Avo 2 Panel Valve Tester Success - sort of!

[Backtoreality]

In January 2019 I bought one of these valve testers from an eBay seller, sold as untested which is usually the case. I had been looking for one for quite a while watching how much they were going for. The ones I'd seen were sold for £120 plus so I felt quite happy to get mine for £100.
When it arrived all of the fixing screws had been previously removed from both units, the valve socket panel had a gap at the bottom right corner and the escutcheon? (frame over the 9 rotary switches ) was broken over the 3 rightmost switches and there were bakelite pieces missing.
After examining both units I removed the escutcheon and made a mould with blu-tack of the good half, which could be used to repair the broken half.
I then filled the gap on the socket panel with a magnoval socket. I also examined the meter which seemed to have real problems with stickiness. The rear spring looked fine but I suspected problems with the front spring and possibly dirt/metal causing sluggishness.
A whole year passed by before Covid 19 gave me a push and I had the time to continue so...…..
Task 1 Check the socket panel and locate breaks and dry joints in the wiring, of which there were 5!
Task 2 Replace the 3 capacitors on the panel, 2 of which were well out of spec. 0.001uF 1000V
Task 3 Check and replace 1 out of spec resistor on the main unit: 100K which had gone significantly high. The others were well in tolerance including the 2 300K resistors which were supposed to be 250K according to the circuit. Replace the mains flex.
Task 4 Wrap the damaged areas of the sheath of the umbilical flex with linen thread.
Task 5 Connect the units and check all the voltages according to the fault finding guide using my recently restored Avo Model 7 - all spot on!
Task 6 Respray the cases and the socket panel with black hammered enamel.
The meter was next on the list so I removed it for examination and it was clear the front spring had been replaced, very badly and was fouling itself. Not wishing to destroy the meter I tried carefully to straighten the spring but gave up and replaced it in the unit.
After reassembly I decided to test the tester with some valves.
First to hand was an EF80. Carefully following the instructions I set all the switches appropriately and eventually was ready to push the rocker switch for the mutual conductivity test. The pointer flew to the left. Puzzling! After reading more on the forum I decided to test a rectifier. Everything set up as per the instructions and as the valve, a 5Z4G, warmed up the pointer went hard to the left again!
The answer was the meter connections had been reversed by a previous owner! I wondered if I could have done this, but no I hadn't, the wires were carefully routed around the magnet and had to be re-routed to reach the correct contacts.
The tester now works and has successfully tested a number of valves including: 5Z4G, 5V4G, 5U4G, 5X4G, 4 x 5Y3GT, 431U, EZ81, 6V6G, 4 x 6V6GTs and a 25L6.
A sort of success because the tester is now operational but the meter has to be regularly tapped like a barometer to get the correct readings!!
Next step? Can I do what Andre did with his meter? Not sure but thanks to everyone who has contributed to any of the forum threads on the Avo 2 Panel Valve Tester, I've read them all again and again!
David

[Silicon]

I have seen 'tin whiskers' on old AVO meter movements.

They are so thin they are almost invisible. There may be some that are interfering with the movement.

A magnifying eyepiece and a portable light source will help you see them.

[Backtoreality]

Thanks, I'll have a look.
David

[Electronpusher0]

Originally Posted by Backtoreality

the meter has to be regularly tapped like a barometer to get the correct readings!!

David

I have just been working on my spare control panel of my 2 panel AVO.
I also had the problem of having to tap the meter to get it to settle.
Turned out to be two of the turns on the lower spring touching each other. Not fouling as such or crossed but just touching. Tapping allowed the coils to slide past each other. I managed to tease the sping so none of the coils touched. (I know the spring is a spiral and thus all one coil but you know what I mean )

Peter

[peter_scott]

Originally Posted by Silicon

I have seen 'tin whiskers' on old AVO meter movements.

They are so thin they are almost invisible. There may be some that are interfering with the movement.

A magnifying eyepiece and a portable light source will help you see them.

You can usually push them out by sliding a little strip of paper between the pole pieces and the moving coil.

Peter

[Backtoreality]

Thanks Peter and Peter, I will try the paper to see if anything changes (Peter_Scott). I will try to take a photo of the front spring Peter (Electronpusher0) to see what you think. I've been having fun testing a variety of valves for Good/Bad/Indifferent results which seems to show the pointer sticking at both ends of the scale. Using the Zero control regularly during the process seems to give me a good indication of whether a valve is ok or not. It does look like I need to change the AO8 valve base due to poor contact despite trying to clean the sockets and crimp them in slightly. The B5 and B9a bases seem fine.
David

[Backtoreality]

No sign of any debris making the meter sticky so here are two photos of the front spring which I think is the cause of the problem.
David

[Electronpusher0]

Looks pretty likely. It is possible to tease the spring into better shape but also very easy to muck it right up.

Peter

[Backtoreality]

I do wonder if its not the correct spring because the turns are not tangled at all they are just being pulled over to the side because of the position of the two fixing points on the same side. I noticed one of the documents downloaded from VMARS mentions another document related to working on the meter. Has anyone seen it / got it? I don't want to destroy the meter.
David

[Stevie342000]

In the same position as you having just got my two panel Avo tester last week.

I think the document you mean is EMER which you can find here: https://frank.pocnet.net/instruments/AVO/index.html

I may be wrong though I do recall seeing a document with VMARS on it but can not locate it. Because I am looking for it.

I have yet to get the actual tester out of the case, currently looking at the socket panel.

[The Philpott]

Agree. It's likely the wrong spring (or the correct spring installed incorrectly.) I can't see how else that sort of cone shaped distortion could happen.

I have never attempted replacement, only untangled conflicting turns, or mechanically biasing to separate chafing turns. (As per Post No.4)

Dave