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Old 17th Nov 2018, 2:46 pm   #10
Uncle Bulgaria
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Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,315
Default Re: Ferrograph 4A: Restoration advice needed

Thanks Bill and Barry.

I'm sorry - I didn't realise the diagram had been so reduced as to be almost unreadable! Here is a Dropbox link to download the full-size diagram which is rather easier to interpret.

Bill - could I prevail on you to check the numbers of the components again when the image isn't so illegible? I can't see how C19 couples from V3, as my reading of the circuit has it only seeing signal voltage between the anode of one half of V5 and ground, with R22 just elevating the grid of the second half. Were you perhaps meaning C15, or have I got confused with the way the circuit's drawn? I can't see where the HT source would be for V5, unless there's something high from the anode of the oscillator via pin 4 of plug 5.

C20 is what I would recognise as a coupling capacitor, which is a 350V 0.05uF 'metal tubular' in the manual. This is another 'Metalmite Visconol-X'. I can't find what they're actually made of and thus whether they're likely to have dried out.

I have a stock of polyester/polypropylene 630V capacitors which I was thinking of using, so it's good to know that they're recommended by the cognoscenti!

Screws: I've found the majority come up nicely with a brass brush, apart from one from the tape-counter which has a stripped head. Coming across the same difficulty of finding such a thing, I propose replacing it with a new M3 (available in raised countersunk Philips) which from my tests is easily re-tapped to 6BA without much fuss.

I've already tapped up Mr Martini-Yates for a head cover and the manual - what a help!
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