What type of pot for focus knob on analog scope?

[dzarren]

I have an older scope with a seized focus knob. The value is 500k ohms.
For a replacement, what type of scaling do i use for a focus knob, linear or log?

Thanks!

[orbanp]

They use linear pots there.
The only thing, that the pot is at a high voltage, 2kV to 10kV, depending on the CRT, and usually they use a plastic (insulated) shaft to the front panel.
If the pot is in the back, there is a plastic extension shaft to the front panel knob, then you are fine to use any pot.
If the pot is mounted directly to the front panel then you need a special pot with a plastic shaft.
You do not want to use there a "regular" pot with a metal shaft.
See if you can free up the original pot. Be gentle, it is easy to brake them.

Peter

[dzarren]

Okay great. I will find myself a linear pot then.


The shaft part is not plastic on the original, it is metal, as well as the screw threads for attaching to the face plate.
I very highly doubt i will be able to find a plastic shafted pot at the right value and right shaft size.


I wont be able to use the original pot, i was not able to free the shaft, and its broken now. it was not in good condition.


Are you sure i need a special pot, it seems like a pretty normal one thats on there originally. Here is a picture of it.

If you can link me to an appropriate pot i would greatly appreciate that.  The pot says 500M1 on it but is really 500k

[CatalinaWOW]

For future reference, and for others who may read this thread, it is not uncommon to find older equipment with frozen pots.  They can usually be freed with contact cleaner applied where the shaft goes into the housing and also in the body of the pot.  Take your time.  Try wiggling instead of a continuous one direction torque.  And once you get it moving, keep applying cleaner while exercising the pot until you get all of the gunk out.

[tautech]

I have an older scope with a seized focus knob. The value is 500k ohms.
For a replacement, what type of scaling do i use for a focus knob, linear or log?

Thanks!

Scope make and model please.

[james_s]

I've never seen a pot I couldn't free up with a bit of penetrating oil or contact cleaner and gentle application of pliers.

[tautech]

Your biggest issues will be wattage but more particularly voltage rating.
The Focus circuitry of a CRO is usually quite some high voltages so dielectric strength should be your first consideration.
After not too long look at what's available it seems metal body and cased pots max out at 500V which won't be enough.
So some idea of the voltages around this pot will help make a wise selection based on the expected dissipation and insulation requirements.
So scope model and study of the schematics should guide this choice.

This should give some idea of the complexities of selecting a suitable replacement:
http://www.farnell.com/datasheets/1718056.pdf?_ga=2.219707737.1371573998.1523346777-177881220.1515106758

[Gyro]

I agree with the others that it ought to be possible to mechanically free up the shaft.

If you do need replace it, then yes you need to be very careful of the voltage and wattage rating. Although the current into the CRT Focus electrode is very low, the track of the potentiometer normally sits in a high voltage potential divide chain comprising the Brightness pot, Focus pot, the Astigmatism pot, and a bunch of other resistors in order to provide the right range of voltages to each electrode. To provide sufficient adjustment range, there will be a voltage drop across the potentiometer track. Avoid the 50mW to 250mW types. From the look of the one in your photo and the relevant page on the Farnell site, I think you're probably looking at a 2 Watt part...

http://uk.farnell.com/w/c/passive-components/potentiometers-trimmers-accessories/rotary-potentiometers/prl/results?st=500k+potentiometer


Edit: Just an aside, virtually every 500k pot on Farnell UK is US stock only???  What's the world coming to?!

[Andrew Morton]

Edit: Just an aside, virtually every 500k pot on Farnell UK is US stock only???  What's the world coming to?!

They're all in their UK CPC warehouse:
http://cpc.farnell.com/c/electronic-electrical-components/resistors/potentiometers-trimmers-accessories/rotary-potentiometers?track-resistance=500kohm

[dzarren]

http://www.myvintagetv.com/Apple%20PDF%20files/C-B%20126%20Instruction%20Manual.pdf

Here's a link to the schematic. It is part number 7.
I thought that i would only need a Half watt pot, since if you look at that voltage divider stack where the focus knob lies, all of the resistors are only 1/2 watt.

I agree, I should have spent more time trying to free up the pot, But i had tried for a couple days spraying cleaner in there, and I could not get it to free up at all, so i took it apart and i destroyed it in the process. yes it was stupid, hopefully I have learned from this experience.

I have scoured all over my tow locally, and I wont be able to get anything suitable here in town, I bought a 1/2 watt regular style common pot today, but it probably wont be good for that to go in the circuit.

Would any of the two watt ones work for me on that farnell site, or would only a specific type work?

Ive narrowed it down to these two, would either of these be fine? One is RV4 and the other is 53 series.

http://canada.newark.com/honeywell/rv4naysd504a/conductive-plastic-potentiometer/dp/01F2649

http://canada.newark.com/honeywell/53c3500k/conductive-plastic-potentiometer/dp/95F5081?st=500k%20potentiometer


Sorry for a the noob questions, I'm just getting started for the first time doing this high voltage stuff.

Your biggest issues will be wattage but more particularly voltage rating.
The Focus circuitry of a CRO is usually quite some high voltages so dielectric strength should be your first consideration.
After not too long look at what's available it seems metal body and cased pots max out at 500V which won't be enough.
So some idea of the voltages around this pot will help make a wise selection based on the expected dissipation and insulation requirements.
So scope model and study of the schematics should guide this choice.

This should give some idea of the complexities of selecting a suitable replacement:
http://www.farnell.com/datasheets/1718056.pdf?_ga=2.219707737.1371573998.1523346777-177881220.1515106758

this link doesnt work for me

[Gyro]

Ha, well according to the Honeywell datasheet, the first one (RV4 series) is Mil spec. and the second (53 series) is the "cost-effective" version, however Newark are selling them for exactly the same price, so take your pick.

I can't actually see any difference in construction or rotational life between them. Both are rated for 500V element voltage and 1kVAC dielectric breakdown. 500V is well within the 2W max dissipation. I couldn't open the Newark links (probably cookies) so I used the Honeywell one... https://sensing.honeywell.com/honeywell-sensing-potentiometers-380-rv4-485-53c-385-datasheet-32301266-b-en.pdf

Looking at the schematic, it's hard to say whether the voltage across the element is likely to be more than 500V (I doubt it) however it is the highest rating you're likely to find so I think it's worth a punt.

Dielectric breakdown shouldn't be a problem as the Focus pot is at the low potential end of the divide chain (it goes increasingly negative towards the intensity control and CRT cathode).

The final check is a mechanical one - check the physical dimensions bush size etc against (whats left of  ) the existing pot to make sure it will fit in the space.


P.S. As the current through all the elements in the chain is more or less the same (ignoring electrode currents), the needed wattage is determined by the voltage drop, therefore resistance. If one of the resistors directly in series with the pot is 500k or greater and 0.5W then your 0.5W pot should be ok, as long as the element voltage rating is sufficient.

[tautech]

dzarren
After a look at schematic I'm in full agreement with Gyro as type 80 rectifier tube has a max rating of 500V.

So 1/2W and 500V rated should be fine.

[IconicPCB]

In nay case the old pot body has identification markings on it cast molded into the plastic shell.
What does it say?
Can you take a photo of the text?