Marconiphone T89DA

[ppppenguin]

I've attached a clip from Trader 1486 which is supposed to cover your Marconi as well as the Ferguson 627U etc. . The cap you're concerned about is C59 on that diagram. It may have a different designation on other service data for the same model. It is indeed 3900pF and as Old Sparky says, its value is very noncritical. I don't know why the designers chose such a strange value. It must be replaced, as any leakage will damage the output valve V5 and the audio output transformer T1.

4700pF (4.7nF, 4n7 in most modern catalogues) would be an ideal replacement. If I didn't have one to hand I'd use anythng from 2n2 to 10n without worries. Voltage rating? I'd use at least 400V which is amply good enough. The yellow axial types are nice but anything with the right vlaue and voltage is fine. I'd also make sure that C63 (1000pF, 1n) is replaced. Again a failure here will endanger the output transformer.

While I'm sure you're aware of it, this set is a live chassis design with the chassi conencted to one side of the mains. Make sure it's neutral and don't trust the set's own mains switch. Always unplug the set before working on it. If your mains sockets aren't protected by an RCD in the consumer unit then I would strongly recommend using a plug-in RCD while working on the set. Here's an example: https://www.diy.com/departments/diall-13...774_BQ.prd