Mullard HSVT Problem

[Alf Fisher]

All,
Since my previous post on this on 7 May last year, I have now isolated and removed the offending contact in the fixed matrix. You can see where the bakelite has charred through where it passes through the metal chassis plate.

Has anyone seen this before and found a solution. I thought it could be something that could be made by someone who was skilled with a 3D printer.

Has anyone got a donor HSVT that could provide a replacement?

Alf

[Electronpusher0]

Take a look at switch J 10 (see the card layout attached).
I spent a lot of time tracing the function of each switch in the matrix. (Excel sheet attached) and could not find a function for J10.
J 1 to 9 select heater secondary tappings while K 1 to 8 select the heater primary tappings.

if it really is spare you could move it.

Peter

[Alf Fisher]

Hello Peter,

Thank you for your suggestion.
After more research and continuity checking, I found some connections on the drawing are missing - notably that of J10. J10 fixed contact is ground and J10 on the moving gate is connected to I1 through to I9 which is the rail that connects to different value resistors to get various grid bias voltages. So I reckon that J10 has the function of grounding the grid bias line when zero volts bias is required so you can now add that into your table of the contact functions. I did think of going through EVERY card to find those that used J10 but thought it would take far too long so I binned that idea!
I see you have described all these voltages in your Excel doc which is absolutely fantastic.

I made a similar template to yours where I scanned a card and on the image, I copied the images of holes into every location and labelled them with their identifying letters/numbers and then printed and laminated it. Then laying any card on top enables you to read the positions through the holes the same as yours.

Alf

[Electronpusher0]

Thanks Alf, I will add that to my Excel sheet.

Peter

[frankmcvey]

Hi Alf,

A forum member, Ti Pwun, was offering a 3d printing service for items such as knobs and Hacker grills etc. I haven't seen him around for some time, but you could try messaging him. If he can help, he'll require the old one as a pattern.

Cheers,

Frank

[Freya]

I found the exact same pin go in mine five years ago, it was in much worse condition than yours. A friend (now deceased) made a replacement on his lathe from PTFE bar.

[Alf Fisher]

Thanks for your replies guys and interesting to hear that you Stephen had the same problem. I thought more people would have had trouble given the numbers of HSVTs that are around in the hands of the likes of us.

I had a think which can be a dangerous thing! It doesn't need to be as complicated as the original as it is fixed. Its the moving gate ones that are spring loaded. The original bush has quite a wide square opening that narrows down to slot at the end which must be very difficult to do. A simple bush with something like a filed down 4BA round head screw would do the job.
I'll try Ti Pwun and see what he says.

Cheers,
Alf

[PJL]

Yes, T10 in my calculator confirms it is used to set 0V on the grid.

[Alf Fisher]

Hi PJL,
I assume you mean J10.

Alf

[Alf Fisher]

Hi again P,

What is your calculator and how does it work.
Alf