Got this meter working and will send a PDF story for Brad to pin to this post.

In it I describe the process of nutting out the many problems.
I found a matching circuit and a EA magazine ad for the unit.
I also had a small disaster as discussed in the story that someone may have experianced and can shed some light on.


Cheers, Fred.

University MVA-6 VTVM

Im in the position of being able to get a philips vtm in very good order. What do you guys think of them. I will try to get the model number when I get home.
I enjoy reading your endeavours Fred.

What do you guys think of them?

I have an AWA Voltohmyst. I don't use it all that often but it's there if I particularly need 1,000 ohms/volt sensitivity for anything.

I have converted it to be rid of the need of a battery for resistance measurements, and hence rid of the possibility of associated leaking and corrosion.

Here's a good write up on the subject: https://www.industrial-electronics.com/vom-vtm_1975_6.html

That link is a good write up on VTVM's.
I guess the thing about a valve type meter is you can stick the probe into a valve radio and will not hurt it when you touch a HT line or a plate circuit.
I am wary of what instrument I use on valve radios having blown the input fets on one of my CRO's.
That was a fun fix as it took out the FETs in the front end and the IC's deep into the dual beam switching circuits.
I always use a 10:1 divider probe on any CRO when working in a valve chassis, my Jaycar DMM's are all ok on valves.
Fred.

Document uploaded.

Die castings like the dial would have residual stresses in them from the dial cooling and solidifying and thus shrinking. Because it is constrained by the die stresses build up. While it is hot these stresses would relieve to some extent. But to get productivity the dial would be ejected from the die as soon as possible and then would cool fast locking in these residual stresses. Residual stress is not related to external loading. Bakelite radio cases are another instance of residual stresses from manufacture.

When popped on to the meter case an external load would be applied causing further stress to build up.

Over time some of these stresses would relieve, but some would remain. And these stresses would drive crack growth if the toughness of the material is exceeded at the tip of any preexisting cracks. Polycarbonates are tough as shown by typical applications like headlight lenses, car bumpers, baby bottles and mobile phone cases

For the dial to sit there for years without cracking means that the inherent toughness of the polycarbonate was adequate, until either heat or vapour or both, decreased the toughness, or increased the stress, or both. It is also possible that the toughness was decreased by some other environmental factor over time. Maybe paint thinners, turps or some other vapour like the electronic cleaner.

The slow growth of the cracks is indicative of a tough material - if it was brittle it would go with a pop. A maker of quality instruments wouldn't compromise quality with a brittle material.

https://www.bpf.co.uk/plastipedia/polymers/polycarbonate.aspx says

"Resistance to Chemicals

Dilute Acid -Good

Dilute Alkalis -Poor

Oils and Greases -Moderate

Aliphatic Hydrocarbons (petrol, turps)-Good

Aromatic Hydrocarbons (stuff with benzene rings)-Poor

Halogenated Hydrocarbons (eg carbon tetrachloride, trichloroethane, some paint brush cleaners)-Poor

Alcohols -Good

Note that these ratings are generalised. The resistance against specific substances can deviate and are dependent on temperature, applied stresses, exposure time etc."

Plenty of scope here for something to get at polycarbonate in a typical radio restorer's workshop.

Perhaps a warning to us to keep painting and cleaning activities separate from electronic work as much as possible, not just to prevent fires.

Applying Occam's Razor, that the simplest cause is the most likely, perhaps the heat expanded the case of the meter, applying a load on the dial cover where it is popped onto the case.

Hi STC, thanks, a good heads up on the subject.

"Next time" I will carefully remove the cover and site it well away from the cleaning spray/gasses/ banging around!

The really cute thing was how the 1 crack at the top LHS went from 3 or 4mm long and GREW before my amazed eyes!
It was like it was alive!
THEN the cracks from the stress points at the bottom appeared and slowly curved across to shake hands in the middle!

EEEK!

After the repair and the last photo in that, I put a dab of contact cement in each corner of the cover, let that dry and then removed the tape.
Of course the tape then left a residue on the cover so I carefully used Turps and a cotton ball to polish those off.
The polystuff did not react to the turps at all and the turps softened the tape residue nicely.
The cover is now useable being sparkling clear just with the big crack line across the front!

Cheers, Fred.

Must have been horrifying to see, slow motion train crash, hae.

If when a crack grows, the stress is relieved, then the crack will stop. The fact that it kept going means that the load was still applied even as the crack was growing. This consistent is with expansion of the metal case with heat.

Outside of my materials knowledge here - my experience is with toughness testing steel 10 -100mm thick.

Hi guys. I just bought a University MVA-6 VTVM in pretty good condition for its age, to add to my collection. In the process of looking for information on this unit I found Fred's article on his repair. All I was really after was the schematic. Mine works 'out of the box' so to speak, with a near new 12AU7A fitted but a weak 6AL5 (luckily I have a valve tester). The battery holder is broken but no leakage from the silver battery still in the holder - lucky, but it is marked 'leakproof'! I thought I would comment on the cracked meter 'cover' episode. By way of introduction, I'm a crusty old electronics engineer and over the years have done a lot of design and repair work. This has involved using plastic enclosures and materials for various purposes. So, I have routinely had to check the chemical compatibility of a range of plastics. A few commonly used in the industry are Polycarbonate, Acrylic and ABS. From my experience, most meter covers are either Polycarbonate or Acrylic, because these materials stay relatively clear for a very long time. Now to the problem Fred had - the cleaner that Fred mentions in his note, contains Perchloroethylene (source = MSDS). Great for cleaning stuff but deadly on Polycarbonate and Acrylic. Even the fumes and vapours will instantly attack both plastics and cause structural failure in the material as Fred experienced - the mysterious growing cracks. The switch from using fluorocarbons to solvent based cleaners has been good for the environment, but bad for the plastics used in electronic devices. My two cents worth anyway...(shows my age I think). I hope this is useful information for those of us who love to restore old electronic stuff. I'm more into valve-based audio and test equipment. I used to own a beautiful RACAL RA17L, but on-sold it to a collector here in WA some time back.

Hi Richard and welcome!

Yes I am an old guy as well but in all my "experience" had never seen anything like the cracking.
Being used to dealing with old style Bakelite era electrics I never gave a thought about such a thing.

Yep I am so careful now about where I slop or spray cleaners.
Most times I now use kero or turps for bulk cleaning of chassis and parts and soapy water for plastic things.

I keep the "safe for electronic parts" aerosols and rattle cans well away from the work bench now!

Fred.