Osram U50 full wave rectifier. GEC BC5050 Receiver.

[Station X]

Originally Posted by Radio Wrangler

The base can be taken off by solder-sucking the pins (or heating all the used pins at once) Clean out the pins. Fit some PTFE sleeving on the wires to make life easier, then try to get the right wires back down the right pins. You may be able to solder on wire extensions to make this easier, but pull the original wire at least to the end of the pin, where it was originally trimmed off. New adhesive at the ready!

David

Here's a valve I did earlier:-

https://www.vintage-radio.net/forum/...1&postcount=96

[DonaldStott]

Originally Posted by Radio Wrangler

That looks to have no tensioning springs at the top. If a heater gas parted, then the ends could be anywhere in the anode cavity.

As part of my ongoing education it would be good to know (see) what a good valve with tensioning springs looks like?

Originally Posted by Radio Wrangler

If the valve's duff, then you need to find a replacement, however the failure happened. With a loose bulb flopping around, having the bare wires inside the base touching seems the most likely issue.

Looks like I still need the base to come off just in case it's bare wire links touching that's the problem

[ms660]

Originally Posted by DonaldStott

As part of my ongoing education it would be good to know (see) what a good valve with tensioning springs looks like?

http://www.r-type.org/exhib/aaa1583.htm

Lawrence.

[DonaldStott]

Originally Posted by ms660

http://www.r-type.org/exhib/aaa1583.htm

Lawrence.

Thanks for that Lawrence.

I've scoured the Internet looking for an image of a U50 valve with tensioning springs but there is such a variety of different styles for the mica top plate assembly?

This picture shows one variation but is not the same as mine: -

[snowman_al]

There are a number of Osram U50 types.
Yours has no springs at the top mica. They have not fallen off, there were never any there. The filaments were tensioned as part of production and this type is the most common I suspect.
Then there are at least two types with springs to tension the filaments, inverted U like your picture and a 'hoop' type - see below. There may be more?

[snowman_al]

Looking at your x10 pictures, is that a break where the filament has folded over the mica?

[DonaldStott]

Alan - thanks for clarifying the tensioning springs issue i.e. my valve never had any!

As for the possible filament break, I think it's just a break in the oxide coating which I don't think is a problem?

Meantime I managed to get the base removed with the help of my soldering iron and manual solder sucker - a couple of the wires may have been twisted around and touching but I've put in some sleeving to prevent that in future. The RED wires are the filaments while the BLACK wires are the two anodes. The glass envelope was marked with a Sharpie pen before disassembly to ensure that the wires went back in the same pins.

[Station X]

Before you replace the base I'd check for continuity between the red leads feeding the filaments.

[snowman_al]

Measure the resistance of the 2 filament wires before you put it back together! A low reading (less than 1 ohm) is what you want, not OL on your meter...

Likewise check from both the filaments to both anodes, you want OL here not a low reading.

Fingers crossed

[DonaldStott]

Unfortunately it's not good news as I am assuming that I should have some form of continuity across the RED filament wires but I am still getting O.L.

And just to confuse matters Pin 4 to 8 is still showing continuity??

So I think we can all now safely agree, this valve is 'goosed' due to an internal short between one of the anodes and the filament - hopefully I've got that right?

[Station X]

Agreed.

[Radio Wrangler]

Sadly it seems so.

You can dissect it to learn how they're put together Nipping off the end of the small evacuation tube below the pinch will release the vacuum, then a glass-cutter will allow the envelope to be broken. Professionally it's done by wrapping one turn of resistance wire round the bulb where the scribed line from the cutter is, then hitting it with a lot of current to red heat. The glass parts with a small click sound.

David

[Techman]

I had a GEC from that era that had an anode short in the rectifier and it had fried the mains transformer windings to a cinder, needless to say the set was scrap. I would strongly advise that before going any further with this set that you test the transformer with all valves, dial lights etc. removed to see if it gets hot or 'sizzles', and that all the output voltages are as they should be. Use the lamp limiter first, as if it's still bright, then the transformer is toast and probably not worth doing any other tests on. I seem to remember that the set had been in an old house with round pin plugs with no fuses in the plug tops and the mains lead had yards and yards of extension bodged onto it, so was probably plugged into a socket in a different room. I can only imagine the thick clouds of room filling smoke that this burn-out must have caused before its owner managed to get to the un-fused plug to pull it out before the thing went up in flames!

Your transformer may well be ok, but best to do this check just in case.

[DonaldStott]

Thanks Techman

All valves and dial lamps removed and powered up using my lamp limiter - bulb did not glow at all?

You can hear the 'normal' background hum from the set but the mains transformer did not get hot or sizzle, even after half an hour?

I was dreading that crackling sound and overheating smell we all know so well!

[Leon Crampin]

The U50 can more easily be replaced with its American original, the 5Y3 (G or GT). I have found that the MOV copies of the octal valves in this series were never as good as the American originals.

I like to keep sets original with their correct valve rectifiers, but if you have a set with a known suspect (by design) rectifier such as the 6X5, you can protect the mains transformer by putting a 1N4007 diode in series with each valve anode connection, A 1A fuse in the plug to BS1362 is a further (but not infallible) safeguard. My Murphy A30C which has a delicate mains transformer has these safeguards fitted - but the original balloon-shaped UU3 rectifier soldiers on...

Leon.