Ultra coronation twin R786

[clockman]

I have just aquired this radio not working have tested it for voltage readings ok but wonder how i can test the valves ?

[paulsherwin]

Bad valves are very rarely responsible for radio faults.

[PJL]

Battery valves...and a basic AC/DC mains supply. Do you have the mains lead that is usually missing from these? Are you testing on mains?

[clockman]

i have tested it with the mains lead and a battery pack I made up. I have replaced some of the old caps and am going to replace the metal rectifier with new parts that i have ordered, the condition is very clean for its age.

Just to mention i have been in my loft and retrieved my faithful AVO 8 which is still working perfectly. This from my early middle age as a radio and TV engineer, a very happy time and very profitable, but long since a memory.

I purchased this radio on a whim to remind me of my first 5 years learning my trade at the ULTRA factory in Acton W3 which has stood me well all my working life.

[PJL]

Check the continuity of the valve filaments. With basic mains supplies like this, If one filament goes it is common for the rest to fail. I would stick with testing on batteries.

You can sort out the mains supply once it is working. The original metal rectifier probably had quite a high forward resistance so a series resistor is required if you replace it with a silicon rectifier.

[clockman]

I have tested all the valve filaments and all are OK. Filament heater chain is the correct resistance so still stumped?

[Cobaltblue]

The life span of these battery valves is lower in my experience than typical mains valves.
When used on a mains supply years on end they do tend to loose emission.

It seems for whatever reason the Frequency changer is often the first to suffer.

Are you getting anything out of the speaker at all?

Hum, clicks and pops?

When used on batteries the filament supply is 7.5V not the usual 1.5V

Have you tried picking up the local oscillator on another radio?

Also this set contains some very high value resistors which by now have headed off toward infinity.

The last Coronation Twin I repaired I had to replace pretty much everything that was over 470K.

Cheers

Mike T

[clockman]

Thanks for the advice so it looks like it will be a lovely ornament in my front room but i am happy with that

[clockman]

I have now have tested the valves in another set and are all OK so I will now start on small value capacitors to be purchased from eBay?

[Ed_Dinning]

Hi, in sets like these the small value caps are seldom faulty and their wholesale replacement is very likely to affect the alignment and render the set deaf.
You would be far better checking the resistors, some of which will have gone high and replace as required.
You also need to use your voltmeter and compare readings with schematic values

Ed

[David G4EBT]

As Ed suggests, a good place to start would be checking the resistors and comparing them with the spec.

Some are +/- 20%, some +/- 10%, three ae +/- 5% and two are +/- 2%. If any are well outside of those tolerances, consider replacing them.

As to capacitors, there are several which may degrade the performance of the set but ought not to stop it working.

It's better to change those one at a time later when you've got a working set. However, if you're powering the set from the mains as you seem to be doing, there's one capacitor which you must remove right away. Namely the mains RF bypass capacitor which is directly across the mains input. The set will work happily without it, but if replaced, must be with an X Class 0.1uF capacitor designed for that purpose, which are widely available. If you leave the old one in place, it risks going off with a bang. On the Trader sheet it's referred to as C25, and on the maker's sheet as C23. If you change any low value capacitors unless you are sure they are faulty, you will upset the alignment f the tuned circuits which will need to be realigned with a signal generator.

In your first post you say you've checked the voltages and they're all OK, but you don't say which voltages you've checked.

The Trader Sheet (1100) states the voltages of the anodes and screens of the four valves. If the anode voltages are low when the set is powered from the mains, it's possible that the metal rectifier is failing. If you have no HT on the anodes of the valves, check the continuity of R21 & R22 (HT adjustment/smoothing). If either are open circuit, there will be no HT voltage when powered on mains. You need to make sure that the voltage input is set correctly for your mains voltage. If R21/21 are sound, and set to the correct mains input, yet the voltage on the anodes is lower than the spec, check the DC output voltage from the rectifier, which I'd guess should be about 220V or so, depending on your mains voltage. If it's appreciably lower, a silicon rectifier can be fitted with the old one bypassed and left in place for originality of appearance.

I don't know if you've obtained the datasheets. There are two - the Ultra one and Trader Sheet 1100, both of which can be purchased for download at the link at the top RH of the page. (It's useful to have both because each has information which is lacking in the other). If in a later post you mention any component numbers, you need to say which sheet you're quoting from because rather unhelpfully, Trader Sheet component numbering differs from Maker's data. For example, the 1 Meg volume control on the makers datasheet is 'R8' but on the Trader Sheet is R11.

The datasheets also give the resistance values of the aerial coils, the oscillator coils, the IFTs, the output transformer and the speaker speech coil. Those could be quickly checked to see if any are open circuit.

For fault tracing, it's useful to use a signal tracer - just a small amplifier, and a signal injector.
(Lots of forum threads on that topic - search 'Velleman K7000').

In a seemingly dead set, the fault may be in the audio stage - an open circuit output transformer for example. If you inject a signal into the slider of the volume control and hear nothing from the speaker that would confirm that the audio stage is faulty. For a quick test, you can simply scratch the slider of the VC with a screwdriver which should cause an audible crackle in the speaker if the audio stage is working. If you have a signal tracer - just a small amplifier, and connect that to the volume control slider if the RF/IF stages are working, you should hear station when you tune across the dial.

The Coronation Twin is an iconic set as it was only produced for the 1953 Coronation year, and as with events such as the Olympic Games or the Millennium, once they're passed, that's it - yesterdays news. No-one is going to want to by anything with 'Coronation' in the title after 1953. Hence, the set commemorates a significant historic event and is quite uncommon. It's neatly designed and originally the mains flex - with its unique plug which when inserted switched the set from battery to mains - had a small 3-pin 5-Amp mains plug at the other end. The flex with the mains plug was stored neatly in the rear compartment of the radio when used on batteries.

When flat pin 13A mains plugs came along and were fitted in place of the smaller 5-Amp one, the lead could no longer be stored in the rear compartment. Thus many have been lost (including mine!). The only option then is to either modify the set to work off the mains, or to make a replica battery. So you set complete with the mains lead was quite a lucky find.

I hope these rather wordy notes might help you to get your radio to work rather than to be as you put it 'a lovely ornament'.

Every success with it.

[nigelr2000]

Is that the one I sold on eBay a couple of weeks ago? if so I hope you replaced those crumbling rubber cables that were on it. I would also check continuity on the output transformer as I have had several small portables, normally the 1.5 and 90 volt ones rather than the 7.5 valves, with an O/C output transformer primary.
Hope you get it going as it was just taking up space on my round to it pile.

[clockman]

No nigeir 2000 it was not your one it cost me £30 and had no mods done. To David G4EBT thanks for your advise i have all the data sheets but no signal tracer just my AVO 8 all my test equipment went to the dump in 1974 so will follow all you advise but time is not on my side ?

[David G4EBT]

Originally Posted by clockman

To David G4EBT thanks for your advise I have all the data sheets but no signal tracer just my AVO 8 all my test equipment went to the dump in 1974 so will follow all you advise but time is not on my side?

Your AVO 8 will be fine for all the resistance, continuity, voltage and current tests.

As I said earlier, while you said in your first post that 'the voltages readings were OK', you didn't mention what the voltage readings were, or whether they were at the anodes and screens of the valves. If you check the voltages as compared to the data and quote what your findings are, there are experienced forum members who will be happy to help and advise based on those readings.

Whilst a signal injector/tracer is a useful aid, you could at least test the audio stage by scratching the slider of the volume control with a screwdriver, which should produce a crackling scraping sound from the speaker if the audio stage is working.

It's your set to do with as you wish, unless you enjoy restoring vintage radios and don't intend to use it but simply put in back on the shelf, you may feel it's not worth the effort. The reality for most of us who enjoy restoring vintage radios is that once we've restored hem to full working order, we do precisely that - put them on display and dust them from time to time. I have more than forty radios on display that I've restored, but only three favourite ones are in use regularly - one in our lounge, one in our garage where I often work, and one in my garden workshop. Some simply like to collect radios that they like the look of or for nostalgic reasons - other such as myself, become 'collectors by default' - having restored a radio, not wanting to part with it.

Your Coronation Twin is not just an icon of the Coronation, but evocative of other significant socio-economic events that year. In February 1953 confectionery rationing ended, albeit the shops were quickly cleared of what few sweets they had and a Mars Bar (if one could be found), was cut into slices so you could enjoy a piece every day. In September 1953, sugar rationing ended, but it wasn't until July 1954 that meat and all other food rationing ended in Britain.

Your 'Coronation Twin' cost £13.10s.4d in 1953, plus Purchase Tax and Batteries.

That would equate to well over £500 today and two weeks wages for a working man back then. Given the cost of the set, the austere times and the fact that the set was only sold from April 1953 and sales probably quickly declined after the Coronation on 2nd June 1953, goes some way towards explaining why this set - if not rare - isn't commonplace. You did well to get it for £30.00 complete with mains lead.

Enjoy it, whatever you decide to do with it.

[clockman]

As i mentioned in my early post i only acquired the the radio to remind me of the first five years of my working life at ULTRA Electronics in Acton from 1953 so having it working is not that important to me. like you i have a hobby but its repairing and restoring old German vintage clocks and i have a large number all over the house that i am not willing to part with so that keeps me well busy in my retirement thanks for your help with my radio quest ?

[marconi_pete]

Just a thought but have you tested the output transformer for continuity? Had this on a r786 coronation twin set I repaired a while ago. Valve filaments were ok, voltages were in spec after a re-cap and changed the rectifier for a diode and series resistor. Turned the set on and got no life out of it whatsoever. Tacked in a known working output transformer from another set and the set burst into life. Worth checking if you've checked everything else..

Thanks,
Peter

[clockman]

Just a question if i replace MR1 with a 1N5401 Diode what is the value and wattage of the limiting resistor i need and is that enough parts ?

[Station X]

No one can give you an exact answer to that question. It's a matter of experimenting with various resistor values until the HT voltage measures correctly. I would suggest that 150R is a good starting point, but other forum members may have other ideas. To be on the safe side use a resistor rated at 5 Watts or more.

[clockman]

I have ordered a selection of 5W resistors from ebay thanks for advice

[G8UWM-MildMartin]

A 1N5401 is unsuitable bacause it is only rated for 100V reverse voltage.