Waking up an Ultra Twin

[Colourstar]

I'm hoping to revive this little set, an example of which was the first old radio I ever owned, having been bought for 50p in a local junk shop by my mum sometime around 1980!

I note on this one that the original rectifier has been replaced with a later device and a clearly unsuitable mustard cap which led a short and unhappy life as a mains filter. Presumably the set continued to function happily enough after this went pop.

I notice there is no series resistor with the replacement rectifier. I'd have assumed that one would be needed to tailor the voltage to that required for the set. I'm wondering whether to leave this arrangement as is, or whether it would be better to fit something like a 1N4007 and resistor.

Sorry about the poor pic!

Steve

[crackle]

Hi Steve
The cathode of the rectifier should go to section C of R24, I think.
But how much of section A of R24 is damaged, the one at the top?

Which model is your Ultra Twin.

Mike

[Colourstar]

Hi Mike

My set is the R786 Coronation Twin.

According to the Trader Sheet 1100, the broken resistor is R20 (1690 ohms, filament ballast). As it has a single value I assumed that there was no missing section and that just the top had been knocked off.

I suspect it may have been a section from the someone's spares drawer, bodged in as a replacement as I'm sure the original one would look like it's neighbour R21 on the right of the photo attached.

Update: Having just had a good look at it, it was indeed part of a multiple value dropper resistor and is a section marked 1480 ohms. I just tested it and it reads a smidge under 1.5k, so I think my theory above would probably be right.

Steve

[paulsherwin]

A resistor is good practice as it mimics the characteristics of the original rectifier. Without it there may be a very sharp inrush current as the electrolytics charge up, and the on load HT voltage may be a bit high. It's not essential though, and some people seem to oppose a resistor on principle, particularly in the US.

[David G4EBT]

Main problem with the Coronation Twin/Ultra Twin (It was only the 'Coronation Twin' in the Coronation year, which - like the Olympic Games - very soon became yesterday's news after the event, and useless as a 'marketing cachet'), is that invariably the mains lead is missing. It had a uniquely designed plug on the 'business end' of it which - when plugged in - switched the set from battery operation to mains. The original flex stored neatly in the back of the set, with a 5Amp round 3-pin plug on the other end. Trouble was that when 13A plugs came along, the flex would not longer stow in the back of the set, so over times, they part company with the set.

If originality isn't a key issue, the set can be modified by by-passing the switching arrangements, and I guess that some restorers will wish to make a repro battery on which to run the set. I dare say that most sets, like mine, once restored end up on shelves on display. I'm a collector by default - when I restore sets I become attached the then and don't and to part with them, and that includes my Coronation Special'.

The plug design is so small that it would be next to impossible to make one. (Don't know if anyone's tried and succeeded).

Good luck with the set.

[Colourstar]

Thanks David.

I recall that the one I was originally given as a lad all those years ago came back from the local tv repair shop sporting a white plastic mains lead exiting through a rubber grommet, the back case having had a hole drilled. There are certainly more elegant solutions around though!

Steve