Named the 'Batman' I suppose because it was battery or mains? but that would be Batmain, lol, who knows. The mild Art Deco look of this radio won me over on first view, it also came with it's original cardboard box with colourful artwork. It was quite expensive, and faulty too!!, but I still didn't hesitate in bidding on the item. I added two miniature model railway bulbs either side of the dial using radio coil copper covered wire and a 6.3 volt heater transformer for the lights. It did add to the weight of the unit, but items from this period are heavy anyway. Half the cabinet was empty, lots of room to add heater transformer for the two small bulbs, wired to the mains on-off switch, only in used with mains.
The repair
The sub miniature valves
DK92, DF91, DAF91 & DL94 used in this model are directly heated at 1.4 volts heater and using only 50 mA of current (100 in sound o/p), 250 total, means that the battery life was much extended using a 7.5 volt battery (the sound output valve is power/output pentode with separate heaters, so wired in series 5 x 1.4 = 7.00, when switched to mains a voltage dropper with a total resistance of 36020 ohms is added to the chain, a 22 ohm R7 is shorted out when on battery use. I removed the speaker and extended the wire for ease of repair, also took a photo of the rear five aerial wires, because, trust me they will fall off while fixing.
I've repaired a few of these miniature valve sets now and they all suffer from open circuit valve heaters, and I'm now thinking that the poorly designed 7 pin valve bases with tuning fork type pins instead of the old tubular grippers is the reason for the failures. My guess is when a valve is inserted with a slight bend on one of the pins (usually the heater ones) it causes the socket to poorly connect and the overheating of the valve pin would cause the valve to fail eventually.
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| Large electrolytic capacitors were open |
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| 2 valve bases were at fault |
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| Broken tuning fork valve pin, snapped when valve pins bent, usually when trying a random approach to inserting the valve. They bend and when valve pin straightened will then snap. |
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When I turned this set on it was dead and the large smoother (32+32uF) was getting hot quickly, checking it, it was reading open circuit, (when 'C's are by hot mains droppers the electrolyte inside will evaporate with time) so I removed it and fitted 2 modern miniature 350 volt 47uF easily found on line, unlike the originals not made anymore.
C32 a 4uF 350v HT decoupling capacitor was also open circuit, heater were burnt out on
DAF 91 &
DL94 and the bases on DK92 & DF91 were also shot, costing me a total £18, 6 for each valve and 6 for 2 holders.
When you remove a valve base make sure you have a detailed wiring diagram to refit new holder, some holders a rivited in, you'll need to drill out the rivets, but not in this case. When cutting the wires cut them under the solder, that way you'll have enough wire to reach new base, don't try to unsolder them as the insulation will melt and you may damage old components.
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| Main chassis showing doggy smoother and the valves at 10 & 12 PM position were o/c heaters |
She's quite a sensitive receiver only let down by the aerial being installed as two loops in the back door of the cabinet, the ferrite rod option had only took off by around 1955. This may have been the last set made by McMichael as they merged with
Sobell in 1956 and then
GEC in 1957. Started as a radio club with Leslie McMichael and 5 friends in 1913 and grew through the 2nd World War, and then when government work dropped off started to struggle.
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| The eBay photo that drew me in, lol |
