Avo VCM163 - Intermittent

[Bazz4CQJ]

The filament volts supply on my 163 has become intermittent. The problem is affecting a several (probably all) of the sockets and does not depend on the valve being tested. The red "Power on" light is on, so there's 6V coming out of the transformer.

The filament voltage is present up to the LINK connector (i.e. voltage is present between the feed side of the link and the cathode supply) but when a AC ammeter is inserted in to the open LINK, it confirms that no filament current is passing. Tweaking the various function or selector switches doesn't seem to have an effect; seems like a bad joint as it will sometimes cure itself without anything being done to it..

I guess that points towards a connection on the "h-" line. Anyone have any suggestions of where to look?

Thanks

B

[Dekatron]

I can only recommend you to look everywhere, trace every wire in the heater circuit from the transformer up to the valve panel.

I have repaired a few VCM163s where there has been more than one dry joint, even unsoldered joints, and also shorts between the wires and the case and also from wire to wire.

All of these defects have passed the inspection, probably due to that they made enough contact with new clean wires but over the years corrosion affected the wires and joints so the contact/joint became intermittent.

I've also repaired broken switches which also gave rise to intermittent function, some where the rotating electrodes passed on top of the outer electrodes on some occasions with no contact at those positions and some where the deck was broken moving around when the shaft was turned so sometimes there were contact and sometimes not.

Very gently pushing and pulling of the wire looms might help you to find where the problem is. Don't push nor pull to hard as you might break switches and they aren't easy to replace or repair.

I keep my fingers crossed that you find a dry joint, or two, and nothing more severe.

[Bazz4CQJ]

Thanks for the comments Martin.

I've been looking at previous threads on the Forum about refurbishing the selector switches, but I think they mostly refer (in detail) to models earlier than the 163.

My impression is that the wheels on the 163 are a bit different to the earlier ones (they do have more wheels, are a different colour and I suspect their construction is a little different, perhaps made by a different company?).

What strikes me about them is the very short/tight connections made to them by the wires from the wiring harness. The wires, usually in pairs, are single core, so quite stiff, and just about long enough to permit one ferrite bead to have been inserted.

I think it's likely that removing the wrapped and soldered wires from the selector wheels would be extremely difficult. It seems to me that it might be better just to cut them as close the selector tags as possible and then insert a new, short length of wire, wrapping that around the old wires and the tag before soldering. This might lengthen the lead perhaps to the point where two ferrite beads might fit. I think that cutting is likely to present less chance of damage to the harness or the selectors than trying to de-solder and unwrap the 15 sets

However, I'd certainly be pleased to hear what others have done.

B

[Craig Sawyers]

I've certainly done the roller switches in my AVO valve testers (CT160, Mk III and Mk II), and yes - you have to desolder the wires. I did the same on an old 2-box AVO, but sold that one some time back.

I don't know the internal anatomy of the 163 roller switch, and certainly could not afford one even if I got the chance. But yes - the colour is different, there are more positions on each roller, and there are more rollers too.

Craig

[Dekatron]

I would only do anything to the roller selectors on the VCM163 if one or more was broken and I had to replace them, otherwise I would leave them alone. It is quite hard to remove the wiring without damaging the insulation and you also risk getting solder blobs and stuff into the roller selectors while de-soldering the wires.

You can't open the roller selectors on the VCM163 to get to the contacts inside as the two plastic pieces were pressed into place during manufacture when the plastic was softer, now they break easily. I think I posted photos of a few broken ones here on the forum but can't find those photos right now.

I have replaced a few roller selectors on the VCM163 and it was a really time consuming process, not only from de-soldering and pushing them off the center rod and the thinner contact rods but also from aligning them again when they were put back.

My guess is that you have a dry joint either at the transformer or at one of the rotary switches on the front or on the brown SRPB on top of the left transformer (as seen from behind) where many wires meet. It can also be a broken solder lug at the back where the wire loom from the tester meet the valve panel on top.

[Bazz4CQJ]

Many thanks for that dire warning, Martin. Like you, I don't believe the intermittent I'm seeing is associated with the selector wheels, but I was contemplating a major "servicing" of the 163. The selector wheels are not as free-moving as they used to be, but from what you say, the only way forward seems to be gentle and patient with them. Thinking about it, I'm pretty much at that stage myself .

B

[Dekatron]

Here are the two photos that show the innards of a broken roller selector. The different coloured arrows show where each piece goes. The white smudges where the bearings balls are placed is the type of grease that dries out making the movement stiffer, not only by blocking the bearing balls but also by floating around inside the wheel itself and making things move poorly. The left part of the wheel is pressed down into the right part of the wheel and is kept in place by three small ridges, which easily break off like on the wheel in the photo, the bottom right one has broken off right where the green arrowhead is - this can sometimes make the wheel intermittent but it is usually kept closed by the other two and the metal piece extending to the wires.

I wish I knew what kind if grease they used and a way to dissolve the stiffened one. Unfortunately the white lettering is painted on and then has a clear layer of lacquer on top and it easily comes off so it is not possible to dip the roller selector into some solvent as all of the lettering would come off.

[Bazz4CQJ]

Mail sent Martin.

[Ed_Dinning]

Hi Martin, the white grease may well be lithium based grease that was used in wet areas on cars in the 50's and for brakes. It was also used on clothes wringers that were attached to washing machines. Alcohol should be a suitable solvent.

Ed

[Dekatron]

Hi Ed, thanks for the information!

I actually have some different lithium based greases that I got for free from Dow Chemicals (I think it was them anyway) and I'll see if it dissolves the dry one properly and get the loose ones I have (unfortunately broken ones) up and running smoothly again.