Customer repairs: McMichael Receivers

McMichael Model 851U, S/No 115292, Repair No. D212

This set was described when introduced as a "Transportable two-loudspeaker, 2-waveband superhet receiver for AC/DC mains, 190-250 volts". This example was born on 7th June 1951.

Initial testing showed the dial lamp to be open circuit. This set is slightly unusual as the 6.5volt 0.3Amp bulb serves to monitor simultaneously, the heater current of 100mA, and the HT current of about 80mA. A new bulb failed with a spate of loud crackling from the loudspeakers which are nothing special being merely connected in parallel. The set uses a 2-wire mains lead which is connected via a non-polarised plug enabling full mains potential to be inadvertently applied to the chassis.

Valve emissions were as follows:-UCH42 triode 60%, hexode 50%; UF41 80%; UBC41 50% with a heater/cathode leak of 1Mohm; UL41 30% and UY41 80%. These would all prove satisfactory when the set had been overhauled.

Discovering the reason for the bulb failure and the crackling was not easy. Operating the set via a variac set to an intermediate level enabled it to work normally. Disconnecting the HT supply and switching on without a variac was also OK proving that the fault was in the HT circuit rather than the heater circuit. Gradually increasing the variac, to a point just below bulb failure, again resulted in crackling which was found to vary as the volume control was moved. Waggling the output valve also produced a crackle and unplugging the valve left a loud hum from the loudspeakers ...very odd. Following a hunch (after checking everything else!) I used a magnifying glass and a strong light and found the output valve's holder had carbonised between the anode socket and adjacent heater socket. As the holder was black bakelite the carbon was not easy to see. I fitted a new B9A holder, no more crackling and the bulb burned steadily.

There were a only a couple of other faults which needed seeing to: one of the tuning capacitor mounting bushes had perished resulting in imprecise tuning and the perennial AF coupling capacitor was leaky (although only when HT was applied across it). This I discovered by measuring the output valves grid voltage which was 3 volts (resulting in a very hot valve and burnt fingers). A new 0.001uF capacitor resulted in no forward bias and a much cooler valve.

Realigning and lubrication of the tuning mechanism produced excellent results and applying a patented scratch removing stain worked wonders on the cabinet.

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