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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 25th Jul 2019, 3:49 pm   #1
jhockridge
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Default Help with a couple of components.

Hello,

I am just restoring an old Knight KG-620 multimeter and am a little stuck on a couple of items:

1) I need to find a 0.01mfd/10nfd/10,000pfd 1600v 20% capacitor to replace a wax paper type one and can only find ceramics in the UK or film shipped from overseas at a big cost. What is acceptable to use ? I know that ceramics can drift a lot with voltage and temperature.

2) The main filter cap has been replaced with a modern alternative at a slightly higher capacitance and it also looks as if the old Selenium rectifier has been replaced by an AEG diode; how do I identify the diode from the markings? All I can see is "0Y" which I believe indicates that it is a rectifier type diode.

Link to manual/schematic: http://rhetoricity.com/misc/tubes/vt...M%20manual.pdf

Any help welcomed.
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Old 25th Jul 2019, 4:34 pm   #2
GrimJosef
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Default Re: Help with a couple of components.

Here's a 10nF 1.6kV film one in the UK (Farnell have quite a few others) https://uk.farnell.com/epcos/b32672l...-pp/dp/2469006.

Cheers,

GJ
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Old 25th Jul 2019, 5:16 pm   #3
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Default Re: Help with a couple of components.

If the original has 20% tolerance then I wouldn't expect the value to be critical, so you should be able to use whatever is easily available with adequate ratings.

The diode will just be a standard Si rectifier, so a 1N4007 should be OK (or anything similar you have to hand).
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Old 25th Jul 2019, 5:39 pm   #4
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Default Re: Help with a couple of components.

When looking for replacement components rather than try to find an exact replacement which can be costly or hard to find it's a good idea to look at the circuit. It could be the manufacturer of the device had a bucket load of 10n 1600v caps to hand so used them. So unless it's near a transformer or inductor where voltage spikes from back EMF may be expected a lower voltage rated part might do.

The few valve VOM's I've worked on have a HT of 250 - 275v, whilst it's a good idea to over spec coupling caps for instance all other caps need not have a WKG (working voltage) of more than 400v or so, unless as previously mentioned if it's near an inductor.

Just a thought, Andy.
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Old 25th Jul 2019, 6:31 pm   #5
Maarten
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Default Re: Help with a couple of components.

Not only condensators near inductors, but also those that are "exposed" to the outside and that can still be charged when reconnected. This one is used to couple the AC voltage ranges, and is both exposed to the outside and can still be charged when reconnected.

I'd say that if this is indeed used in the highest AC range, 1600V is already on the low side.
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Old 26th Jul 2019, 9:30 am   #6
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Default Re: Help with a couple of components.

Good point Maarten, didn't think of that one.

Andy.
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Old 26th Jul 2019, 10:45 am   #7
jhockridge
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Default Re: Help with a couple of components.

Thankyou guys. I am really still just learning the theory side of things so my ability to read what is acceptable from a circuit diagram is minimal at the moment. I assumed that the capacitor was rated at 1600v because the meter was designed to measure high voltage (not that I would ever connect it to anything that high). It makes perfect sense that it is used in the A.C measuring circuit. I am trying to find some online courses to help boost my knowledge at the moment; it has been nearly 20 years since my electronics GCSE ! Will check out that film resistor.
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Old 26th Jul 2019, 10:50 am   #8
jhockridge
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Default Re: Help with a couple of components.

I have tried to attach a picture of the diode. The one in the circuit looks like this so it is fair to assume it was replaced at the same time they did the filter cap
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Old 26th Jul 2019, 12:35 pm   #9
Mike. Watterson
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Default Re: Help with a couple of components.

2KV ceramic 10nF is a good choice for AC coupling or decoupling. Likely even 25% tolerance is good enough as it's not a filter or tuning part.
1N4007 is fine for any application up to 1A and 1000V PIV. I use the 1N5408 for higher current.
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Old 26th Jul 2019, 4:28 pm   #10
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Default Re: Help with a couple of components.

Is the diode bad? No reason to change it if not.
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