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 University MVA-6 vacuum tube voltmeter.
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 Return to top of page · Post #: 1 · Written at 8:47:19 PM on 9 November 2021.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 979

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

 Return to top of page · Post #: 2 · Written at 1:18:59 PM on 10 November 2021.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1727

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.

 Return to top of page · Post #: 3 · Written at 3:17:05 PM on 10 November 2021.
GTC's avatar
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6268

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

 Return to top of page · Post #: 4 · Written at 6:52:45 AM on 11 November 2021.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 979

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.

 Return to top of page · Post #: 5 · Written at 12:41:51 AM on 12 November 2021.
Brad's avatar
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6663

Document uploaded.

A valve a day keeps the transistor away...

 Return to top of page · Post #: 6 · Written at 10:08:44 AM on 12 November 2021.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1052

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.

 Return to top of page · Post #: 7 · Written at 10:50:20 AM on 12 November 2021.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1052

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.

 Return to top of page · Post #: 8 · Written at 2:13:54 PM on 12 November 2021.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 979

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!


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.

 Return to top of page · Post #: 9 · Written at 7:38:34 PM on 12 November 2021.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1052

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.

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