Avo 7 Mk 2 Repair

[Sinewave]

Hello.

Does anyone know how to remove the range selector dials from one of these?

I'm stripping one down as the switch contacts are dirty making intermittent connection.

I suppose I could try cleaning in situ, but if that doesn't work, then I'll need them out.

I've so far removed the finger contacts from the DC range switch which leaves me with the rotary contacts. When I remove the round plastic peice from under where the finger contacts are (which isn't keyed and I didn't mark it, so will have to open another meter to find out its azimuth alignment) from what I can see, there's two brass hoseshoe connections. These are held in with a metal peice which looks like a securing rod is tapped in with a hammer.

How they did this I don't know, or how I can access it to remove it.

Any ideas? Should the plastic dial on the front pull off giving me better access? I have tried with sensible force, but any further force feels a bit much.

Thanks.

[The Philpott]

Pause!

The bad contact is FAR more likely to be in the leaf switch stacks than the rotary sliding contacts. The rotary contacts, unless they have arced or been contaminated, are tough as hell. You do have the opportunity to clean them a little though, now you have stripped the meter down somewhat.

The procedure below needs lots of light, magnification, and a few slips of white paper to slide behind the switch stacks to improve visibility of the movement of the bendy switch leaves:

When this problem happens the first attempt can generally be made with the leaf stack in situ- a small amount of black oxidation on these contacts can completely compromise the switching even if the contacts are correctly aligned. This preliminary cleaning can be done with a thin strip of cardboard cut from a loo-roll tube- it is soaked in isopropyl alcohol and pulled through each of the contacts with tweezers, while the rotary selector is manipulated back and forth to 'grab' the cardboard between the contacts with a little pressure...

Once this has been done a dry piece of the same cardboard can be used in the same manner, which removes any stubborn residue. This grade of cardboard is not aggressive enough to cause any undue wear.

If this does not solve the poor contact, a small amount of tweaking can be done to the affected leaf/leaves, with point-nose or point-nose kinked pliers. This is a recommended avo repair where no new switch stacks are available, but is a little fiddly with the stacks installed in the meter.

I say again a x5 or better a x10 loupe is pretty vital, along with the white paper to act as a reflector. The movement of the switches in operation is quite small and over-tweaking of them can mess the system up. A little at a time is best, then test!

As you have found, the cam discs are not pegged to the rotary switch, so can rotate at will if the central set-screw is loose. A design flaw. They can be reset fairly easily, but having a second meter to hand is an asset...as is the appropriate circuit diagram, as it gives you the open and closing protocols.

Dave

[The Philpott]

I should add i have never looked into the procedure to dismantle/remove the rotary switches since i have never needed to get in that far, only one meter suffered a selector that was defective (it was actually seizing up) and i suspect that some carborundum or grit had got in where it shouldn't, or a steel ball had broken up inside. This particular meter had other faults as well so was put to one side as a parts mule.

[Sinewave]

It's almost certainly the rotary contacts, as playing with the finger contacts has made no difference, yet if I put a screw driver across the rotary contact with the drive also touching the stationary contact which it makes, then the reading stabalises.

I'm going to see if I can get in there with some contact cleaner on a cotton bud.

[The Philpott]

Interesting! I have had one where i had to increase the clearances to allow the rotary contact to go through the sprung contacts, but it sounds like you could have the opposite problem.. Do you have a burr on one edge of the rotary contact where arcing has created little metal balls on the surface? This would tend to knock the sprung contacts out of alignment. Sorry to not be of more help. I can certainly see why you are wanting to dismantle to the extent that you are.

Dave

[Sinewave]

Right, so according to the circuit diagram, I think the cam disc is to only move the finger contacts when wanting to select an AC range. So on any other range on the DC switch, the finger contacts shouldn't be made.

Edit: actually, as there are two sets on this switch, I may be working from the Mk 1 diagram.......I'm going to have to try and find a Mk 2 diagram.

[Sinewave]

Ok, I think I've got it. So it's working a lot better now. The intermittent connection/reading is stable.

I've now just got to address that it reads a touch under on the 10mA and 100mA range, which is unusual, when the voltage ranges tested so far are reading ok. 2mA range reads fine too actually.

[The Philpott]

Good progress. If you are using low voltage to push 10mA or 100mA through the meter, is it worth using a slightly higher voltage and seeing if the situation alters? Not sure. It's usually with larger currents than this, that low voltage causes a drop in the reading.

I have a 7/Mk I that i could compare the current ranges with at the weekend if need be.

[Sinewave]

Having another tackle of this tonight.

10mA range I've got reading within 1% FSD.

100mA range I'm out by 3% FSD.

I've tackled some dry looking joints and cleaned contacts on switches.

I think I need to go through the circuit some more and see what else I can do.

[Sinewave]

I assume these are the wire wound resistors for the mA ranges? The 1A and 10A range look as though they've been burnt out, though I've not tested those ranges yet.

[The Philpott]

Yep. those are them.

What voltage are you using to push current through the 10mA and 100mA ranges? If you can confirm i will replicate it with my early model 7 and check accuracy.

[Sinewave]

I'm using a Time Electronics Microcal 1030, I'm not sure what voltage it uses.

I wonder how the 1A and 10A ranges have been burnt, the dissipation shouldn't be in the meter should it?

I wonder how easy it would be to wind some new ones. Perhaps if I can match the gauge and length, then it shouldn't be too difficult, since the resistance isn't actually very much at all anyway.

[The Philpott]

I do have few meters where the 1A shunt has been roasted, and also one where the 10A shunt has been roasted. Without exception they still seem to function to a degree, although i haven't tested their accuracy..

Rather than replacing individual current shunts it is-arguably- worth swapping out the whole board for a clean one, although this might cause small recalibration issues. When these shunts have cooked they damage the board which swells and stinks- this smell won't go away while that board is still in the meter!

I have never managed to destroy a shunt by misuse, i can only assume that people leave them on excessive current load for too long in order to cause this sort of damage. The 10A range can cope with 8 or 10amps but how long would you leave it running with that load..? (I leave that as an open question!- it's one that has been discussed before)

I have just run a test as follows on my Model 7:

12v @ 85mA, meter set to 100mA range.

The meter reads low as follows- Minus 1.1% of reading, Minus 1% of FSD.

I don't have a circuit diagram in front of me, but i THINK the current shunts are arranged in series on the Mod.7, so your slightly errant 100mA range might be being thrown a little out of cal. by prior heat damage to the higher current shunts; when you are measuring on the 100mA range, if i am correct, you are in effect using 3 shunts- 100mA, 1A and 10A.

This effect would be proportionately much less when using the 10mA range, which could explain why you are within 1% FSD on that range.

Dave

[The Philpott]

I just selected my worst Model 9/II which i know has quite badly scorched 10A and 1A shunts, and checked the 100mA range for accuracy. In my case the 100mA range has not been adversely affected......Not really the result we were looking for!

Other factor/s could be involved.

[Sinewave]

I've just tried my other 7 Mk2 which reads out fine. I was doubting my calibrator, but it's ok.

I've got three 7 Mk2's under this bench to compare with, plus various model 8's and 9Mk2s to compare this to.

So there's still some gremlin inside this 7 Mk2.

Last night I left it reading spot on after my last post and today it would only read correctly when verticle, but not horizontal, which is indeed odd.

[The Philpott]

Do you have any problems getting the needle to zero consistently? If so:

To check the movement is balanced, zero it whilst horizontal, then sit the meter vertical.

Tilt it to the left (about 45 degrees) so that the zero line is horizontal. Check that the needle is still at zero, plus or minus 2%. This quantifies whether the tail-weight is correct, which is the most critical balance weight.

Bring the meter back vertical, then tilt over to the right so that the zero line is now vertical. The needle should now sit on 'zero' plus or minus 2 degrees or thereabouts. This quantifies whether the side-weight is correct, but i have hardly ever had to adjust the side weight/s as the tail weight is far more influential.

I'm clutching at straws here, but it's an easy thing to check, and so could be worthwhile..

Dave

[Sinewave]

The needle zeros ok consistently in the same orientation, but does need to be adjusted for it's zero when going from horizontal to verticle.

I've just had another look at those shunt windings. I had a fiddle with one of the soldered connections, the one which is to the left of the burnt ones. I think I may have removed a short in doing so, as now both 10 and 100mA are back working properly. Perhaps when I was laying it down (with the back off) I was making a short.

Well I say properly, the readings are now around 2% FSD.

[Sinewave]

I've just tested the 1A and 10A range.

Both are also 2% FSD, so there's some consistency. But where I've lost that 1% I don't know.

The 10A range was tested with AC, so just scrapes by within tolerance, but the DC should be 1% max.

[Sinewave]

Hmmm, very intermittent, comes and goes.

Went to reading even worse, a few positive taps to the case with my fingers and it shoots back up to 1% FSD.

It's a strange one.

Mmmm, give the Q dial a ping and it reads better too. There's a sprung connection there which runs to the finger terminals on the A ranges for DC. I think poor connection there needs investigation.

[The Philpott]

(The ac current ranges are reliant on the transformer windings, not the dc windings in the pic.)

The Q prod can be a nuisance. I autopsied and repaired an intermittent one on here recently....and that was on a model 7. No great surprise.