Bush AC34

[kestrelmusic]

This is a lovely set, but it was in a shockingly bad state. The interior was dirty and rusty – it had been stored in a damp shed, I fear. When powered up through a lamp limiter it was clear that the mains suppressor cap was shorting so that was the first thing to go. One side of the smoothing cap was open circuit. The output and IF valves were down to about 50%, the others were OK.

It’s a nice design to work on, with a lot of the components on a large and accessible tag board. The power supply is unusual: it’s AC only with a mains transformer but it uses a 100mA heater chain fed from a section of the secondary winding.

The AF coupling cap was shorting, the result being that the output valve was heavily overloaded and had burned out the output transformer primary. Voltages inside were all over the shop. The IF valve was passing almost no current as the decoupling cap on the screen grid was shorting. In the end I just replaced all the waxies.

Alignment was interesting. One of the IFTs had been replaced with a new one allegedly tuned to 465 kc/s. There were three peaks! But eventually I got it to settle down at 470kc/s which is the correct value. The RF alignment was way out, too. I had to do it three times before it came right.

The case wasn’t too bad, though some veneer was missing and it was very scratched and battered. I stripped it and used Danish oil for the finish. The pointers run on a steel rail; they were corroded hard. WD40 and a light hammer got them moving, but the rail had to be sanded down. The drive cord was filthy and binding on the spindle, so that had to be replaced as well.
So this one was quite a challenge. But well worth doing, because it’s a very nice set, and a handsome piece of furniture as well.

[vinrads]

Well done, these are well designed sets, easy to work on ,sound is good through that large speaker, the dial seem to survive even when stored in a damp shed. Mick.

[Sideband]

I have one as well and it does give an impressive sound. You were lucky with those B8A valveholders. With all that rust and corrosion I'm surprised you didn't find any with broken contacts. I was surprised at the 'U' range valves with a mains transformer! I think it must have been one of those situations where bush decided that one chassis fits all.....same chassis used for AC/DC....just fit a dropper!

PS, did you notice the glaring error on that snippet of circuit diagram?

[Nickthedentist]

Yes, well done from me too. The almost-not-worth-repairing ones are often the most fun.

[Leon Crampin]

DC into a mains transformer - interesting. Bush were a bit sloppy with documentation. The instructions to my VHF61 show the LH knob as on/off vol, when the mains switch is actually on the push button bank. The DAC90A service manual was never amended when the surge limiter resistor value changed - affecting all the derived HT voltages.

The sets are all beautifully built though, electrically and mechanically.

Leon.

[John10b]

Nice set to work on, incidentally my Bush service sheet just states “mains in”.
John