22nf 4000v Capacitor

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sisacs
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Joined: Fri Aug 14, 2015 10:32 am

22nf 4000v Capacitor

Post by sisacs » Fri Aug 14, 2015 11:01 am

I've acquired a vintage Analogue HP 1744 Oscilloscope but unfortunately after initially displaying a bright splodge after powering up, no display is now visible. Although all LEDs and the unit itself powers up.

After a little research I inspected the High Voltage PSU and noticed that a 'Sprague 0.022uf 4000v' capacitor seemed to be leaking.

The problem I'm having is sourcing one of these capacitors (or equivalent). One person in America did have on on ebay but shipping alone was £36.00. As I'm not sure if this would fix the problem it does seem excessive. Is this a relatively odd or higher than usual rating/voltage? I'm still in the learning stage of all things electronic so I may be unaware of the best places to pick up certain components.

Any advice would be welcome, thanks.

piratepaul
Posts: 421
Joined: Mon May 20, 2013 2:45 pm

Re: 22nf 4000v Capacitor

Post by piratepaul » Fri Aug 14, 2015 9:18 pm

There are shed loads of scopes on eBay dirt cheap, I bought a Chinese box that is supposed to be an oscilloscope, it did not want to work on windows XP, I would not recommend it. I have been considering some of the old stuff, SIG gens etc, but don't have much space.

piratepaul
Posts: 421
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Re: 22nf 4000v Capacitor

Post by piratepaul » Sat Aug 15, 2015 10:16 am

DANGER. OF. DEATH. Cahhode ray tubes are high voltage, this thread is not for the inexperienced .

I would advise a wrist strap and a reliable earth connection if you are going to play with this.

May be try to find the same model on eBay etc for parts, at least you can test one against the other.
Are diagrams available?
If you have diagrams pls post them.

I (pure guess) should think the cap is something to do with HV circuits for obvious reasons, a suppressor for radio frequency to the coils perhaps.
When you say it is leaking, do you mean oozing gunk or losing charge?

A PIC of the splodge would be useful, but I would geuss at tube vacuum problem ( new CRT) or problem with CRT coils or circuits to them.

sisacs
Posts: 2
Joined: Fri Aug 14, 2015 10:32 am

Re: 22nf 4000v Capacitor

Post by sisacs » Sun Aug 16, 2015 2:06 pm

Thanks for the reply. I'd class myself as inexperienced at them moment but hopefully fully aware of the danger and hazards associated with the High Voltage unit. Thankfully the unit hasn't been powered on for some time and it won't be until I can get the capacitor I'm after.

The capacitor seems to be physically leaking which the photo doesn't do it justice as it's taken with a crappy phone. Unfortunately I never took a photo of the splodge on screen before the display stopped coming on.

There are a couple of identical value capacitors (22nf 4000v) in the unit and they are giving around the correct the 22nf value with the Cap' meter. The suspect one is showing around 76-79nf so hopefully this is another indication that it's failing.

I've had a look on ebay for the same Oscilloscope but only managed to find one in America and as yet no luck with a 22nf 4000v replacement.
Leaking Cap.jpg
Leaking Cap.jpg (145.21 KiB) Viewed 1774 times

piratepaul
Posts: 421
Joined: Mon May 20, 2013 2:45 pm

Re: 22nf 4000v Capacitor

Post by piratepaul » Sun Aug 16, 2015 2:35 pm

I don't see how it can increase its capacitance? Still its not what I expected it to look like either, I spose seen as its 4000V it must have a very thick dielectric, I suspect that you have an incorrect reading of capacitance, did you use an LCR meter or what?

Pass on this one I'm afraid, let us know how you get on.

piratepaul
Posts: 421
Joined: Mon May 20, 2013 2:45 pm

Re: 22nf 4000v Capacitor

Post by piratepaul » Sun Aug 16, 2015 2:43 pm

I just googled that and it comes up with 10uf for a Sprague 430P.

piratepaul
Posts: 421
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Re: 22nf 4000v Capacitor

Post by piratepaul » Sun Aug 16, 2015 5:20 pm

There are actually lots of different ones, the 10uf version was only 50V. But I am sceptical.

David
Posts: 190
Joined: Sun Oct 04, 2015 6:18 pm

Re: 22nf 4000v Capacitor

Post by David » Tue Nov 24, 2015 9:14 pm

Have you been able to source your 22nF cap yet? If not, try Maplin, Farnell, and RadioShack. The various vintage radio sites (there are quite a number of these) are good for the more unusual components. Don't worry about getting an exact equivalent - caps with a higher working voltage will always be OK. Likewise, in power supply circuits it is usually OK to vary the actual values (these caps have tolerances of up to 20% or so anyway) so you could use 20nF, 25nF, 27nF or perhaps more if those proved easier to find. If it is easier to get 10nF caps of the right working voltage, just use two in parallel.

Hope this helps

Timothycar
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22nf 4000v Capacitor

Post by Timothycar » Sat Mar 09, 2019 5:12 pm

I have 3 amps, totalling 920 Watt Max total. I have a 2 Farad Capacitor for the sub amp which is 760Watt Max. Id think if youre less then 1200Watts you should be ok.

David
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Re: 22nf 4000v Capacitor

Post by David » Thu Mar 14, 2019 4:39 pm

I have looked at this thread again, and also checked out the spec and circuit diagram of the HP 1744. This seems to be quite a good scope, and one that would probably well repay servicing. The 22nf cap will be in the HV part of the circuit, and the circuit diagram in some parts refers to a 3kV potential - just where you would expect to see highly rated components. 22nF at 4kV caps are available on the net - they just need a search! As I put in my previous post, don't bother too much about the capacitance - the main point is to find some with sufficiently high voltage rating. Bearing in mind that all the caps like the faulty one will be from the same batch, I would get a sufficient number, and replace all of them. You would then have a very useful scope at a small outlay

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