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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 30th May 2020, 9:46 pm   #21
Stevie342000
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Default Re: AVT - Avo Two Panel Valve Tester?

I now know that as I have now reached that part of the patent. The 50R series resistor is there to limit current prior to the rectifier. Not completely clear yet but LS3 comes into play as a current source/sink. I get the backing off control and how that works it is acting as a current shunt.

I do know from what I have read when testing valves like an 807 the current goes off the scale in the circuit.

I am not sure that this is all really relevant unless you are replacing the MR (Metal Rectifier). I have not got as far as marking any voltages or notes on the schematic yet working from memory not good errors creep in.

As far as I can tell if you did replace MR then you would need to make sure that the volts were the same, there may however be an issue with increased current due to the lower resistance of the silicon rectifier.

What may be more pertinent is if anyone has replaced MR and what measures they took?
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Old 30th May 2020, 9:50 pm   #22
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Default Re: AVT - Avo Two Panel Valve Tester?

The simple solution would be if there was a data sheet for the MR.

If the part number and type is known then the data should be out there somewhere.
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Old 31st May 2020, 7:38 am   #23
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Default Re: AVT - Avo Two Panel Valve Tester?

Quote:
Originally Posted by Stevie342000 View Post
What may be more pertinent is if anyone has replaced MR and what measures they took?
I have Two - 2 panel testers, both restored and calibrated. One has the original metal rectifier and the other has a 1n4007. No additional resistance was added.

Both give identical results on my test valve. I have not done further tests I am afraid.

Peter
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Old 31st May 2020, 12:53 pm   #24
Stevie342000
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Default Re: AVT - Avo Two Panel Valve Tester?

Thanks Pete good to know, I think that part of the circuit is current limiting or limited but only a guess.

I get how the whole circuit works and how it functions the patent is really useful.

There should be a patent for the roller switches as well, as it does say on the one I have that the patent is applied for.

More work to do reading and researching until I fully understand it all.

I do not think I can put in a B9A socket on mine as the it appears to me that the two spare socket plates are where the divide by 7 heater transformer is in that spot.

So my thinking is to swap out the B7G socket and put in a B9A instead and to make up some adapters/socket savers.
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Old 31st May 2020, 1:20 pm   #25
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Default Re: AVT - Avo Two Panel Valve Tester?

If it's any use here's a simplified diagram for the mA/V test shown with a triode valve I did some time ago.

Lawrence.
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Old 31st May 2020, 1:52 pm   #26
Stevie342000
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Default Re: AVT - Avo Two Panel Valve Tester?

Thanks the simplified diagram does help.

We have made a little progress, in the current environment I left the tester alone for the last few days.

I have now washed it gently with a soft toothbrush, some mildly soapy warm water.

Before I did notice that the legends are readable but most of the white paint in the lettering appeared to be gone.

It was not completely the case it was mostly dirt.

I gave it a going over a few times wiping off with paper towels never applying much liquid either.

Most of the lettering is intact, but faded.

The two worst controls are of course the Zero Set and mA/V controls - nearly all gone.

How can I clean the surface further without damaging the Bakelite?
Still looking for the patent for the rollers - they do need a thorough clean the lettering is gone off all the thumb wheels.

Once I have got the panels clean I assume I just use furniture polish or some other proprietary polish?

Taking slow steps not rushing like I normally do - thinking this through as I go through each step.
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Old 31st May 2020, 2:46 pm   #27
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Default Re: AVT - Avo Two Panel Valve Tester?

I think this page is useful https://jacmusic.com/Tube-testers/An...l#Valve-Tester

There is a calibration document as well which I have not come across before. It is rather simple really (not read document yet).

But at this point my understanding is the meter has a 600 micro Amp FSD. The scale is 1mA. The two precision wire wound resistors are made up or down from nominal value so the meter gives 1mA FSD, thus you read mA/V directly from scale.
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Old 31st May 2020, 5:06 pm   #28
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Default Re: AVT - Avo Two Panel Valve Tester?

The calibration procedure is also given in this document (attached). That's the one I used.
If the meter has not lost too much sensitivity adjusting the 40ohm swamp resistor can be effective in restoring sensitivity.
Reducing the swamp resistor too far seriously affects the functioning at other mA/V settings. I posted some calculations here:
https://www.vintage-radio.net/forum/...d.php?t=162366

The meter amplifier I posted about in that thread was never used by the way, I suspect that the fact the 2 panel AVO puts a half wave rectified sine wave through the meter would saturate the op-amp (it worked at dc!)

I built a re-magnetiser (posted about separately) and while this worked one of my meters was so far out I had to replace the magnet with a modern rare earth one. It then calibrated correctly.

Peter
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Old 31st May 2020, 5:46 pm   #29
Stevie342000
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Default Re: AVT - Avo Two Panel Valve Tester?

Thanks that is the right document the one I attached is the 1st part, the 2nd part is missing on the Jac website.

This makes more sense than the one I posted, the two together are useful. I will digest your link another day, brain is tofu today.
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Old 1st Jun 2020, 2:14 pm   #30
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Default Re: AVT - Avo Two Panel Valve Tester?

I found this whilst looking for the patent for the roller switch which I found but to save time here is it and some more:

https://www.vintage-radio.net/forum/...ad.php?t=86837

I have tried to take the main unit apart I removed the 5 of 6 screws(1 is missing) - which would suggest I am not the first person in there as would chip from the top of main panel.

I had no success in getting in as it is a very tight fit, any ideas? I do not want to chip the front panel any further.

I may be able to use the cable entry point as well as the chipped area as points that I can lever gently?

As for the valve holder roller switch panel I was able to get into that.

All seems well apart from one point there is an orange cable at the panel end that is not connected, I could not see it at the plug end either when I just had a peek in there.

There are 3 capacitors all of which I think I should replace?

I gave the valve holder unit a quick run over with the vacuum cleaner (gently) - the cobwebs and detritus are now mostly gone.
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Old 1st Jun 2020, 2:49 pm   #31
Leon Crampin
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Default Re: AVT - Avo Two Panel Valve Tester?

As I recall, you need to remove also the 2 screws adjacent to the 9 pin socket on the main panel before removing the metal case. You can also remove the underside cover giving access to the mains voltage adjustment panel - this gives you something to push on (with care).

Leon.
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Old 1st Jun 2020, 3:27 pm   #32
Stevie342000
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Default Re: AVT - Avo Two Panel Valve Tester?

Thanks Leon I could not remember if I had to remove the two screws on the 9 pin socket and good thinking on the mains voltage adjustment panel.

Will attempt to take some better pictures soon....
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Old 3rd Jun 2020, 2:11 pm   #33
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Default Re: AVT - Avo Two Panel Valve Tester?

Can anyone suggest a better way then using a Chinagraph pencil to refill the letting?

It works but you tend to get more on the body of the case having to scrap it off and reapply. Is this the only way?

I am thinking tippex but suspect that may react with the bakelite or you will face the same issue. Being left with problems of how to remove it from the fascia?

Additionally I have ordered some bakelite cleaner the type used in restoration of hard or early plastics - awaiting its arrival.

My research suggests you should not put water anywhere near bakelite. As bakelite is porous to water. Additionally it removes the finish which is difficult to return.

Bakelite may contain sawdust used as a filler I assume, any thoughts?
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Old 3rd Jun 2020, 5:10 pm   #34
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Default Re: AVT - Avo Two Panel Valve Tester?

The way I use Chinagraph is to rub it in hard so the wax is pushed into the lettering. Any surplus can then be rubbed off, this has the added benefit of burnishing the infill.
Any stubborn overfill can be removed with a small amount of solvent on a tissue. I us Evostick glue remover.

Peter
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Old 3rd Jun 2020, 5:56 pm   #35
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Default Re: AVT - Avo Two Panel Valve Tester?

You also have to manually remove all the old infill and clean out well before using chinagraph to refill. Peter's advice is good regarding use and overspill removal. I did exactly that process with some old chicken head bulgin knobs pretty recently.

Regarding Tippex. I used it a very long time ago to fill in the lettering on an AVO8 mk 3. But that was back in the days that Tippex used a volatile solvent, which has since been banned. Tippex is now solvent free, and I have no idea about its suitability for filling lettering.

Craig
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Old 3rd Jun 2020, 6:03 pm   #36
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Default Re: AVT - Avo Two Panel Valve Tester?

These guys http://www.ukengravingservices.co.uk...nt_filling.pdf recommend either a paint stick (ie chinagraph) or oil based enamel, then clean up with mineral spirit. I have not done the enamel process personally.

Craig
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Old 3rd Jun 2020, 6:15 pm   #37
Stevie342000
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Default Re: AVT - Avo Two Panel Valve Tester?

Thank you one and all for your help.

I think I need to clean the front panels first of all.

What is people's experience with using brasso in cleaning bakelite?

Or should I consider an alternative?
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Old 3rd Jun 2020, 6:20 pm   #38
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Default Re: AVT - Avo Two Panel Valve Tester?

Check google with the search term "cleaning bakelite".
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Old 3rd Jun 2020, 8:52 pm   #39
Stevie342000
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Default Re: AVT - Avo Two Panel Valve Tester?

I had already checked online I was looking for other people's experience plus not taking all that I find on line as gospel.

The search elicited this: https://www.wikihow.com/Clean-Bakeli...ular%20motions.
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Old 3rd Jun 2020, 9:13 pm   #40
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Default Re: AVT - Avo Two Panel Valve Tester?

Hi Steve, the proprietary stuff for filling engravings (available in several colours) was "Kwikfil", a small ingot shaped stick of a waxy material.
Not sure if it is still available.

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