Marconiphone 538 Electrolytics

[cathoderay57]

Hello folks, just started on a Marconiphone 538 Quality Receiver (=HMV 469, the one with the KT66). I will do a success story when it's done but meanwhile a quick question on the electrolytics. There are 3 screw type cans (2 x 16uF + one x 32uF). Two of these have reformed OK using the set's own HT transformer and rectifier supply through a 10k resistor providing about 450v. One of the 16uF cans will not fully reform such that the leakage current remains stubbornly at about 3.3mA. According to the notes for the Military Capacitor Reformer No. 1, the leakage current for that capacitance at that voltage should reduce to just over 1mA. However, it does retain a decent charge and the capacitance measured using the Fluke is about 25uF. The thing is, when the set is on load the HT on this cap is likely to drop to some 250v and so the leakage current will drop somewhat compared to that measured at 450v. Would you recommend leave as-is for now, or re-stuff the can with a modern 16uF? I'm favouring the latter because if it goes s/c I do not want to burn out the speaker field coil or HT choke (although I am going to fit a fuse below the chassis). Cheers, Jerry

[Stylo N M]

Hi,

I Think instead of running the risk of damaging something else, that's going to be hard to find a replacement for a new capacitor would be the best option.

You could indeed restuff the old capacitor can, you may also be able to buy a replacememt 16uF can capacitor. If you decide to restuff the old capacitor, the modern replacements may well be eather 10uF at 450 volts or 22uF at 450 volts, but you will need to check the higher rated capacitor at 22uF and ensure it will be OK with the rectifier.

It might also be better to avoid re-using any of the original contacts on the original capacitor, and replace the old rubber bung with a peice of paxolin cut to size, with sleved wires threaded through pre-drilled holes. This will ensure that no corrosion will accur, from any liquids that may work their way out of the original capacitor.

But I'm sure others will be able to advise you better than I can.

Paul

[cathoderay57]

Hi Paul, thanks for the advice. It needs to be 500v rated as off-load the HT reservoir rises to 460v DC via the U50 directly heated rectifier. I can get brand new screw-type 16/16uF or 22uF 500v from the BVWS Spares Dept but the new cans are 50-55mm tall which is considerably shorter than the original (110mm - see pic). I'm not obsessive about it but I'd like to keep the appearance above chassis as original as poss. Therefore I think I will attempt a re-stuff. I can't find any sensibly priced 15uF or 16uF alu can electrolytics at RS or Farnell rated at 500v but RS does a 22uF 500v capable of 560mA ripple which for the reservoir will do nicely (to avoid confusion with post #1 and my comments re: choke burnout there are 2 x 16uF caps, one reservoir before the choke and one after. Obviously I could fit the restuffed one in either position) . It will fit inside the original can. 22uF is OK for the U50 which can cope with 32uF max. The Trader service sheet #703 dated 16 Dec 1944 says the HT caps are wet electrolytics but I think mine, while maybe original, are more likely later replacements and appear to be dry. As far as I know if the electrolytic is wet it usually says so on the can and any of this sort of age have corroded the seal and leaked out or are showing signs of doing so. However, I will be careful when I cut open the can. Cheers, Jerry

[Stylo N M]

That sounds great Jerry, the RS electrolytics sounds a great way to go and a good choice to, brilliant' and as you say you can move them arround acordingly

Paul