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Old 5th Oct 2007, 7:26 pm   #1
Steve_P
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Default Unitra Figaro - I always wanted one, but...

I don’t know why a basic 3 valve radio from Poland should fetch the money I’ve seen them go for, both at Radiophile Auctions and on Ebay, but they do. So when one came up cheap, on Ebay, I yelled ‘Eureka’ and a few days later this small set arrived, not terribly well packed, in a massive box!

The seller said she hadn’t tried it, so things looked even better. Just like my pills don’t look as dreadful tasting as they are. So, I took it into the workshop, back off, quick check and plug in and see what happens.

Loud Mains Buzz and crackles from speaker matched by pops from inside the Mains Transformer. Not going to be easy this one, I thought. Switch off Quick. The Mains Transformer was faulty (and boiling hot, leaking wax onto the chassis), the diode in these is the flat metal type that I’ve had fun with in the past and the smoothing cap was doing nothing. I don’t know why a 3 valve radio should create so much fuss. I chose to fix/modify it because after 10 years looking for this model, I wasn’t going to let it go.

So, after all this time, I get one with, shall we say, problems. Ah well, c’est la vie. C’est B****y Awful sometimes isn’t it, oui?

Well, let’s have a look. The circuit is below, and it’s an Auto Transformer System. Mains goes into it and rather than Transformer Action, it is more like a choke system. Well this Auto Transformer was up the creek, so out it came, along with the diode and this gave me an area to work in. The new power supply had to provide rectified DC for the H.T. and a 6.3v AC supply for the valves. While I was at it, the mains cable went out too, so I could see which strand was Live when the thing was re-wired.

NOTE: This two wire cable that isn’t marked Blue + Brown is now illegal. That isn’t why I changed it, but the fact that the set had a 50/50 chance of having a live chassis was.

A root in the scrap box brought me a suitable transformer for the heaters. A diode was found to fit the job (Solid State - sorry!) and Suitable Caps were found for the reservoir and smoothing.

The new mains cable was fitted, the newly fitted transformer wired up, and with the valves out and just the bulb in place, via the variac, the set brought up. The lamp lit, so something was working. I then put the first two valves in - ECH81 and EBF89 - and they glowed. I put the ECL82 in and this glowed too.

So the L.T. for the Heaters was OK. Power off, valves out, back under the chassis. Put the two caps in place of the Smoothing Can. The can was left in place for appearance purposes. While I was under here, I also changed C29. Just to save the ECL82 from going into grid current.

Now for the H.T. side of things.

The new diode would pass a lot more voltage than the previous one, and there are two ways of going about finding what the value of the resistor should be. One method is to guess, and the other way involves pen and paper. Just for once, I chose the second way. So I did it like this….

The resistor had to carry all the H.T. to all the valves. So, get the data book and look them up.

ECL82 - Pentode - 35mA. Triode 3.5mA.
EBF89 - 3.3mA.
ECH81 - 14.5mA.

The total current was 58mA.

The ECL82 has an Anode Voltage of 200v. So now I needed to know what the voltage off the Diode I was putting in would put out - on load. Then it was a simple matter of finding out the voltage drop and then the value of the resistor needed. So, valves in, connect up diode and put meter in place with croc clips. Then it’s a matter of speed. At 70% of mains voltage, there were noises. So I gave the tuning control a tweak and got 5-Live. I then wound it up to 240v and quickly measured 400v or so on the diode. The ECL82 began to glow where it shouldn’t, so off quick.

The Resistor was to drop 200v. Therefore, by Ohms Law, it worked out at 3.5k. Power worked out at just over 11 Watts! A quick trip up to CPC brought me a 50 Watt 3.9k resistor. Bit more, but it’ll do I thought. Bolted to the chassis, wired in before the Diode and we tried the set again.

Success! The volts were rather low, but it worked fine. And that’s what counts. The speaker was ripped, so another speaker was found and fitted. On again, listened to somebody moaning on 5-Live, and let’s see how hot the resistor gets.

Well, it didn’t get very hot, and everything seemed stable. Now there came another job - putting a chassis with differences back in the set. This took about half an hour, and I ran the set again to make sure everything was OK. Fortunately it was, so the set was finally finished and although it was not entirely the same as the set that arrived, it worked now and didn’t drip from it’s transformer…

Anyway, a dead power supply mustn’t mean a dead set, must it?

As a postscript, there have been some discussions on here lately about using diodes in place of valve rectifiers. If a reservoir cap failed on a set with a valve rectifier, the valve will restrict the current and you’ll see it drop. With a diode in this way, it will just provide more current, and hence more damage will occur. This set was an example of this.
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Old 5th Oct 2007, 7:36 pm   #2
howard
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Default Re: Unitra Figaro - I always wanted one, but...

Well done Steve,

I think these Figaros are pretty little sets in their lacquered wooden cases. I did win one off eBay some time ago but had to send it back cos it had bits missing off of it and I didn't think I'd find spares for it. Good to see one working well again too.

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Old 5th Oct 2007, 7:57 pm   #3
paulsherwin
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Default Re: Unitra Figaro - I always wanted one, but...

Quote:
Originally Posted by Steve_P View Post
The ECL82 has an Anode Voltage of 200v. So now I needed to know what the voltage off the Diode I was putting in would put out - on load. Then it was a simple matter of finding out the voltage drop and then the value of the resistor needed. So, valves in, connect up diode and put meter in place with croc clips. Then it’s a matter of speed. At 70% of mains voltage, there were noises. So I gave the tuning control a tweak and got 5-Live. I then wound it up to 240v and quickly measured 400v or so on the diode. The ECL82 began to glow where it shouldn’t, so off quick.

The Resistor was to drop 200v. Therefore, by Ohms Law, it worked out at 3.5k. Power worked out at just over 11 Watts! A quick trip up to CPC brought me a 50 Watt 3.9k resistor. Bit more, but it’ll do I thought. Bolted to the chassis, wired in before the Diode and we tried the set again.
I was a bit puzzled by this at first - a metal rectifier shouldn't be dropping 200V! Then I had a look at the circuit and realised that the autotransformer would have reduced the voltage quite a bit before rectification.

I suspect your estimate of HT consumption may be a bit off, as the valves are likely to use less current than their rated maximums (especially the ECH81). All you can do is adjust the resistor until the HT voltage is correct.

Quote:
As a postscript, there have been some discussions on here lately about using diodes in place of valve rectifiers. If a reservoir cap failed on a set with a valve rectifier, the valve will restrict the current and you’ll see it drop. With a diode in this way, it will just provide more current, and hence more damage will occur. This set was an example of this.
It's always a good idea to fit a fuse of some sort when subbing a silicon rectifier. Some people like to choose a surge limiting / voltage reducing resistor rated very close to the expected power dissipation, so that it can act as a fusible resistor if disaster strikes.

Paul
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Old 5th Oct 2007, 9:11 pm   #4
Steve_P
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Default Re: Unitra Figaro - I always wanted one, but...

You're right Paul, the Auto Transformer does drop the H.T. so the resistor was in its place. Also, modern solid state diodes drop less!

In this situation, I try and estimate high rather than low. A lot of sets, and a few tellies too, let the H.T. drift about while the taps only vary heater volts. There is quite a bit of leeway here, and you can get away with a lot. Battery Radios operate with an H.T. down to about 75v sometimes!

If it was too low, I'd have put another resistor across it. There was room.

Yes, I fitted a fuse!!

Cheers,

Steve P.
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Old 5th Oct 2007, 10:51 pm   #5
Tazman1966
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Default Re: Unitra Figaro - I always wanted one, but...

Hello Steve.

A nice success story and an interesting and easy read too.

Cheers,
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Old 6th Oct 2007, 2:22 pm   #6
ianj
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Default Re: Unitra Figaro - I always wanted one, but...

Does this little set have maic eye tuning?ianj
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Old 6th Oct 2007, 6:08 pm   #7
igranic
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Default Re: Unitra Figaro - I always wanted one, but...

Yes. A DM70.
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