AVT - AVO Two Panel Valve Tester?

[Stevie342000]

I finally took the plunge and bought an AVO Two Panel Tester, for a reasonable price. It has yet to arrive so I know very little about it other than what I saw on the pictures on the auction site.

This is the first time I have had both parts I did have the main panel but got fed up of waiting for the socket part to turn up so I sold the main panel some time back.

I have a number of projects on hand and a valve tester although not essential is one piece of test kit I thought I could do with and yes I know the best form of test is in the circuit. I know this tester is limited as it does not check for gassy or leaky valves. The shorts test frankly scares me but I think I have a work around for that.

I assume it was made in June 1941 from the number on the meter panel 3129-641, no idea if it is the Admiralty version or not. However I suspect it might well be. I have a number of questions:

1. what are the differences between the standard version and the Admiralty version?

2. I assume the sockets for Anode, Screen, Grid and shorts tests are all 4mm banana?

3. to make shorts tests safer (as I know these should be performed lest you should damage the tester by testing a faulty valve) - I was considering making a break out box with 4mm bananas to plug into tester. This would be similar to the socket panel with the sockets of the valves I am likely to use on it UX4, 5, 6, possibly 7, IO, B7G and B9A with a couple of 12 way single break before make switches to select pins under test paying attention to valve data in relation to pins with Internal Connections - not that it would make much difference or would it?

4. I the silicon rectifier in the meter circuit (its not a case of if but when will it fail) what should it be replaced with i.e. 1N4007? Does the 5R resistor in series need to be adjusted to make sure the volts stay the same? If so what volts am I looking at? Assuming DC1V.
5 and I think this is the final question on the resistors and the capacitors in the main unit and the socket panel unit - my assumption is all the capacitors should be replaced. Is replacing either the capacitors or resistors likely to throw it out of calibration - my assumption is not - as its function is to supply the correct voltages?

Any help much appreciated and I am pretty certain that replacing the rectifier was discussed recently...thanks I am reading info on Golbourne about the tester - which is where I found the manuals......

I know this tester has limited functions compared to other Avo testers but they are too rich price wise for me. I have the parts for the Radio Constructor Tester and was going to use that to test for leakage, gas etc and to compare the two although until it is built I will not know if the latter tests accurately. Only yep afraid so interested in Audio Valves to no fancy tester was really needed.

[Craig Sawyers]

These are still really useful testers.

The three sockets are 1/8" by the way. The later testers are 3/32". I found a particular size of split pin suited the 3/32" ones on my AVO III, and you can probably find ones that work with the 1/8" ones on the two panel. I found my 3/32" ones on eBay and bought ten. Other suppliers were cheaper on a per-pin basis but you had to buy 500.

Craig

[Stevie342000]

Thanks Craig that is useful, in new money 1/8th " is 3.175 mm and 3/32 nds " is 2.38125 mm.

A further correction I said Silicon Rectifier and I meant Selenium realised my error but had already posted.

[Stevie342000]

Duly arrived safely yesterday and unpacked.

All seems to be in order so far, I have not as yet taken any screws out to inspect the insides or powered it up.

One item I did not notice is that the roller switch index is a stiff on the two left hand rollers and the furthest right hand roller.

How do I go about making them less stiff?

I need to replace the mains lead and grommet (which has disintegrated) and put a new plug on the item.

I have a number of projects but this is higher priority as it enables me to complete those.

[AC/HL]

The Bakelite swells a little. Loosening the longitudinal bolts just a tad may help.

[Electronpusher0]

Whatever you do, do not be tempted to use wd40 to free the roller switches.
On mine I stripped the whole assembly down, cleaned it all and reassembled with petroleum jelly applied to the contacts and rollers.
I took the opportunity to redo the numbers with white chinagraph as well.
I also removed the "0" rod so that selecting a switch position of 0 means the pin is open rather than shorted to Cathode. Later AVO testers had this mod.

If you decide to go down this route be methodical and lay each piece on a large sheet of paper in order, and take lots of photos!

Peter

[Stevie342000]

Originally Posted by AC/HL

The Bakelite swells a little. Loosening the longitudinal bolts just a tad may help.

I know what you mean by longitudinal bolts just where they are located will be revealed once I take the screws out(putting them in a safe place).

I intend to take lots of pictures as I bring this back to life. I will test using an ohm meter where I can to check for any O/C in all leads from point to point.

It's going to be a long haul I think.

I should have my work bench (Ikea Trestle table) cleared in the next week or two so I can get to work on this and then following projects. I may work carefully on one or two more at the same time.

I knew about chinagraph pencil to make the legends legible but could not recall what it was, I know you can get something from Halfords that you can use but can not recall what it is. None of the legends on mind are readable even in daylight it has all gone or very dirty....a clean up with reveal more.

I have never seen the insides of an Avo socket panel from or the roller switches of an Avo before other than pictures.

I know there are threads on here but there so many it is taking me some time to trawl through them all. I am making notes as I go so I know where to go and what to do.

I do recall that you can dismantle the roller switch assembly and the mention of petroleum jelly. I have however not made it as far as that thread yet.

I assume there are springs in there as well and to be mindful that they do not ping across the room?

I will do a search on roller assembly to see what I can find. Thanks so far, it is helping me to stay focused.

I recall that I can snip out the selenium Rectifier and wire in a 1N4007, do I need to increase the series resistor? As I know it is likely the DC volts out will be higher, will it does it make a difference?

[Craig Sawyers]

No springs, but just lots of parts that need to be assembled in sequence. There are usually a spacer washer or two, which I think were possibly selected when the switch was assembled, that you need to keep track of.

AFAIK there is no way to resurrect these other than to disassemble, clean off old gummy grease, re-lube (good old lithium grease - not the white stuff, the brown) and reassemble.

Craig

[Craig Sawyers]

The two-panel is disclosed and explained in detail in a patent, from 1938.

Craig

[Stevie342000]

Thanks Craig all the documents was my first point of call in researching how to get the tester back up and running in fine fettle.

That appears to be my reading sorted out for the weekend.

[Stevie342000]

Originally Posted by Craig Sawyers

No springs, but just lots of parts that need to be assembled in sequence. There are usually a spacer washer or two, which I think were possibly selected when the switch was assembled, that you need to keep track of.

AFAIK there is no way to resurrect these other than to disassemble, clean off old gummy grease, re-lube (good old lithium grease - not the white stuff, the brown) and reassemble.

Yes so I found out with a simple search on how to service the roller switches in the Avo valve testers. I found several posts not all here, there is one with pictures and instructions.

Care needs to be taken with the paxolin washers as well as what you lubricate it with. The impression I got was lithium grease was not the right option but they did not specify if it was white or brown. Consensus was pure petroleum jelly as a lubricant.

The other impression I got was you could reverse the side of the wafer that comes into contact so what you are using is the side that has not been used.

Methods of cleaning are isopropyl alcohol or white spirit, what is the consensus here from those that have cleaned the roller switches?

What did people use to clean the bakelite panels? Is there anything that should not come near them?

[ms660]

If you replace the rectifier with a silicon diode I would try and keep the same minimum circuit resistance value otherwise the transformer might start to grunt.

Lawrence.

[Stevie342000]

From what I read yesterday there is no need to replace the Metal Rectifier, yes I am correcting myself again it is not Selenium either.

The if it ain't broke don't fix it rule applies. I assume the same is to be said about the carbon composition resistors.

Is there any benefit to replacing all the carbon composition resistors?

The only benefit I can see is that carbon film resistors today are a lot tighter tolerance 5% rather than 10 or 20% of olde.

The wire wound resistors are all close tolerance and as a rule do not need replacing.

Are my assumptions so far correct based on others experience?

[ms660]

If you don't have to replace the rectifier that's fine but if you do keep in mind what the maximum current draw from the transformer will be, it's likely to be more with a silicon rectifier, you can tell that by looking at the schematic.

Lawrence.

[Stevie342000]

It's only a theory but can ohms law not be used to calculate what the current is flowing through that part of the circuit?

Or is it more complex than that?

If the diode had been a selenium rectifier than it should be replaced by research informs me that Metal Oxide Rectifier is a much more reliable military grade part.

As an aside it might well be a metal oxide rectifier made at SEI (Salford Electrical Instruments) who were part of GEC it was their forte - the former SEI site (the office part which is heavily contaminated) is just at the end of my street). The factory part is now a housing estate built in the mid-late 90s.

There were not many manufacturers of metal or selenium rectifiers in the 1930 but I do not it was a research area for SEI. They worked on radar as well using the office roof to site equipment to test the equipment on traffic on the street.

They worked on some guidance thing for missiles as well, I forget what it is called.

I know this is slightly off topic now but is it possible that Avo had access to the Enigma machine, the roller switch does have a strong resemblance to the roller switches in that machine. Did Bletchley not crack enigma circa 1939 or was it later? Avo may well have been tasked given their close ties to military to work out how the tumbler switches functioned.

[Craig Sawyers]

The Polish cracked Enigma before BP (in 1932). https://www.bbc.co.uk/news/magazine-28167071 . Even BP's name for the electromechanical breaking machine - the Bombe - comes from the name for the Polish machine that predated it - called the Bombas.

Craig

[ms660]

Originally Posted by Stevie342000

It's only a theory but can ohms law not be used to calculate what the current is flowing through that part of the circuit?

Or is it more complex than that?

If the diode had been a selenium rectifier than it should be replaced by research informs me that Metal Oxide Rectifier is a much more reliable military grade part.

You would have to know what the forward resistance of the rectifier was when the maximum current flows through the transformer, in a schematic I'm looking at (EMER manual) the transformer load, in effect, at maximum current is a 500 Ohm resistor in parallel with a series resistance comprising of a 50 Ohm resistor, the rectifiers resistance and a 1 Ohm resistor.

Also remember that it's not a steady DC out of the rectifier but halve wave, so the RMS value of the voltage across the rectifiers load will be approx. Vpeak/2.

Used to know Manchester/Salford area quite well, worked Manchester area many moons ago.

Lawrence.

[Stevie342000]

Originally Posted by Craig Sawyers

The Polish cracked Enigma before BP (in 1932). https://www.bbc.co.uk/news/magazine-28167071 . Even BP's name for the electromechanical breaking machine - the Bombe - comes from the name for the Polish machine that predated it - called the Bombas.

So ergo it may well have been possible that AVO based their roller switch on the one from the Enigma machine. I watched the programme on Bletchley on BBC 2 last Sunday. A further aside my friends father who moved to BT research at Martlesham worked for the GPO at Dollis Hill and was there in Bletchley in 1946 dismantling what I now assume was the Computer that was moved to GCHQ.

I never got the full story, as I moved away but it did crop up in conversation it was all early 80s late 70s era, so it was still very much not talked about at that point. It is now no longer possible to get his full story as he is no longer with us the last 2 years.

[Stevie342000]

Originally Posted by ms660

You would have to know what the forward resistance of the rectifier was when the maximum current flows through the transformer, in a schematic I'm looking at (EMER manual) the transformer load, in effect, at maximum current is a 500 Ohm resistor in parallel with a series resistance comprising of a 50 Ohm resistor, the rectifiers resistance and a 1 Ohm resistor.

Also remember that it's not a steady DC out of the rectifier but halve wave, so the RMS value of the voltage across the rectifiers load will be approx. Vpeak/2.

Used to know Manchester/Salford area quite well, worked Manchester area many moons ago.

We should be able to work it out though from the Emer & the Patent docs. The maximum current flowing in the anode valve under test is 100mA, if I recall correctly.

From what I have read of the patent so far the ester is simulating Normal static DC working conditions. The grid voltage is change by 1 volt when valve is under test.

The backing off control essentially shunts the 100mA, when you do the test it then reads Mutual Conductance in mA directly on the meter. It is all very very clever.

The patent documents give you the paxolin former shapes for the variable resistances used in the backing off control and the mA/V control. I suspect the figures are in the patent.

I have to say it is a very well written patent as well, I've read a few being a policy geek and with a degree in environmental sciences plus an understanding in some electronic principle, as well as being able to read a schematic that I do not find it at all dull, some of the documents are though especially government policy documents.......however that is what an executive summary is for......

My main aims here are to get this working, understand how and why it works as well as increasing my understanding in electronics. I get the valve stuff a lot, I can even figure out working conditions but only have a basic understanding in solid state, it leaves me a little cold.....

Someone that has a working two panel tester with the original metal rectifier in it could measure the forward resistance?

[ms660]

The maximum current that flows through the valves anode is not the same as the maximum current that can flow through the 20 volts winding.

Lawrence