Just bought this radio from eBay. I only concentrate on 30's and 40's radio's with the odd interesting/unusual 50's sets. This one is dated (on the smoothing can) Oct 1945 so only just Post War. It's an AC/DC TRF. There has been some bodgery around the mains dropper which I will have to un-bodge and it's fitted with a three pin plug and an EARTH!!!! It's purported to be working but there is no way I'm going to check it yet until I've investigated the bodgery. At present it has a number of wirewound resistors in series, placed inside a plastic????box with a length of wire connecting them to the circuit. I'll probably take all that out and use a capacitive dropper. I have yet to work out how he's managed to earth a non-isolated chassis........the mains earth is connected directly to the chassis!
Not sure what colour the cabinet was. It looks as if he has stripped the top down to bare wood, the front has a dark wood stain and polished, the sides are painted a dark brown. I'm inclined to think it should all look like the front unless anyone on here has one and knows otherwise. Whatever the original finish was, I'll have to do some work on the cabinet as well.
I believe Defiant was a Co-Op brand?
Pretty good condition
The rather fetching red lead is the mains lead
with a blue earth lead connected direct to chassis....
The 'dropper' is contained in the plastic box on the right
What a curious thing to do.... Or rather, curious things! Is the rear panel missing?- I suppose that might have something to do with the thought process involved if so, though not really excusing it. Perhaps they reasoned that if they had a PME supply and no ELCBs that it would work. Unfortunately, when this sort of thing comes to the attention of fire investigators/coroners, it tars anyone involved with "old radios" with a bad image- and legislation gets ever more restrictive and forbidding for everyone, not just the bodgers and/or irresponsible.
Yep, Defiant was a name chosen with feeling by the Co-op.
Still, it looks as though it should come up well, and there's a certain plain and unpretentious appeal to it- that control layout is reminiscent of pre-war sets. The big wood box should help for a mellow sound. And big bottles and their sockets are easier to work with!
Definitely a name chosen with feeling, some information at this web address.
https://hardylane.blogspot.co.uk/2013/03/defiant-radio-story-1.html
The circuit is in Radio Museum if you need it, it’s not in the library here.
Frank
No the back isn't missing, I just removed it. I managed to find the circuit...neither Trader or RTV seem to show it but ERT did. I've investigated further, took the chassis out and it seems that he's fitted five 100 ohm resistors in series so 500 ohms altogether (the original was 490 ohms so that's OK) and taken a tap off the first one to feed the rectifier so basically he seemed to know what he was doing....but I still don't know why he's earthed the chassis. Anyway I'll lift the earth and maybe try some power via a variac just to see what the state of play is. One thing....he hasn't changed any of the waxy caps or the one across the mains so some replacements will be fitted before any mains is applied.
I'll probably use a capacitor dropper with a separate 50 ohm to feed the rectifier anode (original was 55 ohms).
Should be interesting once it's working...it has a reaction control! The 'volume control' alters the cathode bias of the RF amp. V2 acts as a leaky grid detector. Valve line-up for those interested V1/V2 VP133, V3 (output) PEN383, V4 (rec) U403
Still find it curious that hot-running resistors were apparently placed in a plastic box, though- with a rear panel present, it would have made more sense to screw those alclads to a piece of (say) metal angle inside that ample case.
Well whatever his reason, I'll probably use a cap dropper. Works out to 3uF so I'll need to find a motor-run cap of around that value.
Defiant M770 service data now resides in the data library
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I'll post some more pictures of the under chassis later but a general check shows that all the original waxies and electrolytics are still present. The smoothing cap appears to be OK (at least the ESR meter says it is) but the 8uF decoupler (C7) and the 30uF across the output valve cathode bias (C12) are both O/C.
I have found a couple of 1.5uF 450V AC motor run caps of the flat plastic type that could be connected in parallel and used as the dropper but not sure about the mounting method yet unless I can hide them neatly under the chassis somewhere. I may obtain a 3uF motor run type that will bolt to the chassis.
That's about it for now. Update later.
Here's another Defiant radio set. This is the 1939 model MSH950. It's a bit bashed up but is restorable.
Till Eulenspiegel.
I was hoping to post some pictures but my camera is playing up....I think the USB socket is b*****d!
Anyway I spent an hour or so this evening removing the bodged dropper and then wiring in a couple of 1.5uF 450V AC caps (in parallel). I was pondering the best way to mount the caps and as they are the block type approx 1.5" x 1" by 1/2" I found a small tagboard, soldered them to that and fitted it under the chassis with a 6BA bolt. Not entirely happy with the result and will give it further thought. I may use a 3uF motor-run cap mounted above chassis. However it proves it will work as I now have the valve heaters working and both pilot lamps. I've deliberately not connected HT at present. Two sections of the original dropper were also used to feed the anode of the rectifier. I can't do that with the capacitor dropper so will fit a separate 100 ohm wirewound resistor from the mains feed to the anode of the rec. There are a number of capacitors that need replacing before full HT is applied...hopefully that will be later this week.
So far it all looks rather promising.
Unless I'm wrong, I'm thinking a capacitor dropper can pass on a rather nasty surge/spike from the mains due to switch arcing etc so some series resistance is still good to have. Perhaps an NTC thermistor?
To understand the black art of electronics is to understand witchcraft. Andrew.
Here are the photos I wanted to download last night.
underside at present seems all original
The 'dropper' made up by previous owner now removed
The dropper caps fitted at present...2 x 1.5uF 450V AC. I'll probably change this later
The cap dropper works....scale lamps and valve heaters lighting
Unless I'm wrong, I'm thinking a capacitor dropper can pass on a rather nasty surge/spike from the mains due to switch arcing etc so some series resistance is still good to have.
That's what Paul recommends here
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I've personally never found it necessary to fit a surge limiter...my Pilot Model 10 has been running for around 5 years with a cap dropper and no surge limiter with no problems. The Pye 11U has a cap dropper in place of the heater section of it's dropper, no surge limiter, no problems. I do agree that a discharge resistor is a good idea which I will fit at some stage.
When Paul first published the cap dropper calculations, I think there was a discussion about the surge limiter and it was decided that it probably wasn't necessary but does no harm to fit one anyway.
I've noticed that with a cap dropper fitted, there is no initial 'surge' on the valve heaters....with the Pye 11U, a couple of valves used to glow quite brightly for a few seconds at switch-on and then settle down to normal....it had always done that. Now with the cap dropper, that doesn't happen and all the valves come up nice and gently. With the Pilot radio, the scale lamps used to be quite bright for a few seconds at switch-on until the valve heaters started to warm up then they would die down to normal. Again with the cap dropper, there is no bright surge and the pilot lamps stay constant.
I think it's up to the individual as to whether a surge limiter is fitted or not....some circuits may require it.
However it is good practice to fit a discharge resistor.
A capacitor dropper sounds like a nice way of preserving dial lamps in those series circuits where both valve heaters and dial lamps are involved, as the chain resistance is low to start with, there is high phase shift and little real power in the CR circuit and thus less stress on the resistive parts. As the bulbs/heaters warm up, the phase shift decreases and power absorbed bumps up. Now that there are lots of mains-rated, uF-range film capacitors available in a good permutation of values quite cheaply, it seems difficult to find an excuse not to use them!
A few years ago I fitted a capacitor and used it as a dropper for the HT in a Bush DAC90. The original resistor dropper had burnt out and the heat had damaged the back cover. The capacitor was around £2.50 from Maplin. I had done the calculation for the value but switched on and run it up via a variac at first. The HT was within 5v of my calculated value. The set also runs very cool now.
as the chain resistance is low to start with, there is high phase shift and little real power in the CR circuit and thus less stress on the resistive parts. As the bulbs/heaters warm up, the phase shift decreases and power absorbed bumps up.
So it effectively compensates for the differing resistance during warm-up of the valves, thus limiting excess current through the heaters ?
To understand the black art of electronics is to understand witchcraft. Andrew.
Does a HT capacitor dropper reduce the power consumed by the radio?
More information about the how Defiant radio was started.
https://www.vintage-radio.info/download.php?id=363
Frank
Does a HT capacitor dropper reduce the power consumed by the radio?
Given that a reactive element has been introduced into the equation by fitting the cap, it will involve AC power theory that's beyond my knowledge (this is why we talk of VA rather than W in AC power, because the power factor may not be unity)
