AVO Mk2 Valve Tester - Cut out mechanism

[adeftereos]

Good afternoon everyone.


I've been working on refurbishing this uniyt both electronically and cosmetically.


As advised in the Euan MacKenzie in his excellent Radio Bygones article I have replaced all the carbon resistors, both wax capacitors, and the electrolytic capacitor.


As recommended, all the metal rectifiers have been replaced with Philips BYW96E types. I have also subsituted a BYW96E for the EA50 valve diode.


I have also completely dissassembled and cleaned the roller switches on the valve base panel at the top of the tester.


Thproblem I am having is that the cut out mechanism wont stay engaged. It was working oefectly well before the restoration but now when i press the reset button the meter lights come on momentarily and the meter needle twitches slightly. As soon as I let go the reset button the cut out mechanism points all disengage and the unit stays off. When the reset button is pressed there is a slight buzzing sound, presumably from the contact points.


I have tried cleaning the points with some 1500 sandpaper and contact cleaner but it's made no difference at all.


Disconneting the top panel with the valve sockets also makes no difference.


I'm a bit stumped and would be grateful for pointers as to where to start with the trouble shooting!



Angeos

[retailer]

It's been a long time since I played around with one of these cut-outs and my memory may be a bit hazy but I believe the cut-out is held closed against spring pressure by residual magnetism, excess anode or screen/G2 current reverses the magnetism causing it to weaken and the spring opens the contacts breaking the anode and screen/G2 circuit, pressing the RESET pushes the movable contact arm back against the coil and also remagnetises the cutout with DC current via a diode, so residual magnetism will once again hold the contacts closed, perhaps check that the remagnetisation is taking place. I'm not 100% sure but I think the switch contacts for the remagnetisation circuit are not actually mounted on the cutout itself but in close proximity.

[adeftereos]

Thanks retailer! So there is a specific remagnetisation circuit?! That sounds like it could be the culprit.




It's late now but I will look at this again in the morning to see if I can determine which is the remagnetisation circuit and whether there is any power going to it.

[adeftereos]

I'm afraid I haven't made any headway with this issue and could really do with some help understanding this circuit.

If I understand it correctly, and I'm sure I don't the relay has two coils, one fed from the Anode supply and the second is fed from the screen supply. These coils are wound in opposition to each other. The Anode circuit must supply greater current than the Screen supply for the relay to energise and the contacts to close.

The voltage supplied from the Anode and Screen supplies is derived from a transformer, so it is initially an AC waveform. The valve under test acts to rectify the current and transforms the current into a pulsating DC supply.

The only way the cut out is triggered is if there is an internal unit short, or a short between the screen and the anode of the valve under test. This would act to prevent rectification by the valve. Consequently the anode and screen supplies would revert to an AC waveform and then, according to the AVO manual, the first half cycle of the AC in the reverse direction to normal will trip the cut out mechanism.

So on to my questions:

When the VCM is originally turned on, without a valve being tested, there shouldn't be any current flowing through the coil so the relay will stay open? If the reset button is pressed then there still won't be any current flowing through the coils. Is that right?

As well as the two main relay contacts which control the Anode and Screen circuits there is a third set of contacts to the side of the relay. This set is not actuated by the relay but the do for a connection when the reset button is pressed. Does anyone know what their function is?

[Waverider]

I have a couple of Mk II testers, one was purchased recently and I found that the protection relay would throw out often, for example sometimes when I was switching the set switch to get the mains position set on the meter before a test. I cleaned the contacts (emery paper isn't ideal for any contacts, it removes platings) but I still found that it was too sensitive. It did get a little better after leaving the AVO on for a few hours (when I bought it it had not been powered on in years) and I suspect that this may be because the energised coil may have re-magnetised the armature a little.

I had to adjust the return spring tension screw a little to reduce the force acting to pull the armature away from the pole face. There is also a calibration procedure to set the correct spring tension to set up the cutout to operate at the correct point.

The reason that the relay doesn't cut out when you power down the AVO is that it uses residual magnetism in the relay pole-piece and the armature to keep the armature against the pole piece when there is no current flow.

The contacts at the side of the relay are for the anode and screen voltages and the set of contacts underneath are for the meter backlight lamps.

[adeftereos]

Thanks Waverider!

Great to hear from another Melburnian. I live in London now but hail from Elwood.

I'm going to have another go at the unit tonight. Thanks for your thoughts and the benefit of your experience.

I'll report back on how I get on. Thanks also for clarifying the function of the contact points underneath the main set.

[Waverider]

Happy to help! There are a lot of odd iterations of the Mk 1 and mk. II with all kinds of small variations between them so restoring them takes some detective work. I'm about to repair a wire-wound pot in one of mine and also to design an op-amp circuit to compensate for a small loss of permanent magnetism in one of the meters.

The Mk II is a fantastic unit, I prefer it to the Mk III and Mk IV for a lot of reasons. If you PM me your email address I will send you some docs.

[adeftereos]

Thanks waverider. Sending PM shortly.



I've tried to be as thorough as possible in my reasearch and, from What I've found out so far I think the Mk2 is perfect for my needs. (I mostly play with radios from the 30's to the 50's.)


What really struck me is the brilliance of the design in terms of allowing the testing of pretty much any valve, with just the valve manufacturer's data.


I've never come across that facility before. It's pretty neat. I'm looking forward to becoming more familiar with how to use the unit over the long Winter evenings to come!

[Waverider]

Originally Posted by adeftereos

Thanks waverider. Sending PM shortly.



I've tried to be as thorough as possible in my reasearch and, from What I've found out so far I think the Mk2 is perfect for my needs. (I mostly play with radios from the 30's to the 50's.)


What really struck me is the brilliance of the design in terms of allowing the testing of pretty much any valve, with just the valve manufacturer's data.


I've never come across that facility before. It's pretty neat. I'm looking forward to becoming more familiar with how to use the unit over the long Winter evenings to come!

Yes, the Mk II VCM is a great tester, it tells you everything that the Mk III and the MK IV will tell you but it's easier to use and it has a less fragile meter and it's physically easier to get internal access to service.

I use it in conjunction with an etracer computer controlled curve tracer. The Avo does all of the hot testing and if the tube passes the Avo testing I move it to the etracer for high resolution testing and printouts.

[Craig Sawyers]

Originally Posted by adeftereos

Thanks waverider. Sending PM shortly.



I've tried to be as thorough as possible in my reasearch and, from What I've found out so far I think the Mk2 is perfect for my needs. (I mostly play with radios from the 30's to the 50's.)


What really struck me is the brilliance of the design in terms of allowing the testing of pretty much any valve, with just the valve manufacturer's data.


I've never come across that facility before. It's pretty neat. I'm looking forward to becoming more familiar with how to use the unit over the long Winter evenings to come!

The only thing it won't test correctly is high power triodes like the 6080, often used as a series regulator. With an ra of 285 ohms and a current measuring resistor in the usual series of AVO valve tester of 200 ohms the measurements will be incorrect. Usually a perfectly good 6080 will measure low gm because of this.

The AVO settings usually have an asterisk next to any valves of this nature.

Craig