AVO Mk 2 (Return to srevice).
 

Hi Guys
I think it best to start this story at the begining. About eight years ago my father who was a radio, and electronic engineer past away. In all that time his work shop was not used and at times got a little damp. Well just recently I to have become interested in restoring old radios and have aquired all of dads test gear, ie megger, capacitance meter, transistor tester, and his Mk2 AVO valve tester. Before switcing it on i took the case off to check that all was ok. It all looked fine except that one of the coils had a small amount of mould growing on it so i cleaned it off using a soft brush. Switching it on all is ok, no pops or bangs. The problem it has is in the setting up. I have followed the instrution book to the letter, but the needle will not zero to the red line. I have tried altering the mains tappings at the rear (as the book says) all to no avail. It does test valves fine but with low readings (or should I say will not give a full swing to the right). I am a relative beginer but i have tried the set up procedure about twenty times just in case i had made a mistake. I hope that one of you out there can offer any help.
Terry

Re: AVO Mk 2 (HELP)
 

If it's been kept in the damp it first needs to be dried thoroughly, such as by leaving it in an airing cupboard for a couple of weeks.

After that, well damp is pernicious and you could find you have resistors which have changed value, contacts which won't make properly, meter movements sticking. You just have to go through the whole thing thoroughly, cleaning switch contacts and checking that everything is as it should be, with no grey joints, drifted components or broken wires etc. .

If you are relatively new to this, it might be an idea to set it aside for a few months and have another look at it later.

Pete.

Re: AVO Mk 2 (HELP)
 

Hi Pete
Thank you for that sound advice. I don't think i will tackle this one just yet, although at the time I did spray all contacts and pots with switch cleaner. Taking your advice, I will take the case off and leave it in a warm room for a while. Thank you
Terry

Re: AVO Mk 2 (HELP)
 

I don't have any first hand experience of calibrating a valve tester (yet) but there seems to be a well-known problem with the magnets in the meters losing their field strength. I imagine that this means they read low, which sounds like your problem. There's a discussion here http://www.radiomuseum.org/forum/wha...be_tester.html and there may be much more info available on this. Maybe someone who knows more than I do can advise ?

Cheers,

GJ

Re: AVO Mk 2 (HELP)
 

Hi GJ
Thank you for that thread, I found it most interesting.
Terry

Re: AVO Mk 2 (HELP)
 

Hi There,
Can also recommend www.vintagewireless.net there is a lot about avo valve testers on there.

Robin

Re: AVO Mk 2 (HELP)
 

In theory, AVO testers look pretty straightforward, a tapped transformer, a switching system, resistors, pots and a meter.

In practice, they are expensive and not quite so simple, so no one wants to bodge them by doing things like taking any panel meter which fits and can be adapted and making up a home-made scale. Spare parts are expensive and hard to find although there do seem to be some specialists around dealing in them.

Judging from some of the threads on this site, they can throw up some challenging repair jobs. They're not things to pitch into unless you are confident, and if you decide to, it makes sense to do your homework on the WWW before, so you don't fall into a hole unnecessarily.

Pete.

Re: AVO Mk 2 (HELP)
 

I'd be happy to help with this but I'm a long way from you.

You'd probably do better to try

http://www.crowthornetubes.com/

Very helpful and stocks of spares and information.

Regards,

Re: AVO Mk 2 (HELP)
 

Hy Guys
This is just a final update on my "AVO Mk2" valve tester.
Well i did exactly what "XTC Pete" said (thank you Pete) and left it in a dry atmosphere for about two months. Before plugging it in I took the case off once more just to check all was ok. I am glad I did. The same coil which is wound using cloth covered wire had mould growing on it again. I brushed it of and sprayed it with switch cleaner,I hope it does not come back. I sprayed all the switches and contacts again and now the tester works fine. I can set up the meter and am now checking every valve in sight.
Thanks to all. Terry

Re: AVO Mk 2 (HELP)
 

I'm pleased to hear that it works and you have no problems with a sick mains transformer, demagnetised meter or other expensive horror story.

Taking a piece of equipment which has been left in a shed, or stored in uncertain conditions for years and just powering it up, is so tempting, but you have to fight it. You were lucky to get away with it.

As I understand it, mould requires high humidity (80%?) and still air to grow. It may have been growing on the moisture coming out. I'm pretty sure that coil is insulated resistance wire, rather than an inductor.

I'm not sure how good switch cleaner is as a mould proofer, but if you keep the tester somewhere dry and check it every now and then, I don't think you'll have any problems with mould.

Congratulations on a right result.

Pete.