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Old 17th Mar 2021, 10:03 pm   #1
vintage_8bit
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Default Eddystone S640. Mains primary open circuit.

Good evening.

Just starting work on this receiver. I have noticed the 200 to 240 tap on the mains primary is open circuit. A previous owner has just soldered a dropper resistor across the winding. I have removed this mod.

Looking at the output transformer I'm guessing the metal surround is pitch filled? If not I will drill out the top rivets and see if I can do anything with the open circuit. If not, its a rewind, replacement or perhaps use the 110V tap and a 240 to 110 step down transformer.

So does anyone know how the transformer is built up please?.



With just the rectifier and output valve fitted I slowly powered using the variac to 200V, with the 200V tap selected. The H.T. measured 250V and I had 39mA through the output valve and no hum, so a good result here.

I will now be looking at all the electrolytics and paper caps. I see the audio coupling cap to the output valve is mica. Colin
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Last edited by vintage_8bit; 17th Mar 2021 at 10:12 pm. Reason: Post in wrong area?
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Old 18th Mar 2021, 11:12 am   #2
turretslug
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Default Re: Eddystone S640. Mains primary open circuit.

Another way out- though still a bit of a kludge- would be using a transformer with 40V or 20-0-20V or 2x 20V secondary as a bucking device to produce a 200V supply from incoming 240V mains. I don't think that this set would consume more than 80W, possibly even 60W, so a 20VA bucking transformer should suffice.

A second-hand transformer will likely be the cheap'n'easy part, I find the safe and sturdy enclosure for things is often the time'n'money bit!
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Old 18th Mar 2021, 12:05 pm   #3
vintage_8bit
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Default Re: Eddystone S640. Mains primary open circuit.

Good idea Turretslug, I know I have a 110/115 transformer but I think I have a 20V-20V somwhere as well and that's smaller. With just the output and rectifier fitted it used around 40W. Colin.
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Old 18th Mar 2021, 12:06 pm   #4
turretslug
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Default Re: Eddystone S640. Mains primary open circuit.

Oh yes- the mica grid coupling cap to the output valve grid. Is it a thick square (almost cube!) dark brown 10nF component with classic strangely thick lead-outs? Both the 670A and 750 here featured one of these in this part of the circuit (in the 670A, retained in place with a natty little plated brass clamp- very Eddystone). I glared at it initially, sure that a 10nF Bakelite-case capacitor would be one of the notorious "Micamold" paper types. However on removal, they felt strangely heavy (mica/silver sandwich?) and proved impeccable on a 500V tester. I changed them anyway, on the grounds that they were nearly 70 years old and a 10nF 630V polycarbonate cap was much cheaper than a UL41 or a N78.

Eddystone were a funny outfit, using some rather rubbish passive components, interspersed with a few very good ones- I suspect that the latter may have come from surplus sources, the micas in question may even have been transmitting or some other arduous spec. types.
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Old 18th Mar 2021, 12:32 pm   #5
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Default Re: Eddystone S640. Mains primary open circuit.

Quote. "Is it a thick square (almost cube!) dark brown 10nF component with classic strangely thick lead-outs?"

I would say yes. The audio components sit on a sub chassis with very limited access due to short wires. I may remove this sub chassis when I have gone through the rest of the set and replace most of the components in there so I don't have to remove it again. This was suggested some where else on this forum I think. Colin.
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Old 19th Mar 2021, 8:25 am   #6
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Default Re: Eddystone S640. Mains primary open circuit.

I tried the bucking transformer and it worked really well hardly any voltage drop when loaded. I ended up with around 215 volts, it was a 0-17V 0-17V secondary.

In the end I found an autotransformer with a 200 volt tap so will use that while I go through the restoration. Noting of course no mains isolation is given by either method. Meanwhile I will keep a lookout for an origional replacement or a transformer that will fit in the shield of the origional. Colin.
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Old 13th May 2021, 10:04 am   #7
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Default Re: Eddystone S640. Mains primary open circuit.

To finish off the thread.
I changed all the usual paper caps. The main two smothing electrolytics were re-stufed and the rest replaced. Quite a few resistors had to be changed. 3 of the valves were changed as the metallic coating had come off. The only problems other than the mains transformer, were intemitant toggle switches and a broken slug. To aid allignment, I made up a temporary plate to replace the origional R.F. shield. This plate had 9 extra holes for the coil adjustments. The origional plate only had holes for the trimmers which seemed strange. The set is now working well. Thanks to the Eddystone user group, who helped with some of my questions. Colin
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Old 13th May 2021, 4:53 pm   #8
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Default Re: Eddystone S640. Mains primary open circuit.

The bucking-transformer is indeed a cunning way to deal with this - though I'd be fitting the radio with a 'non-standard' mains plug so it doesn't inadvertently get plugged-into a normal supply.

[I've done this with an Eddystone 840A which had an open-circuit in the bit of the dropper-resistor needed to run off 200/240V: it now works with an autotransformer, has a US-style 2-prong plug to reduce the risk of inadvertent misconnection, and an admonition on the rear of the case saying "FOR USE ON 110-120V AC/DC SUPPLIES ONLY!"]
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Old 13th May 2021, 7:59 pm   #9
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Default Re: Eddystone S640. Mains primary open circuit.

I am using a non-standard mains plug and socket as well, otherwise I would probably forget.

I am pleased with its performance. The BFO works well, as does the noise limiter and crystal filter.

Colin.
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