Silver Ceramic Capacitor Replacement

[Bushbaby]

Hi All,
I need to replace a silver ceramic capicitor (C50 in a Bush VHF90 - yes, I'm still working on it!).

It might seem a daft question - what type of cap should I replace it with?

Thanks

Rob

[pwdrive]

Hi Rob, I would say any ceramic of the same value and a suitable wkg voltage. You might get away with a bog standard modern foil cap if you put the marker band end to the earthy end of the circuit.

Lawrence.

[Skywave]

Hi Rob: generally speaking, to select a suitable replacement capacitor in any application, several bits of info. are required; I am not familiar with that particular wireless cct., etc.
1. what's is its nominal capacitance value ?
2. does it have a tolerance figure stated on it ?
3. do you know its rated voltage ?
4. is it a polarised capacitor ? (Usually limited to electrolytics: obviously not in this specific case).
Also its function in the overall scheme of things is needed too.

Al.

[Bushbaby]

In this instance its described as a silver ceramic cap 2200pF and 350v - its not polarised and appears to connect to triode section (if I am correct?) of the UABC80 valve, A.F amplifier section.

I do have an old NOS 2200pF silver mica RS cap (350v) which was considering using.

Regards

Rob

[pwdrive]

In this instance its described as a silver ceramic cap 2200pF and 350v - its not polarised and appears to connect to triode section (if I am correct?) of the UABC80 valve, A.F amplifier section.

I do have an old NOS 2200pF silver mica RS cap (350v) which was considering using.

Regards

Rob

That might do Rob but be aware that if it is big in area it might pick up a bit of stray hum, if it does try reversing the connections or altering its position relative to any nearby wires that are carrying AC, heaters for example.

Lawrence.

[Bushbaby]

It does seem huge in comparison to the cap its replacing. Do I have to use a ceramic cap??

Thanks

Rob

[Skywave]

No, a ceramic cap. is not your only option here. In actual point of fact, since this cap. is only passing audio frequencies (if my understanding of what you have written is correct), I am surprised that a ceramic cap. has been fitted at all. Usually ceramic or mica caps. only appear in circuits where there is substantial r.f. currents flowing, such as the mixer, local oscillator and I.F. amplifier anode circuits.
Now if this cap. sits between the anode of one valve and the control grid of another, then the most critical parameter is its leakage current under the d.c. and signal conditions. As is well-known, in that position a capacitor with zero leakage is required, and although a 'good' ceramic or mica capacitor will be 99.9% guaranteed to meet that requirement, there are other types that will also me suitable - especially new, unused, modern ones. But whatever type (or vintage) of cap. you choose for such a coupling capacitor, I would subject it to a high-voltage test first and measure the leakage current at that voltage. Anything over about 20 µA I would reject as unsuitable. But if that cap. is doing duty as a screen grid or HT decoupler, then the leakage current requirements are far less stringent.
That is why I asked "what job does it do in the overall scheme of things ?"

Al.

[Joe]

Here's the blighter in question:-

   

I suggest you right-click and select 'Open in New Tab.'

- Joe

[Bushbaby]

Hi Joe,
Yes, that is the little blighter. Unfortunately, to get to rewire the other side of the tag strip I had to cut this one out (it was very burnt anyway). Every ceramic cap I have is at least 4 to 5 times the size for the same value as the one in the radio. I must admit this isnt the most pleasant radio I have worked on just yet. I really dont like stacked components on tag strips (badly soldered to boot!)

I'm waiting for someone to come back and say, wait 'til you've had a go at so and so.........

Regards

Rob

[Skywave]

Thanks Joe. Now I have a better appreciation of what is going on: it's a tone correcting capacitor: cuts down the treble response. Its performance is not critical; it's C51 that is the 'critical cap.' For the cap. in question, almost any type will do. I suggest you make your choice of replacement based on three factors and in this order:
1. Suitable voltage rating,
2. Physical size,
3. Physical appearance: e.g. do you want it to look original or will a modern-looking one appear O.K. to you?

I would fit a 0.0022 µF 300-v. tubular polyester.

Al.