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Old 11th Nov 2022, 12:00 am   #1
woodchips
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Default MHSVT - leave on between valves, can the CRT deflection be interpreted

Not found these points described in either the CT80 or HSVT documents or forum question, but there is rather a lot of search results, some with lots of pages.

If testing lots of valves, can the tester heaters be left on so no preheat needed for every valve. Idea being to reduce wear and tear on the heaters.

It is a go-no go tester, but can the CRT deflection be interpreted so so many millimetres is so many mA/V? Obviously different for every valve type, but in reality if bulk testing then the valve Ma/v etc can be determined on a VCM and the deflection noted.

Using a VCM does seem a very slow way of sorting valves.
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Old 11th Nov 2022, 9:26 am   #2
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Default Re: MHSVT - leave on between valves, can the CRT deflection be interpreted

As far as I am concerned it is OK to leave on between valves, I understand that when in use back in the day they woud be left on all day.
The MHSVT cannot measure mA/V (gm). I have done some work on modifying one to read gm.

https://www.vintage-radio.net/forum/...d.php?t=149561

Peter
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Old 11th Nov 2022, 11:38 am   #3
woodchips
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Default Re: MHSVT - leave on between valves, can the CRT deflection be interpreted

Ah, sorry, mA emission not mA/V.

All my testers and paperwork are miles away in storage, trying to work out if it is useful to quickly sort valves. Rather what it was designed for of course, but mine isn't working, and like the VCMs not trivial things to fault find.

Any comment on using the CRT as a analogue output tester? How repeatable would it be?

But, the gate switch includes the mains in its contacts, so how does the tester stay on with the gate open?

Last edited by woodchips; 11th Nov 2022 at 11:57 am. Reason: Mains on gate switch query
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Old 12th Nov 2022, 2:05 pm   #4
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Default Re: MHSVT - leave on between valves, can the CRT deflection be interpreted

The power switch turns on the valve heaters, the gate does everything else. Cathode poisoning from leaving it powered on kills the EL37 and other valves. The deflection is fairly linear and repeatable but difficult to read accurately.

Valves that have not been used for years can't be quickly tested as they need time to settle although you can weed out the catastrophic failures.
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Old 12th Nov 2022, 6:37 pm   #5
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Default Re: MHSVT - leave on between valves, can the CRT deflection be interpreted

If you should kill the EL37 an EL34 can be sucessfully substituted providing you add a link between pins 1 and 8. You may find pin 1 (not used on the EL37) has been used as a spare tag for components, if so lift them off and isolate (still connected to each other of course)

Peter
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Old 13th Nov 2022, 10:18 am   #6
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Default Re: MHSVT - leave on between valves, can the CRT deflection be interpreted

The quickest way to test valves I've found is wire up a valve base to a couple or more bench supplies with a DMM set to ammeter, Test conditions lifted from Avo or a valve datasheet/graph. Leave every voltage as is bung in a valve, note emission on the valve with a sharpy, onto the next. To speed things up further wire up a few more bases for heaters only.

After testing a few you get a feel for it, EG how a valve reacts when first inserted, how quickly the emission comes up etc.

Andy.
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Old 13th Nov 2022, 11:35 am   #7
woodchips
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Default Re: MHSVT - leave on between valves, can the CRT deflection be interpreted

Thanks to all.

I had forgotten it had an EL37, and an EL34 isn't much cheaper. Looking through the valve data book, why not use several of the lots and lots of old goggle box valves? It is not a very arduous job it has, just lots of mA and be able to withstand 300-400V. Hate to say this but even a transistor might work!

Even I have several boxes of them, collected in the 70's for my dad, now inherited. The line output valves are pretty meaty, need something to manage 400V at 100mA to replace the EL37, but most of the time you are not going to be doing emission testing on big rectifiers.

Valves will quite happily run in parallel, just look at the old 500V 250mA power supplies from the 1950/60's. Some load sharing resistors, and wire six PL802's together. Just use a bench PSU for the heaters, since the tester won't supply 16V.

In fact the ECC88 could do if you have lots of unwanted or failing ones.

Using bench PSUs is an alternative, just that the tester already has a bunch of mounted valve bases, and all insulated! Quite a bit of work even with Q-Max cutters to make up a panel for the bases.

Did hope that as mentioned, after a few dozen you get to know what a good or bad valve looks like.
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Old 13th Nov 2022, 6:39 pm   #8
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Default Re: MHSVT - leave on between valves, can the CRT deflection be interpreted

Quote:
Originally Posted by woodchips View Post
Thanks to all.

Hate to say this but even a transistor might work!
I have an on going project to fully convert a MHSVT to semiconductor. The CRT has been replaced by an LED bar graph display and all the rectifiers are silicon, the voltage stabiliser is now a zener.

Just got to change the HT regulator to silcon while retaining the card programming.

Oh and I will one day finish adding a gm function.

I have reported the CRT replacement on the forum somewhere.

Peter
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Old 6th Dec 2022, 8:23 pm   #9
woodchips
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Default Re: MHSVT - leave on between valves, can the CRT deflection be interpreted

Saga continues.

Now have the HSVT in my hands, a type /4. First problem was a Bulgin mains plug, solved by finding one. Next problem is the overload won't latch closed, so no power. Looked through my copy of manual, CT80, and a search on the forum and not turned up any description or repair info on these. I am sure I had seen something. but where.

Anyone else repaired these please? Or even how it works. Being valves can't be something simple like a 200mA, AC or DC, circuit breaker can it? Pretty inaccessible buried in the tester, does it explode in bits of metal and springs if try to remove?

Why do you have to search through a whole tin box of cards to find the test cards?
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Old 6th Dec 2022, 9:21 pm   #10
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Default Re: MHSVT - leave on between valves, can the CRT deflection be interpreted

First, put cards in numerical order, simples!

Try running up the unit on a variac or lamp limiter with the cut out held closed. This will show if the is a mains transformer(s) fault. All valves should be out for this test.

Note that the HT voltage selection is from the HT transformer taps. A fault on the switch can short out a section of the HT winding. It is well worthwhile fitting additional fuses in these leads as it is virtually impossible to rewind these transformers

Good Luck

Ed
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Old 7th Dec 2022, 7:40 pm   #11
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Default Re: MHSVT - leave on between valves, can the CRT deflection be interpreted

Make sure that the gate lever is vertical or that you have one of the test cards in the gate when doing initial tests.
A faulty interlock on the gate lever and no card would result in all the gate switches being closed with multiple shorts.

Peter
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