Another one completed

The Bush DAC90A is finally complete and on its way to a new home.

The final fault that had to be sorted was a duff volume control, which cut out at a point around three quarters from full volume.

No amount of cleaning made it any better, even after I stripped the pot down to expose the track. Luckily I had was able to get a similar potentiometer for £3, which had the added advantage of an insulating plastic shaft.

The audio distortion I noticed earlier was cured by giving the speaker a really good clean to dislodge a load of dirt and muck that had built up between the chassis and cone.  There were also a couple of tears in the speaker cone which required a bit of glue to stop them tearing further.

After a good clean and polish of the case it looked great, and ready for many more years use.

More radio recycling tales will follow soon.

Keep listening!

DAC90A valve failures

The DAC90A powered up without anything unpleasant happening, but it was obviously not receiving anything. I switched the power off for a few minutes while I made some room on the bench, but when I switched it back on again it was completely dead. None of the valves heaters were lit.

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!

Bush DAC90A progress

The DAC90A has been in my workshop for a couple of weeks now. I've not spent as much time as would have liked on it, but it is coming on well.

After cleaning up the chassis and removing the valves for safe keeping, I started off by replacing all the wax paper capacitors and a lot of the wiring. This included the wiring around the mains dropper, the dial lamps and the loudspeaker wiring.

The flexible wires were replaced with modern silicone insulated wire which looks very much like the original rubber insulated stuff.  With solid core wires I just snipped one end, pulled off the old rubber, slid a new silicone sleeve in its place and then re-soldered the wire in place.

Dial lights & reflector removed for easy access

Several  high value resistors measured considerably higher than their original value, and I replaced any that were outside a 20% tolerance. The 2.2MΩ resistors in the AGC circuit were way beyond this, nearly 3MΩ.

I spent a while reforming the smoothing capacitors before applying power. I did this with the capacitor in place, with the negative tag disconnected from the chassis.  When I started restoring old radios, one of the first items of test equipment I built was a capacitor re-former, which applies the capacitor's working voltage via a current limiter.

Reforming the smoothing capacitors

Both the 16uF and 32uF sections of the smoother reformed well, the initial leakage currents of around 5mA dropping to less than 1mA after around 30 minutes of reforming.

I fitted a two core mains lead, ensuring that the neutral was connected to chassis. A test with the multimeter between neutral and chassis gave around 5 Ohms  (the combined resistance of the the dial lamps and the bypass resistor). This check is very important to avoid a live chassis!

New components & wiring

I always use a mains isolating transformer when working on an AC/DC set.  I also used my lamp limiter when applying power first time, if there had been a problem the lamp would have lit and reduced the current somewhat.  All was well, so I took the lamp limiter out of circuit.

I'll tell you what happened next in the next blog entry.

Keep listening!