23-09-2014, 08:28 PM
(This post was last modified: 23-09-2014, 08:32 PM by Old Sparky.)
I've now completed the repair of this set, which involved, besides replacing the usual waxies, 3 resistors and the blown reservoir/smoothing capacitor, fitting a substitute o/p transformer and LF smoothing choke, the latter two items being obtained from a scrap EKCO/Ferranti, and BUSH VHF 41 respectively!. With a 1N4007 diode as the rectifier, and an 820 ohm 5 watt resistor in series, the AC at the 4007 anode is 208 v., and HT at reservoir cap. (22uf 450v) 200v, with 180v at the Smoothing cap (also 22uf 450v) Whilst these, and other voltages are a tad lower than those specified in the service sheet, I'm inclined to leave things as they are to hopefully prolong the life of the by now 67 year old valves, especially since the radio works beautifully, putting out a little over 2watts into a 4 ohm load from several MW stations ( I haven't yet measured the o/p using my sig. gen!) Everything runs cool after maybe 1.5 to 2 hours operation, and All DC conditions remain stable, with the set out of it's cabinet. Incidentally I've fitted a 2n2 class 'Y' disc capacitor across the 1N4007 to bypass any 'spikes' on the supply. This cap. is rated at 275vac (ca 780vDC if I've done my sums correctly!) which I feel should be adequate for the job. Anyone care to comment on any or all of the above? NB As the AZ 31's heater was O/C - hence the use of a 1N4007 - I've removed the wires from the heater winding on the Mains TX to it's heater pins. Presumably no harm will result from there being no load on the (4v) heater winding?
