AVO Two Panel Valve Tester

[williamsunique]

This AVO Two Panel Tester came my way from a friend after her brother passed away. He was an electronics engineer.
It wasn't working, but it was complete, although grubby. It was missing most of the screws that hold it together.
Before I powered it up, I visually inspected it to see if there was anything obviously amiss. Several things surprised me. It was certainly beautifully made, but where were all the components, there must be more than one carbon resistor and one capacitor. There wasn't. The checking of valves is accomplished with AC voltages.
I fought my way through the wiring diagram and I went over it many times before I began to understand it. There was one rectifier diode, several wirewound resistors and some high quality switches and wirewound potentiometers.
Upon switching it on there were no nasty burning smells or any signs of anything becoming hot. The neon lit and the "set zero" potentiometer worked. The valve socket and switch tumbler panel was not plugged in. I checked voltages, having first established a common earth point. Voltages were as expected. Why was it not working then? I connected the two panels together and checked voltages entering the valve base panel. Here sometimes there was the expected voltage present and at other times there wasn't. This was confusing as I was unsure whether the intimitancy was due to my meter probe not making proper contact. I checked again to see if anything was working, no it still was not reading any current flowing through the valve I had plugged in.
The 9 wire cable that connects the two panels was cotton covered. As soon as I started to check continiuity between the two panels the connections broke, the cable was fragile. I made up a new loom and connected this between the panels. The Tester now worked. A little bit of tidying up and redoing the lettering on the control knobs. One waxy capaciter was replaced, it wasn't leaky but open circuit.
I now have what I regard as a lovely functional valve tester. The meter is easy to read, it is large and clear. The Tester is easy and quick to use. I will put it to good use.
I scavenged my "odd screw biscuit tin" for the missing appropriate BA screws. Enough were found to at least hold the thing together.

While researching this in the very beginning I downloaded David Williams re-drawn circuit, this helped immensely, thank you David.

Images show. Brittle cotton covered cable. Rotating key positions for EL33 valve. Valve socket panel. The meter showing the tested valve to be good.

PS. In the second and third image. I've just noticed the heater switch is in the wrong position for the valve tested, it was in the correct position when the valve was tested.

Paul




https://www.vintage-radio.net/forum/...1&d=1679429313
https://www.vintage-radio.net/forum/...1&d=1679429313
https://www.vintage-radio.net/forum/...1&d=1679429313

[Craig Sawyers]

The roller switch lettering is very white and crisp. These two panel testers were introduced in 1938, and the switch lettering is usually faded, missing bits, or yellowed.

The guy who owned this before he went to electronics shop in the sky, clearly refilled the numbers somehow.

If you look very carefully you can just about see witness marks on the screws fitting the top bezel, which would be consistent with someone working on the roller switch.

Craig

[williamsunique]

Thank you for your comments. Yes it was me who refilled the numbers.

Paul.

[David Simpson]

Paul, you've done a great restoration job. The 2 Panel is an ideal basic tester for all VR enthusiasts, and well worth keeping & maintaining with TLC. It's value has been boosted in recent years by the massive increase in the price of it's larger, newer, AVO VCM siblings. But even those are 60+ years old, & genuine replacement parts are like rocking-horse manure.
I see that you've got a B9A wee valve holder, but not a wee B7G one. Best bet would be to either use one of the blank valve holder spaces, or build a small extension box with two or three valve holders in it. If you can get your hands on a big duff old B9 valve & use it's commodious bakelite base to build a nine wire extension lead that would do just the job. The B9's base has heaps on room to remove the old electrode wires from the pins & then re-wire with 0.5 or 0.75mm sq. pvc insulated wire. Also there is room to fit ferrite beads over the wires. Test it all out & then back-fill the B9 base with Araldite or similar non-conductive compound. Or you could fit the ferrite beads over the ext. box's internal wiring.

Regards, David

[Andrewausfa]

I bought one of these at the last BVWS meet and have been working on it and like you was surprised at the lack of components inside when I opened it up. I believe mine dates from May 1944. I also refilled the switch digits using white acrylic paint which is easy enough to scratch off where it's not actually infilling.

The only real issue with mine was the meter glass was loose so I filled the gaps with black sealant and tightened the retaining screws up. I also made up a test lead to go on the front panel as needed. There are three capacitors shown on the circuit diagram, one wax paper in the meter unit and two ceramic in the valve holder unit. The only thing I have left to do is infill the three A/G/S wander plug sockets with white paint, the letters need rescribing which I will do with a plastic pyrograuvre.

Andrew

[williamsunique]

There were two adapters which came with the Valve Tester. They are for valves that weren't catered for in the original design.

Paul

[David Simpson]

Hello again Paul, re post 4, I've attached a picture of the valve holder adaptors I use.
Rear left = B7 to B7A (Septar)(QQV-06's etc.)
Rear right = IO to B8B & IO
front, L to R :- B9/B9A, B7/UX7, B9/B9D(Magnoval) (PL500's etc.)

Fitting Ferrite beads prevents PO(Parasitic Oscillation), particularly in "High Slope" valves like PL500 types,TT22's, and also KT's & powerful EL's "don't like it up em". PO sends Valve Testers & VCM's haywire. Particularly CT160's in my experience.
However, the AVO VDM for the "2 Panel" (R/H tabulations) kindly gives lower Va's & Vs's & Vg's - - thus working amplifying valves at the lower regions of the Ia/Vg (mA/V) "Slope".

Regards, David

[williamsunique]

Hi David,

Thank you for the information on your valve adapters.

I have just recently started to work my way through the electronic equipment I have accumulated over the past years. Test equipment, radios, amplifiers, tape decks, amplifiers and other HiFi stuff.
I have a lot of valves (mainly unboxed) that I will be checking and sorting. I do have another Valve Tester that is functional. A Mullard High Speed Tester with two boxes of cards.
Looking back to when I first started repairing, mainly TVs, but some radios it was mainly other components that were at fault, valves I found were quite reliable.

Paul.

Originally Posted by David Simpson

Hello again Paul, re post 4, I've attached a picture of the valve holder adaptors I use.
Rear left = B7 to B7A (Septar)(QQV-06's etc.)
Rear right = IO to B8B & IO
front, L to R :- B9/B9A, B7/UX7, B9/B9D(Magnoval) (PL500's etc.)

Fitting Ferrite beads prevents PO(Parasitic Oscillation), particularly in "High Slope" valves like PL500 types,TT22's, and also KT's & powerful EL's "don't like it up em". PO sends Valve Testers & VCM's haywire. Particularly CT160's in my experience.
However, the AVO VDM for the "2 Panel" (R/H tabulations) kindly gives lower Va's & Vs's & Vg's - - thus working amplifying valves at the lower regions of the Ia/Vg (mA/V) "Slope".

Regards, David

[David Simpson]

Aye Paul, seeing as you've also got a Mullard HSVT, you're quid's in for testing & sorting out your valve collection. The 2 Panel, despite being by far the older of the two, does give a basic mA/V (Gm) metered indication. Whereas the HSVT just gives a basic "Red to Green" fail/pass indication. HSVT's are a great set-up, 'specially if you have the complete steel desk/drawer caboodle with all the cards. However, it's heffing heavy, and best left in a permanent position in your workshop. The AVO 2 Panel is much lighter & portable, and AVO's Valve Data Manual & it's 9 numeral code for setting the thumbwheel switch is spot - on. HSVT's have a big Forum following, and no doubt you've seen the massive ongoing Card project thread. So you'll always have plenty of helpful advisors.
Likewise there is a similar Forum following of AVO VT/VCM enthusiasts. 2 panels have a propensity to "Whang" their meter's needle to fsd, but those old meters are very very robust. However, AVO's range of sensitive 30uA meters in the VCM Mk 3 & 4, and the CT160 can be easily damaged. So, I've always advised folk who have recently acquired a VT or VCM - to set the -ve VG control to a higher voltage than the book value before the "Test" procedure. Then if all is well, apply the correct Vg.

Regards, David