Universal Testmeter Unit No. 107

[Paul Adams]

This has been a long and drawn out battle, I acquired this Universal Testmeter No. 107, (like this one of PMM’s) but mine didn’t have much going for it, it had extensive damage to the Bakelite down the left hand side, a cracked front, movement damaged and if that wasn’t enough there was also another problem, the original fixed Bakelite terminal at the top LHS was broken.



I first looked at replacing the fixed terminal which I thought would be easy! With one from an Avo 7 Mk1 but the height of this was different to that of the Universal Testmeter.

I cut the Avo 7 mk1 terminal to the same size as the one on the RHS of the Universal Testmeter as this was undamaged, the only tool to hand was that of a junior hacksaw and some masking tape, (a lathe would have been ideal) after wrapping the tape around the donor terminal at the correct height, I proceeded to cut around the terminal until I reached the centre, the spoil was removed and finished off with a file, unless you look very closely you cannot tell the difference from the other on the RHS.

The crack nearest to the Pheon on the LHS of the front was repaired using Araldite, sash clamped and left to set for 24hours.

Now to tackle the front panel damage down the LHS, I tried cutting a replacement section from an old Avometer using a ‘Dremil’ and a cutting disc, but the dust and the smell of burning Bakelite was horrendous.

Plan B!…… I purchased some latex dripping rubber, in order to make a mould using another Avometer, a number of coats of this liquid rubber was applied to the LHS of another donor Avometer and was left to set for 3 days, also a coat of Araldite was applied to the top surface of the rubber mould to try and give it some rigidity when removed from the donor Avo.

Finally the rubber mould was removed from the donor meter and transposed the mould onto the damaged Universal Testmeter, the difficulty came when trying to keep this mould in the correct position ready for filling with epoxy resin.

I tried a number of methods of clamping the mould into position, but found the best was with ‘clothes pegs’ and a battery alligator clamp, as other methods were distorting the rubber mould.



Now came the fun bit, filling the damaged area with epoxy resin, I thought about using all of the dust collected from the Dremil experience, mixed with slow setting Araldite, but dismissed this idea as air bubbles were trapped in a ‘test’ piece that I tried.

I eventually opted for a sealed two pack black epoxy resin, with the same viscosity of Araldite, and the same curing time, with no air bubbles also it’s black and almost the same colour of the Bakelite, which I obtained from here. (no connection with the company, just a satisfied customer).

The most difficult part was aligning the mould in the correct position, in order to retain the continuation of the arc to the left of the movement aperture, it took 4 attempts at this due to some slight shrinkage of the rubber mould, each time removing the set epoxy resin, I was ready to pull out my hair! But got there eventually.



I then looked at repairing the meter movement, which at first I thought was just the rear hairspring had broken away from the centre collet/pivot assembly, but after successfully re-attaching this, the coil was found to be o/c, also I couldn’t see the break using an 60x magnifier, so I needed a replacement movement.

In the original Universal Testmeter No. 107, the FSD was 100uA, not one to give up on this, I decided to use a movement from an Avo 8 mk III, but in doing so the FSD on this is 37.5uA, so a bit of experimentation was required with shunting and series resistance of the Avo 8 mk III to 100uA FSD, (checked on a current calibrator) I would have liked to have kept with it’s original movement, but this was not possible. (unless someone wants to volunteer to rewind the original coil)

The meter is looking a darn sight better now than when I acquired it, although the repairs to the LHS of the Bakelite are not totally invisible, I am more than pleased with the outcome and it‘s functioning, although probably not within calibration due to the non linearity of the scale plate/movement, well worth all of the effort in repairing this Special Avometer.



Many thanks to PMM, for his extensive knowledge on these type of meters and his encouragement via emails.

Looking forward to any comments on this monstrous repair, and hope that I haven’t bored everyone by now.

Paul.

[newlite4]

Nice job Paul, this is one of those exceptionally difficult repairs, well worth the effort since as much of the original case has been retained as possible. Araldite repairs can prove to be challenging to say the least.
Neil

[Phil G4SPZ]

Paul, that repair is nothing short of heroic! Fantastic work. There can't be many of these instruments around, so well done on repairing and preserving it.

[threeseven]

Thats an epic job, you have more patience than I do!

[pmmunro]

Paul,

It's heartening to know that this rare meter found its way to someone who was able and willing to persevere with such painstaking restoration work.

As we have discussed, there may have been only a few tens or possibly hundreds of these meters made and almost certainly all for the Air Ministry. Who could say how many have survived?

Your write up shows what can be done with care, determination, skills and patience. This should be an example and encouragement to others with several useful tips for cases where they are needed.

All of us who can't help wishing we could save all the Avometers (or other worthy equipment) which are in need of attention will feel grateful to you.

PMM

[Paul Adams]

Wow, I wasn’t expecting such kind comments regarding this repair, I really enjoyed repairing and restoring this meter, and your comments make it all worth while.

I did ring around a couple of Meter repair companies, to see if I could get the original movement repaired, one company in particular (Herts Meter Co) wanted…hold your breath, £100 +VAT, it would be nice to have the original movement back in, but I cannot justify that cost on this type of instrument.

Paul.

[Phil G4SPZ]

Originally Posted by Paul Adams

...one company in particular (Herts Meter Co) wanted… hold your breath, £100 +VAT...

Put in perpective, Paul, that probably represents less than two hours of a skilled technician's time. For example, my car dealer's hourly rate for a mechanic is £110 per hour, I kid you not. The real cost of manufactured goods has fallen so far in recent times, to the point where the sustainable cost of repair appears extortionate, but it's simply the fact that the repair costs are linked directly to present-day wage rates whereas most manufacturing is done by robots and computers. As a result, things get written-off pretty quickly nowadays.

The other thing is that when your meter was bought new by the Air Ministry, I bet they probably paid the monetary equivalent of the best part of a thousand pounds for it. A repair costing £100 would therefore have seemed worthwhile at the time.

We're lucky in our hobby, in that we can continue to enjoy using skills and workmanship that have become commercially unaffordable.