two necessary additional dogbone resistors (compared the 6A8) look terribly original-soldered.
I'm still without a Mains transformer & scrounged through my hoard only to find one that's physically too huge (but has the 5V winding for an 80 rectifier) only to discover it is a 115V Primary. I guess though i could run it up & use it to get things going.
A search for a brandnew one will set me back NZD $250 for the Hammond 291AEX from Digikey, (free shipping) that specs a 650ct or 550ct @100ma, 5V @ 3A & 6.3A @2.5A. Its a generic for a guitar amp (so i guess if everything goes wrong i can always maybe make a guitar amp)!
Of course that could be a bit of an excessive purchase & i'd prefer to recycle one not being used that needs a new home, so may I ask please Phil whether you've rediscovered your Beacon R04 (no urgency).
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Steve
Society Members
vintageradio.co.nz
Posts: 732
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Post by Steve on Jan 14, 2024 15:19:33 GMT 12
Hi Pete, and welcome!  The Popular is a nice little set - but I would always be wary of taking a white finish off a 1938 or '39 Courtenay or Columbus set from Radio Corp - some left the factory that way. Any 5-valve vintage radio transformer should work from that era - You shouldn't need to bring in something from overseas. And re: the tails of the HT and mains wires losing their insulation - Radio Corp used rubber insulation on those (I believe the cloth covered wire wasn't safety rated for high AC voltages). Rubber insulation from that era is almost always crumbled, or nearly ready to crumble, regardless of brand. The wiring of the transformer is as follows (this is from an official Radio Corp memo on transformers)  I'd also think the ECH35 was a replacement later in it's life (possibly during WW2) - Radio Corp didn't often, if ever, use Philips - predominantly due to cost. They imported direct from the US in bulk. It's possible they might have used the odd few to fill a shortfall in production, of course... I know Philips often approached Radio Corp offering to sell them valves, but the pricing never worked out in correspondences I've read. I'll have a look through my transformers and see if I have one suitable - a T57 or T58 should work just fine. Possibly even an earlier one like the T53. Cheers, Steve |
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Post by trombone on Jan 14, 2024 18:22:37 GMT 12
Hi Peteb2 ,I have a spare RO4 transformer salvaged from an Ultimate EB6 if you would like it. Pay postage. Phone (03)3272421 Cheers,
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