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Old 18th Jul 2010, 11:45 pm   #1
Kat Manton
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Default 1968 Electronica S632A3 'Carmen 3'

Good evensocks, ladles and jellyspoons,

Sometimes I fix things. Sometimes they're radios, fairly old and even have valves inside!

'Electronica' what?
When this Electronica Carmen 3, otherwise known as the S632A3, turned up I couldn't resist. I have a soft-spot for Eastern Bloc sets like this; they're often an exercise in building a functioning radio set using as few parts as possible. From a technical point of view, that can be just as interesting as the all-singing, all-dancing sets with everything. They're simple, they're fun, they're unusual; dozens of them can be accumulated for less than the cost of a round Ekco...

Manufactured by Electronica of Romania, this particular Carmen 3 was made in 1968 according to a date-stamp on the cabinet. It's a fairly basic LW/MW/SW superhet using ECH81, EBF89, ECL82 with a silicon diode half-wave rectifier and the added bonus of an EM80 tuning indicator.

Going by the dial markings, it's an 'export' set. There's one station on the dial which is either a place I've never heard of or maybe that's the little-known BBC Ligth Programme..?

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Electronica of Romania produced quite a few different sets, more can be seen here. I might keep my eyes peeled for another one.
Inspection
Off with the back and what have we here? Two studs sticking out of the baffle-board roughly the same distance apart as a typical small transformer and several loose wires, some twisted together. Apart from that it looked complete and unmolested.

Underneath, there's a hardboard inspection cover. Removing that revealed nothing untoward, the only sign of previous attention was a replacement output stage coupling capacitor ('That Capacitor') attached to the leads of the original.

With the chassis removed I could trace those wires. Out of curiosity I'd noted which ones were twisted together before untwisting them. One side of the speaker was connected to HT and the other to the ECL82 pentode section anode. A slight impedance mismatch; an ECL82 isn't going to drive a 4 ohm speaker directly!
Testing, Testing
Selection between 220V and 240V is achieved by fitting the mains fuse in one of two positions; it was in the 220V position so I moved it, as one does...

Now, a few more checks. Remove ECL82, apply mains (via isolating transformer); remaining heaters all light up, EM80 'magic eye' lights up and there's no DC on the pentode section control grid. So there's some life in there and the replacement 'That Capacitor' is healthy.

HT measures somewhere over 200V but... those heaters look a bit bright. Ah. LT measures somewhere over 7V. Disconnect mains. Scratch head, put fuse back in 220V position, apply mains. LT now 6.4V. Ah.

How are the reservoir/smoothing caps? No sign of the dual-section can getting warm, though it'd only been powered up briefly. Quick and dirty test; stick meter on HT and switch off. Nice slow discharge rate, taking minutes to drop. Seems safe to leave it powered for longer.

Now, the output transformer. Not got any (must do something about that.) Let's try a 100V line transformer. Dig one out and lash it into circuit with clip leads and reinsert the... oh, hang on... the ECL82 is cracked!

Interesting; cracked and showing other signs of distress. Vacuum intact, though. Wonder if this and the missing transformer are the aftermath of a 'That Capacitor' failure?

Rummage through box of random valves. Lo and behold, an ECL82, 'Zaerix' branded, 'Made in G.D.R.' That'll do nicely, don't want to put a capitalist Western ECL82 in there, it might not like it.

Insert valve, apply mains, wait and... loud shrieking noise. Ouch. Power off, reverse chassis and neg. feedback connections on transformer secondary, apply mains.

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This is going to be easy...
Excellent! Slight mains hum and a crackling noise. Well, it's a start; though MW ought to be a bit less silent. LW is the same. Try SW and... faint traces of life. Attach clip lead to aerial socket. Stations! It's alive!

Check obvious things first. The coils on the wave-change switch assembly look like a good place to start. Is that a couple of wires touching? Could be... Gently prod with cocktail stick and try again. Now we have LW and SW. Let's have a closer look. Aha! A break in a very fine wire leading from a coil on the MW switch. Solder that back together and that's all three bands working.

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This is going to be easy. But, what's this? Surely the 'magic eye' shouldn't respond to modulation? OK, I give in, let's find a schematic. No Carmen 3 schematic. Carmen 1 looks close. 0.1µF AGC capacitor looks like a suspect. Except it's 0.01µF in this set though everything else is identical. Disconnect one end, tack a new 0.01µF in place, hang 'scope on AGC line. AGC line bouncing around. Remove 0.01µF, tack 0.1µF in instead. AGC line now behaving.

Given the mains transformer was wired up wrong, shall we assume other mistakes may exist? Like 0.01µf where there should be 0.1µF. Remove original 0.01µF and neatly fit nice NOS Russian 0.1µF. So, that's that. All it needs now is an output transformer...
Just fit this and we're done
Fast-forward a few weeks and a small but heavy box arrives. You know how it is; look for one transformer, find someone selling a job-lot of thirty-odd unidentified HT/LT and output transformers...

Divide pile into 'HT/LT' and 'output'. Lash up quick test circuit with two bench DMMs and a generator, calculator to hand. One of them ought to be close.

Voltage measurements and the ECL82 data sheet tell me that, in this set, the ECL82 wants to 'see' 5.6k and the speaker is 4 ohm. That's an impedance ratio of 1400:1. Hook up transformer, calculate voltage ratio and square it, rinse, repeat. Three transformers later and bingo! We have a winner and it's not just close, it's bang on 1400:1. Nearly done!

Transformer lug spacing even fits the studs. Find some nuts and washers, bolt it in, solder the wires on, apply mains, wait and... loud shrieking noise. Ouch. Power off, reverse chassis and neg. feedback connections on transformer secondary, apply mains.

Relax with smug feeling of satisfaction and nice cup of tea and tune around a bit. Working nicely but... odd... the tone changes as the volume is increased. Never mind, maybe they all did that.

Tune into quiet station on SW and turn volume up. Loud shrieking noise. Erm... we've done that, haven't we? Back off volume. Shrieking stops. OK, it's not the feedback connections then?
Amplifiers oscillate, oscillators don't
That's one of the things one learns somewhere around the same time as Ohm's Law and "high voltages bite".

Check everything. Transformer definitely wired correctly. Short top of volume control to chassis with clip-lead. Determine oscillation starts after around five minutes with the volume control around three-quarters (or half-way in terms of resistance.) So it's oscillating when the triode control grid 'sees' the highest impedance but not straight away. It needs to get warm first. Interesting.

All components testable in-circuit seem to measure correctly. Voltages around ECL82 look good too. Layout leaves much to be desired, rearrange it a bit to position control-grid bits away from anode circuit bits. It oscillates. Tack 68µF across HT smoothing cap. Tack 0.1µF across HT smoothing cap. Try another valve. Screen triode control-grid connection. Re-route HT wiring so everything connects directly to HT cap rather than 'daisy-chained' around the chassis. Nothing stops it oscillating!

Give up. Put it back together; sort it out another day.

Half an hour later, take it apart. I shall not let this thing defeat me, it's a simple little ECL82 amplifier! Check more components by substitution. It still oscillates. Consider contacting Rev. Lionel Fanthorpe. Does he exorcise radios?

Let's think. Everything else has been eliminated apart from the valve-holder? Could it be that? Desolder everything (again.) Drill out rivets, bolt in nice NOS Russian ceramic valve holder, solder everything back on.

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Connect output transformer and speaker (still in cabinet) with clip leads. Apply mains, wait and... loud shrieking noise. Ouch. Power off, reverse chassis and neg. feedback connections on transformer secondary, apply mains.

Tone no longer changes as volume is increased. Promising. Let it warm up a bit more. Still behaving. Leave it on for half an hour and it carries on not being an oscillator. So it was the valve holder!
And Finally...
The observant (or those who haven't fallen asleep yet) will note I didn't mention re-arranging the mains tappings. I left that until the thing was working.

Best check everything, though. It's a 'live chassis' set with an auto-transformer. According to the schematic for the Carmen 1, one end connects to chassis, the other end to 240V; with 6.3V, the HT tapping and the 220V tapping in between.

As well as the 240V and 220V tappings being swapped, the chassis and LT connections were swapped. Putting that right dropped the LT to 6.3V (from 6.4V) and reduced the HT to something like the voltages on the Carmen 1 diagram with the fuse in the 240V position.

I should really replace the cap across the mains and the aerial isolation caps with X and Y ones respectively along with fitting a colour-coded mains lead. It's caused enough hassle, so that can wait. The original mains lead is in good order and I've fitted the plug so neutral connects to chassis.

It works, it wasn't as easy as I thought it'd be and that warrants a 'Success Story' even though it's technically not quite finished...

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Now I think I'll have a little lie down then tackle something easy. Like a huge Grundig made of speakers, knobs and buttons...

Cheers, Kat
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Old 19th Jul 2010, 6:06 am   #2
mark pirate
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Default Re: 1968 Electronica S632A3 'Carmen 3'

Well done Kat, i admire your patience, i usualy put sets like this aside for later when i cant seem to find the fault!
Quote:
I have a soft-spot for Eastern Bloc sets like this
I have a polish Diora calypso wanting a new home

Mark
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Old 19th Jul 2010, 7:41 am   #3
JoshWard
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Default Re: 1968 Electronica S632A3 'Carmen 3'

I likes that I do

Well done

Josh.
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Old 19th Jul 2010, 8:39 am   #4
Steve_P
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Default Re: 1968 Electronica S632A3 'Carmen 3'

I've got one of these in the Kitchen here, tuned to Radio 4 and given time off during the cricket. Very nice sets they are too. Didn't give me as much trouble as yours though.

Handy site that. Thanks. These sets came into the UK via a catalogue by the way.

Another tip - Electronica Carmen 4 : Very sensitive set, but difficult to get apart. Speaker is always open circuit. Got one in the shed.

Cheers,

Steve P.
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Old 26th Jul 2010, 6:58 pm   #5
Kat Manton
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Default Re: 1968 Electronica S632A3 'Carmen 3'

Hi,

Quote:
Originally Posted by Kat Manton View Post
Everything else has been eliminated apart from the valve-holder? Could it be that? Desolder everything (again.) Drill out rivets, bolt in nice NOS Russian ceramic valve holder, solder everything back on.
This got me thinking; is there a bit of test gear which would've identified the valve-holder as the cause of the fault? It would've been rather annoying if I'd found it wasn't, having drilled out rivets and bolted a non-original one in.

That's led to a another acquisition and this post. I've still got the original valve-holder. I wonder if the Megger BM8/2 will measure some leakage between pins, and which pins? I'm still somewhat curious as to how much leakage was present and where.

Close inspection under a bright light reveals some tiny cracks in the insulation around some of the pins. My suspicion is a leaky coupling capacitor overheated the ECL82, damaging the valve-holder and the valve itself and may have fried the original (missing) output transformer.

Maybe I'm strange, but I like to see if I can deduce what happened to leave a set in the state in which I found it. Perhaps I've watched 'CSI' too often...

Cheers, Kat
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