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Yet another Mullard HSVT restore...

 
Toibs
(@toibs)
New V-Ratter Registered

Spurred on by @Crustytv 's Thread from a good few years ago (2012!), have dug out a Mullard HSVT Mk4 in the hope that i can get it functioning alongside my Taylor 45D - therefore allowing a quick Go/No-go test on a couple of boxes of valves i have kicking about...

Hopefully i can gain a bunch of knowledge from you guys here who are FAR more knowledgable on these than i am... I'm a engineer that was trained on post-valve tech, and not dark arts as Valves - something i would love to (and am slowly) learning more about.

So - a bit of history. This has sat, unloved and uncared for in the corner of my workshop for a couple of years, after picking up a couple of years ago from a guy in Devon. Full (as full as you can get) set of cards... including the all important Mains/Reject/HT cards.

Thought i should bring this up properly and slowly since im unaware as when it was last used... so used my Variac to wind up to 1/4, 1/2 and then full value over the course of a few hours. Stuck the mains card in and pulled the "Bandit Handle"... and... nothing.

Guessed that i should really be doing this properly and replacing at the very minimum all the electrolitics as they have a life span of 25 years, and after all... the machine is at least 50 years old.

Caps replaced and finally changing the two large 4uf elects, decided to give it a fire up again... and i get a Dot!!! it was about 5mm too far south, and therefore i tried to tweak using the mains adjust and the x adjust to centre... Over a period of 2-3 minutes i watched the dot migrate off to the left of the screen, and then disappear altogether... 🙁

Calling on the kind nature and experience of the people on here... I have a few questions please...

1. Does anyone have a list of expected voltages on a working HSVT please? I would like to, if i can, break the cct into sections and confirm what is being seen in each stage.

2. Has anyone had experience of the above, and can advise on their experiences to Investigate/repair?

3. Any knowledge greatfully received on these 🙂

Many thanks

Paul

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Topic starter Posted : 23/02/2021 12:08 pm
Marconi_MPT4
(@marconi_mpt4)
Reputable V-Ratter Registered

Hi Paul  and welcome to the forum.

A dot, albeit for a short time is a good start. Given the age of this HSVT, it might be a good idea to first check and replace every electrolytic and wax capacitor showing dielectric insulation leakage.

There are a few voltages mentioned in Crusty's thread here , particularly around the differential Y amplifier V2 and V3 (EF91). The anode voltage for each should be the same for a centred dot. A quick check might be to temporarily link the two anodes together to ascertain if the spot centres prompting further checks around the amplifier.

There may also be a few high value resistors (typically those marked >= 47k) that have drifted out of tolerance.

Rich

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Posted : 24/02/2021 8:29 pm
Toibs liked
Toibs
(@toibs)
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Many thanks Rich (and for the Welcome!)...

All caps have already been replaced... Including the infamous 4uf beasts 🙂

Intention now is to, as you mention, Check voltages around the Differential...

With respect to resistors, i have tested a few of them, most of which are within tolerance, however i wasnt sure if there were any resistors that needed special "attention" so to speak...  I have replacements for the 3.3M/2.2M/1.5M on the way... thanks for the information around the high value resistors... 👍 I'll double check the current values with special attention to those greater than 47k...

Is there any type of resistor that is more prone to "failure" over others?? eg High Stability/wirewound/Carbon??

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Topic starter Posted : 25/02/2021 11:36 am
freya
(@freya)
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Wasnt there something about transformer saturation/leakage  causing the dot to go off the screen, might be the early version im thinking of.

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Posted : 25/02/2021 1:14 pm
Toibs
(@toibs)
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Posted by: @freya

Wasnt there something about transformer saturation/leakage  causing the dot to go off the screen, might be the early version im thinking of.

Thanks for the heads up - i cant see anything, however i suspect im not looking in the right place 😀

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Topic starter Posted : 25/02/2021 2:50 pm
Marconi_MPT4
(@marconi_mpt4)
Reputable V-Ratter Registered
Posted by: @toibs

Is there any type of resistor that is more prone to "failure" over others?? eg High Stability/wirewound/Carbon??

Usual suspects are carbon composition resistors. They do have one advantage in being non-inductive but for 95% of applications modern high stability resistors are far better. When replacing  carbon types in valve equipment be careful to choose a modern resistor with adequate voltage rating particularly around CRT power supplies. DG7-5 is typically operated at around 800V between anode and cathode.

I had a quick look at the schematic for E7600/4 and it is possible for leakage between mains transformer windings to affect the amplifier balance! So, T1/H winding  supplies rectifier diodes V9 and V10 to produce ±300 to 400V where the centre is connected to C5/RV5. If this connection becomes biassed either through a leak to ground or a voltage injected, it will cause the voltage on V3 g1 to change and so unbalance the differential amp  and move the spot.

My suggestion would be to initially disconnect and isolate all circuitry from windings T1/H, T1/F and T1/G. Might also be a good idea to disconnect all secondaries. Check insulation between these windings and all the other transformer connections with no leakage allowed at 1000V.

Rich

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Posted : 25/02/2021 8:13 pm