Golborne Vintage Radio

When I got my ivory DAC90a I found that it's UL41 was leaky, having the best part of 2v on pin 6.

I got another off e-Bay which was even worse, having 3.6v. I can't complain as the description said it had been tried in a radio and worked well. This is perfectly true and it's only when the voltmeter comes out that the trouble is found.

I don't need telling that buying it was silly but it may not go amiss to tell me that it's even sillier leaving my lap-top by the bed where things can be ordered while half-asleep - I had no recollection of bidding on this until I got the request for payment. At least it wasn't a telly!

I've done bit of research and found a couple of techniques for clearing these internal shorts. One is to apply a sustained high voltage, in the 3kV region, and the other is to blast them with the spark from the ignition of a gas cooker.

Now I don't have either a gas cooker, though will have access to one next time I'm cat-sitting, or a high voltage source. I therefore tried wiring the pins, except number 6, together and discharging a 32uF capacitor across them but it it wouldn't discharge. I then tried the piezo igniter off my flame-thrower and this did produce a few internal flashes but had no other effect.

I was wondering if any of our members had the necessary gear and fancied a fiddle - the TV bods have got access to voltage-tripplers for a start. I certainly won't yell if the valves end up utterly wrecked as they're no use to me like they are. Certainly having two would seem to give a chance for one to be practised on and then the other one properly sorted.

Any other ideas would also be interesting.

Cheers
- Joe


may be only leaky when hot trouble is 47volt heaters i think what happens is the grid sags when it gets hot ,heater cathode shorts i have cleared witm a charged cap but not grid cathode
if all else fails i have a nos ul41 you can have
rob t

I'd heard about that and wondered if it was a joke but if a man of your wisdom suggests it it's got to be worth a try. I'll have a pop (probably not the best way of saying it!) tomorrow.

Cheers
- Joe

I've only just got around to trying this and, at least as I did it, it made no difference for good or ill so I'd like to check if I'm doing it properly - if such a technique can ever be regarded as 'proper.'

I wrapped the valve in foil leaving just the bottom ⅛" exposed and put it in a mug of water. I then gave it lengthening exposures starting at two seconds and progressively stepping up to thirty at which point my nerve failed me. I tried it both pins up and down.

I saw no flashes and heard no noises beyond those of normal operation. The resistance between pins, including 3 and 4 which shouldn't be connected to anything, is around 3MΩ; I can't measure it more accurately than that due to the scale on my megger not being marked for high accuracy.

Yes, I admit I'm a tight-fisted Tyke but there's also a certain satisfaction in fixing anything, especially when it's supposed to be beyond hope, so any other suggestions would be most welcome.

Cheers
- Joe

Have a look at this link to Alan Lord web site at Dundee College

http://old.dundeecollege.ac.uk/Sections/...igvolt.htm

It is the design of a high voltage insulation tester, which he used to remove the conductive coating inside a leaky UL41 valve.
I have tried this method and can confirm that it does work, although I used a more lethal mains EHT transformer powered by a variac.
Regards Stan.

Thanks Stan.

I'll shortly be scrapping a television so should be able to nick the trippler out of that.

I tried further microwaving this morning but when I got a flash and a bang thought it best to desist in case I blew it up - at least before having my dinner!

- Joe

I don't have a licence to operate it, it's too big and is occupying too large a part of a shed.

Actually given how our membership has grown recently I ought to offer it again before it gets the chop.

- Joe