Universal Avo Test Set 1 zero problem

[MelMeter]

This meter is the high sensitivity unit and was previously owned by REME Workshops and last calibrated 1984..! When I use it to measure DC voltages of 3v - 12v and 15v it under reads ( 1v instead of 1.5v - 10v instead of 12v )
The needle reads hard over to the left on zero and when I adjust the zero screw, either way, nothing happens.
If I adjust the zero x 100 ohms or zero / 100 ohms the needle zeros fine using the relevant adjuster buttons.
Can anybody let me know what is the likely problem please?
Cheers
Mel

[The Philpott]

I don't think we can address the under-reading problem until the faulty zeroing is resolved. The Z screw in the front facia has quite a narrow slot in it and, for a start, it's vital to use a slightly undersize screwdriver otherwise stress can snap the bakelite screw cleanly in half! Easily done.

OK. There is more than one type of Z assembly behind the Z screw. Yours is likely to be a brass coloured coiled spring, the end of which pokes up and intercepts an elongated stirrup which adjusts the movement. (You won't know what the precise problem is until you look inside the meter, of course.) The second type, if i recall right, is a metal disc with a peg at it's periphery, and this disc is threaded (and usually glued!) onto the back of the Z screw.

If you get a little rattle inside when you GENTLY shake the meter it is possible that this Z assembly has come apart..... or it might be the case that someone has had the meter apart and failed to engage the spring with the stirrup.

It will help you to look at richard's radios website before you even open the meter up; he has some good internal shots of various avometers which will make what i have said clearer. There is a good chance that your meter is fixable. Just take it one step at a time.

i can do some pics myself if that will help in due course.

Dave

[The Philpott]

..Other (less likely) problems causing a bad zero would be

-tangling of the coil springs either side of the movement. (sometimes fixable)
-distortion/damage to these coil springs (not really fixable)
-movement balance weights need adjusting (not likely on this era of meter, but fixable)
-damage/stiffness of meter jewels and their spring loaded mounts (not good)

I still think we're looking at the Z screw simply not engaging with the stirrup though, as per my last post.

[The Philpott]

Typical stirrup and spring/peg assembly. (this one is older than yours) The spring is a squeeze fit onto the rear of the bakelite Z screw on this particular meter.

Stirrup SHOULD NOT be levered back onto the peg if the two of them are not engaged, although on this particular design it might be possible to persuade the Z screw out slightly through the front of the facia, then re-engage everything.

Viz:

[MelMeter]

Philpot, many thanks for taking so much time to help me out, now it is the weekend, I will carefully open up the meter and check the components you mention. Many thanks, as you are an expert on these meters, I also have acquired a Multi Minor Model 1, which is in perfect condition with a little manual in several languages. It doesn't explain though what the black button at the top of the ranges button is for? Would appreciate solving this one as well.
Cheers Mel

[MelMeter]

Philpott, managed to check the meter this morning and it is exactly as you described, the stirrup had come of the peg. Gently persuaded it back in position and zero now working perfectly. All the voltages now reading correct, obviously a big offset due to the bad zero. Many thanks for your help, it seems I am bound to become a collector now I have two meters..!!

[The Philpott]

Excellent, Mel, another one saved from the bin! (Expert? Nah, i've only been doing it a few years!)

I haven't got a multiminor myself but i think the button above the selector switch is likely to be the 'ohms zero' adjuster. Shorting the leads together (on the ohms ranges) you adjust the needle as close to zero as possible to ensure (relative) accuracy when you are measuring resistance. This meter will probably not excel at measuring resistance but it should give a reasonable indication.

Dave

[MelMeter]

Thanks again Philpot, it is as you say, ohms zero
many thanks