A few questions regarding the restoration of a Pye 76

[platinum95]

Hi all,

Title is a bit of a lie; it's actually a Pye P74 which is not really documented. As far as I can tell, it's (nearly) identical to the Pye P76, but manufactured at the Dublin factory. I'm working off the service manual for the P76, which I found online (not sure what the rules on posting snippets of the service manuals are so I'll leave them out).

I've been looking over the schematic and deciding what capacitors are to be replaced. It's all point-to-point soldered so will take a bit of work. The RF tuning side is an absolute mess of connections, with capacitors hidden beneath layers of wires, so ideally I'd want to avoid unnecessary replacements in here. Most of the caps are 100pF, but a few are around from 2.2nF to 50nF. Ideally, these would be replaced, but given that this is a low-voltage side of the circuit, is it completely necessary?
Of course, all electrolytic caps will be replaced, and everything 1nF+ from the rest of the circuit.

There's also a big ol' canned capacitor in there, markings say its a 16mF one rated at 350V. Thing is, this seems to only be in the P74 (Irish model) as it's not documented at all on the P76's service manual.

Another point of note is that the IF tuning is done through cans which have both capacitors and variable inductors inside.

Finally, I'd like to do some basic tests on the leakiness of the capacitors I pull out, to get an idea of the state of the radio. I haven't got a fancy leakage-detector (and don't really want to buy one), but I do have a low-current, high voltage generator (~250+v), an oscope, a signal generator and some other bits, and figured I might be able to do some basic tests using these.

My questions on the above are:

1. Am I correct in saying that capacitors below 1nF don't really need replacing? I've seen that mentioned in a couple of places, but I do note a 100pF coupling capacitor between the cathode of the IF amplifying tube and the first stage audio amplifying stage, so I'd be wary about some leakage current getting through here.
2. Am I going crazy, or is a 16mF 350V capacitor not unreasonably large? I couldn't find a direct replacement for it, and the closest I could get to it was a starter cap for those fluorescent bulbs.
3. What can I do about the IF tuners? The capacitors within them are 100pF, so ideally I'd like to leave them as-be. On the offchance that they need to be replaced, is my only option to just but the caps and variable inductors individually and frankenstein myself up a sit-in replacement?
4. Is there much I can do to test the caps with the equipment listed above?

Thanks!

[ms660]

16mF is very large, 16uF isn't.

Lawrence.

[platinum95]

For sure, but there are ~16uF electrolytic caps in there too which are significantly smaller, and all axial. This is a canned cap and is relatively large (in size).
Was it common to denote 'micro' as 'm'? Seems like a poor choice in wording.

[ms660]

mfd was common, mfd = uF, both are microfarad.

Lawrence.

[platinum95]

Fantastic. I haven't had a chance to go at it with a continuity meter yet, but once I do I'll find out where this cap sits in on the schematic.

[camtechman]

Was it common to denote 'micro' as 'm'?

Yep, in some companies, mF meant uF.

When I worked for Ferguson, back in the 60's, for pF they said "Puff", took me some time to work that one out LOL.

[TonyDuell]

The unit 'mF' meaning millifarad is not commonly used. I've seen it in some HP service manuals but almost nowhere else. It is much more common to write 1000uF (1000 microfarads) than 1mF, for all they are the same thing.

I think in any vintage radio service manual, it's going to be microfarads, no matter how it's actually written.

[platinum95]

Originally Posted by camtechman

Yep, in some companies, mF meant uF.

In retrospect, it both seems reasonable and seems like an awful convention at the same time.

When I worked for Ferguson, back in the 60's, for pF they said "Puff", took me some time to work that one out LOL.

I must admit that I've always called them piffs and niffs in my head.

Anyway, good to know I'm not dealing with some monstrous impossibility of a vintage cap.

[G8HQP Dave]

'mF' should only ever be used for millifarads; it is standard SI, although not used as often as perhaps it could be.
'mfd' is a common way of writing microfarads, especially in older American stuff.
'mf' should be avoided, because it is unclear whether 'mF' or 'mfd' was intended.
'uF' is a quick way of writing microfarads for people (like me) who can never remember how to insert Greek letters when typing into a forum or making a quick note.

[julie_m]

I don't suppose anyone ever expected to be making capacitors in the millifarad range, so the (mis)use of "m" instead of µ (which, by the way, is alt gr + m on Ubuntu; probably different on Windows and Mac, if not on other Linux distributions) was blatant enough not to take at face value.

[paulsherwin]

Beware though: the Pye Ireland models made in the Dundrum factory are often surprisingly different from the supposedly related UK models. They seem to have had a lot of autonomy with regard to model development, for no obvious reason.

[Silicon]

When you don't have the correct schematic for a radio I would suggest that you proceed with caution.

Replace or reform the electrolytic capacitors. Replace the audio interstage isolating capacitor and then see if the radio works. Use a lamp limiter and monitor the HT current.

Radios will often work with several partly leaky waxed paper capacitors. Once you have found that the radio is healthy you can then replace the waxed paper capacitors one at a time and test the radio after each replacement.

[Boater Sam]

Flat silver mica caps rarely leak, occasionally the rivet slackens and they go O/C.
16uf is common in 350v and 450v, you can re-stuff the can with a modern one if you wish.
It's the round waxed paper caps that usually leak. Replace one at a time and test then you don't make untraceable errors.

[platinum95]

Had a chance to poke about at the circuit the last day, and found a few things of note.

Looks like what paulsherwin said is true, the circuit does deviate a fair bit from the schematic that I have. The overall circuit 'flow' is generally the same, but there are a few undocumented capacitors thrown in, and most of the caps have values different to what's stated. Not a huge deal, since it just means ordering replacements based off the circuit itself rather than the schematic.

Thankfully, there appears to be very few paper-wax capacitors in it. The non-polarised caps are all either mica-wax, ceramic, or (as far as I can tell) the 'Hunts' style which have been cracking and breaking all over the place, so they'll need replacing.
As for the electrolytic capacitors, I think I'll replace any easy-access ones, as well as any used for the likes of mains filtering etc.. The rest I'll try to reform. I have a bulb current-limiter from a previous repair, and also have a high-voltage current limited supply that might be suitable.