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Post by Bob Killip on Jan 29, 2024 18:22:50 GMT 12
I'd welcome feedback on the mains supply feeds on this "made-for-export" U.S. Pilot 1936 S-623B. Attached are pics for understanding. I generally stay away from transformer-less sets and same goes for this one so I've given up servicing it until I get better advice. The cabinet's in reasonable nick so I'm wondering if the thing is worth progressing. I have no idea what needs attention but tried to fire it up thru dimbulb no go. It has a wire-wound radiant heater looking AC dropping resistor bolted to the speaker, which must be an afterthought; I'm guessing this arrangement was only done on export models to drop the mains 230v down to the 110v it runs on without adding a transformer. Would I be correct here? Anyway, the fat wirewound "heater" resister is open circuit and beyond repair. What would YOU do? I'm happy, and so is the client, to return it as a showpiece but interested in others opinion. Look forward to comments, Bob Attachments:
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Post by Bob Killip on Jan 29, 2024 18:26:47 GMT 12
Note that mains Neutral connects directly to chassis via ON/OFF switch, and no earth! If mains supply gets swapped (say thru an incorrectly wired extension lead) we have live chassis (OUCH!). Being a qualified test & Tag guy, I don't like it at all and would never give it a PASS.
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Post by Peter Walsham on Jan 29, 2024 19:35:32 GMT 12
If you were to restore the radio & connect it to the mains through a double wound 230 Volt to 110 Volt isolating transformer (ideally with the radio permanently connected to the transformer) then all should be OK. I would connect the radio chassis to the mains earth though). The radio will be quite safe to use on a regular basis with a transformer so fitted . I am horrified to see that the mains on/off switch is wired in to the neutral lead. This makes the chassis live when the radio is turned off - extraordinarily dangerous! The dropping resistor certainly looks like an afterthought. Are you sure this set was originally designed for export? As it is, I certainly wouldn't pass it on an electrical safety test either. The 'odd' valve line-up is normal, as they're all have 0.3 Amp heaters. They (plus the dial lamps) need a voltage of 87.8 Volts, leaving only 22.2 Volts to be dropped across a resistor (at a 110 Volts supply). If the rectifier valve is connected to the 110 Volts as a half wave rectifier, then the radio's performance won't be too flash. However, if you wire it as a half wave voltage doubler, then performance will be much better (although the 25A6 output valve is designed for a relatively low screen & anode voltages). You could get a special isolating transformer wound with a centre tapped 230 Volt secondary winding - or a 230 Volt winding with a 87.8 Volt heater tapping.
Peter
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Post by Philconut on Jan 29, 2024 20:34:06 GMT 12
I fully concur. This is an extremely dangerous beast in its present condition and I too would not give it any pass. Peter's suggestion of a double wound transformer is a tidy solution but beware that most of the 230V/110V transformers around are auto types so the dangerous situation is perpetrated.
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Post by Richard on Jan 30, 2024 15:43:31 GMT 12
I inherited a 1937 AC/DC "National" radio manufactured by "radio Ltd". It used a dropper resistor , with the heaters in series. Because it had a lovely cabinet and a chassis that had plenty of room, i converted it by adding a power transformer and changing the heaters to parallel, changing the rectifier and output tube. works a treat and is safe now.
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Post by Peter Walsham on Jan 30, 2024 16:38:04 GMT 12
Richard
I bought a Columbus console radio in really nice order about 35 years ago that was factory wired for 230 Volts AC/DC (using a dropper resistor). As I hate the live chassis concept, I converted that set to a power transformer set and, like you, changed the rectifier & output valves to suit. That set was almost in daily use until the power transformer recently burnt out. It is currently in the 'queue of jobs to be done' - with the intention of getting the transformer rewound.
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