Replacement for high voltage resistor?

[buggies]

Resistor in 3" CRT display has about 1.2kV across it. The resistor (pictured) has of course gone high to nearly 800k. I am trying to keep this 1940s unit looking original inside.

Do you think this resistor will continue to increase in value? If not I could parallel it with a couple of resistors (in series) hidden in some black sleeving.
Otherwise would have to paint a cardboard tube or something to look like an old carbon rod type?

[TrevorG3VLF]

I would keep the resistor which if 560k should dissipate about 2W.
Newer resistors to be used in parallel will dissipate about 0.7W but sufficient should be placed in series to limit the voltage on each to within the voltage specification. You will need about 2M.

I have an amplifier which has a voltmeter, the only reliable high voltage resistors I found were of the type which you have. I bought the amplifier cheap because the voltage indicated low. It was just high resistors to the voltmeter. I used five 1M resistors in series to show 5kV full scale. The amp uses much less than this.

[buggies]

Thanks for the reply.
I will have to make up a CPC order and get 3 x 680k 500v which will give 2040k with a resultant of about 554k.
They don't do a 180k (x3) in the same range which I would need to do a replacement.
Strange how some old components don't seem to have a modern equivalent.

[M0FYA Andy]

Although the resistor has gone high, is it actually causing a problem?
What is the unit?
Andy

[beamcurrent]

Just looking at the resistor in question and musing on the difficulties of putting modern resistors in series.

These body tip dot types are tough old birds, It occurred to me that it might be possible to shorten it a bit? possibly by extending the solder end fixings.

Has anyone experimented with this? I also thought that the going high might be caused or partly caused by the end contacts failing?

Interested to know what others think

Brian

[G6Tanuki]

The old body-tip-spot carbon-stick resistors: in my experience the bit-of-wire-wrapped-round-the-ends-and-soldered always seemed to be attached rather firmly to what appeared to be a plating/metallising of some form on to the surface of the carbon-stick itself.

What this plating was, and how you'd recreate it if you did shorten the resistor, I don't know.

[It's the same with the "Brimistor" type thermistors used in series-heater circuits to control inrush current: the ends of these are definitely plated with something-metallic-that-is-not-solder]

[M0FYA Andy]

Rather than struggling to 'repair' the resistor, why not simply replace it with a good one, if such can be found, or maybe a nominally lower valued one which has gone high to 560k? Hence my questions above. I may be able to find one.
Andy

[buggies]

Originally Posted by M0FYA Andy

Rather than struggling to 'repair' the resistor, why not simply replace it with a good one, if such can be found, or maybe a nominally lower valued one which has gone high to 560k? Hence my questions above. I may be able to find one.
Andy

Hi Andy,
To answer your previous question - the unit is a "Labgear Viewing Unit" circa 1944. The previous owner has done some repairs including what looks like a 470k added in series with a 1M in the focus chain. There is no circuit available and the provisional sketch I made contains errors which are only now coming to light as I dismantle the unit for capacitor and rubber wire replacements.

The 560k is about 1.75 inches long, .25 inch diameter and about 35-40% high instead of +-10%. I have nil stock of anything remotely like it and Google has not found that value with 1.5kV rating.
One of the questions in my first post was if anyone knew if these resistors continued to increase in value as I would not like to leave it in circuit for the next owner.

[Argus25]

Get four 2w rated common garden metal film resistors in series, two 560k and two 470k would do it, in some heat shrink sleeve and tack that across your resistor to establish the correct value. It would be a shame to throw out that lovely old resistor.

I guess near one end you could scrape off the paint and make a small circumferential brass clamp and screw/nut to tap down the value.

[Diabolical Artificer]

Might have one these somewhere, though it's likely a pull. I'll see if I can dig it out.

Andy.

[M0FYA Andy]

The dimensions make it look like an Erie Type 2 resistor, according to the datasheet I have the maximum voltage rating is 1000 volts, so its pips are being squeezed at 1.2kV anyway!
I don't think there is any evidence available on whether the resistance will continue to increase, I've asked this question a number of times and drawn a blank.
As I said earlier though, unless you conclude it is affecting the performance of the unit I'd leave it alone. Maybe an experiment with parallel resistors tacked across to note the effect on the unit before going for a repair.
I'll see if I have a 560k example in my stock of 'dogbones' and check its value.

Andy

[trobbins]

I've surreptitiously placed a parallel resistor 'bahind' an original - difficult to see - so that may appease the aesthetic appeal ?