bwd 821 CRO: sourcing a 3Meg 1kV pot (and were all designers this mad?)

[Whales]

BwD 821 analog oscilloscope from the 80's.  Nothing fancy, 2 channel 50MHz, seems to be in working condition apart from a dodgy focus pot.

The old pot is FUBAR.  Probing around the carbon track on the inside reveals sudden discontinuities (suggesting cracked or worn away).



It's 3Meg according to markings and the schematic (thankyou archive.org).  My brief measurement using a totally-not-600V-max DMM (woops) reveals there is most of 1000V across this pot.

Question 1: where do I source 3M panel pots from?  eBay has nothing, digikey wants a good chunk of 30AUD each or more and seemingly does everyone else who has one in stock.  If I pay for shipping and some spares then I end up out of pocket by around 100AUD or more just for a 3Meg panel pot replacement 

At this point I'm contemplating putting three 1M pots in series and only making one accessible from outside the case.  Suggestions welcome.


Question 2: what madman puts thin insulated wires carrying 1kV touching (let alone near) the 0.5Vpp squarewave cal output? 

White wire = 1kV. 
Metal stud out of green insulator = cal output.



Seriously, am I the sane one for once? 

[Ian.M]

Make a chain of 10x 300K resistors, replace the pot with it and see which tap of the chain gives the best focus when fully warmed up, then fit a 1Meg pot with fixed resistors in series with the track ends for a total of 3Meg, calculated so that the in-focus position is near mid-travel. 

As for the wiring proximity to the Cal output, its not great, but it looks like the cal circuit is low impedance (I can see a resistor to chassis), and the 1KV feed is almost certainly fairly high impedance, so even if it flashes over its unlikely to cause permanent damage.   However I would strongly recommend clear heatshrink or clear silicone sleeving over the paired 1KV and Focus wires.

[xavier60]

I owned one of those many years ago. I used a TV focus pot. They are usually 10MΩ.
I can't remember exactly what I did, I guess I put a  resistor across it.
https://www.wagneronline.com.au/attachments/Catalogues/CAT-TV-NP.pdf

[Whales]

Doh, of course I didn't check WES.

They have a 3Meg pot but in an anti-log taper. 

I might go for whatever linear value and then parallel some resistance, exact linearity and such is probably not too important.  Thanks Xavier.

[CaptDon]

I think you only see 1kv because the pot is open. A typical focus voltage
for a low budget scope is around -250 to -350 volts referenced to chassis.
That pot was never designed to handle 1000 volts, the spacing is not great
enough for a metal chassis mounted pot!!! Maybe 500 volts max on any
terminal referenced to the shell, and maybe no more than 500 volts max
from end terminal to end terminal.