After a few measurements with the ohmmeter I found that the heater of the UY41 rectifier valve was open circuit...most strange, because it had worked initially. I pulled out the duff valve and happened to notice some movement inside it. The wire that connected one end of the filament to pin 1 had become detached. I've never seen this sort of valve failure before! Fortunately I had a scrap Ekco set in the garage which has the same valves, so I had a spare on hand.
On powering up the set again, all I got was a little hum and noise from the speaker. Touching the wiper of the volume pot with a screwdriver gave a nice loud hum, so the audio stages were obviously working.
I fired up my AVO Allwave Oscillator (a previous restoration project) and poked a 465 k/cs signal into the IF amplifier valve grid. Nothing happened. With suspicion now firmly on the IF section, I checked the anode Volts of the IF amplifier valve and found they were a bit high, suggesting that the valve wasn't doing much work.
At this point I decided to take a short cut and just change the UF41 IF amplifier valve for one of the spares I had just liberated from the scrap Ekco. As I pulled the valve out of the DAC90A, I noticed something funny; there was a lot of white debris inside the valve!If you look at a radio valve you will normally see a silver coloured deposit inside the glass, usually at the top. This is created during manufacture, and is the result of a process which removes any gas that remains inside the valve. If this silvering changes to white, its a good indication that air has got into the valve. In this case the white substance fell to the bottom of the valve where it couldn't be seen easily.Otherwise the valve looked OK.
Another UF41 was fitted, and the radio started to work. Here's a picture of it on the bench, tuned to Smooth Radio, my local AM station. The audio is a bit distorted, so I will have to investigate this further.
More to follow soon
Keep listening!
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