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Old 31st Jan 2006, 12:44 am   #1
Cerberos
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Default "It's been got at!"

Note from Moderators; this is the remnants of a thread which has been saved to illustrate, out of general interest, a radio which has been 'Got at' and the problems faced in trying to repair/restore such a radio.

Hi,

I've recently acquired an Ultra Coronation Twin as a non worker.

A quick look a the pictures shows why it is not working. The 2 dropper resistors r21 and r22 are totally knackered (trader service sheet reference numbers).

However the trader sheet shows r20 as a fixed value resistor. This set has a 1k pot and a fixed resistor in series.

Non of the valves are the mazda type shown on the trader sheet. They are a mixture of Brimar and mullard. I am afraid I don't know where to look to see if they are direct replacements. The 1k pot looks as if it there to set the filament current. What should that be assuming the original valve line up?

Looking at the set the resistor arrangement looks to be original with this set. Has any one else seen one like this?

The trader sheet shows R20 as filament ballast resistor at 1690 ohms, presumably the reason for the pot. 1690 does not seem to be a standard value,therfore it should be possible to set filament current with the pot.

R22 is tapped at 195, 410 and 370, how would I replace this?

R21 is 1450 ohms again I would like advise on replacement values.

It would seem I need something close to 195 ohms as a a smoothing resistor and a place to connect C24, a 50 micro farad ht smoothing C and something close to 2425 ohms ( the sum of r21 and r22 ) as a dropper resistor to generate the HT.

As a relative beginner this is all new to me, so any advice would be appreciated and if any one has seen this arrangement before again I would be gratfeul for your input.
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Last edited by Darren-UK; 23rd Jul 2007 at 10:02 pm. Reason: Ebay reference removed and forenote added.
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Old 31st Jan 2006, 1:18 am   #2
paulsherwin
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Default Re: Ultra Coronation Twin - What on earth is this?

I think you should be very careful with this set. It has fairly obviously been Got At, possibly a very long time ago.

The wirewound pot doesn't look original from your photo, and the DL96 in the photo is wrong - it has the wrong heater current. Possibly somebody has replaced all the valves with Dx96 types and modified the dropper arrangements accordingly, but you will need to check everything out. There is no problem replacing Mazda types with equivalents but the Dx96 series *aren't* equivalents

The resistor values aren't as critical as you seem to think. Generally, going to the next larger preferred value will be OK. Check the circuit carefully to work out the power handling required by the different resistors. You can combine resistors in series or parallel to get the values you need if necessary.

For info on valve equivalents, the first thing you should do is download the free TDSL-PE database from the Duncans Amps website: http://www.duncanamps.com/tdslpe/index.html. I use it all the time. It's not so good with Mazda types though. The correct Mullard valves for your set are DK92, DF91, DAF91, DL94.

Good luck with this - it looks like quite a job for a beginner.

Best regards, Paul

Last edited by paulsherwin; 31st Jan 2006 at 1:42 am.
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Old 31st Jan 2006, 1:38 am   #3
Cerberos
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Default Re: Ultra Coronation Twin - What on earth is this?

Looking at the valves;

In place of the mazda IC2 is a mullard DK 96

In place of the Mazda IF3 is a brimar BVA, D7, DF 96

In place of the Mazda IFD9 is something with the markings completely worn away

In place of the Mazda IP11 is a mullard DL 96.

I assume all these valves ending in 96 mean thay are part of the same series? If that is so what should the heater current be?

Leaving the set in battery mode, I can measure about 1.3k between the LT battery terminals when the set is switched on. So that would seem to suggest the valve filaments are intact.

Inspecting the rest of the set, there are a couple of cracked hunts capacitors, some very badly tarnished switches, a dodgy looking wax capacitor across the mains ans looking at where the main dropper resistor has failed I would think the HT smoothing cap is leaky.

So I'm not frightened off yet, I hope with some advice and guidance I can get this going again, what do you think?
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Old 31st Jan 2006, 1:48 am   #4
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Default Re: Ultra Coronation Twin - What on earth is this?

It's certainly restorable. Your set has been fitted with the Dx96 series of valves which take half as much heater current as the originals (25mA in a series string, compared with 50mA.) This would have made battery operation more economical, but the filamant ballast value will need to be changed to take account of this. This is probably why the wirewound pot was fitted.

The mains wax cap you mention should be replaced with a modern X2 rated component. The Hunts mouldseals should be replaced with modern film capacitors.

The HT smoothing caps may actually be OK, you never know. It may be best to change them anyway though, since you'll be doing a lot of recapping anyway.

Good luck, Paul
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Old 31st Jan 2006, 11:48 am   #5
Cerberos
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Default Re: Ultra Coronation Twin - What on earth is this?

A quick look at the circuit and where the dropper resistor has failed. (see attached image)

The dropper has failed at the bottom end where C24 is attached. This is why I think the HT smoothing C has gone.

But the first thing I need to do is work out some resistor values and get them ordered.
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Old 31st Jan 2006, 12:22 pm   #6
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Default Re: Ultra Coronation Twin - What on earth is this?

Quote:
Originally Posted by Cerberos
The dropper has failed at the bottom end where C24 is attached. This is why I think the HT smoothing C has gone.
You are very possibly right. The metal rectifier may also be in a poor state, but this is easy to replace with a diode and some extra resistance.

Have a look at eBay. You often find assortments of power resistors there at very good prices, and you can always combine several resistors to get the correct value. It's worth stocking up on power resistors in the 47R-1K range if you're planning to work on old radios in the future.

If you post your requirements in the 'Parts Wanted' section, a forum member with a good resistor stock may be able to supply you for a couple of quid. Most of us will have suitable diodes for the rectifier too.

Best regards, Paul
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