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!