Bush DAC90 Repairs

 
 

"On the Bench"

 

10th February 2004.

I have recently completed repairs on two Bush DAC90 sets, both of which had the same general problems, so I'm putting them both on this page.

The DAC90 is a 4 valve plus rectifier superhet with an AC/DC supply, and was introduced in 1946. Generally, they are a reliable set, though they do have a couple of irritating design faults:

1. The mains dropper resistor is to close to the fibreboard back, and tends to burn a hole in it.

2. The tuning control is sited next to the frame aerial, so placing your hand on the tuning control alters the tuning (before you turn the knob!).

The set is quite compact, using octal valves, but is generally easy to work on.

In this picture you can see the mains dropper resistor to the right of the chassis. The white material around it is Asbestos - take the relevant precautions when working in this area. The dropper was originally fixed to a paxolin strip at its top end, but this has burnt away, and been replaced by a piece of stiff copper wire in a previous repair.

Just below the dropper, you can see the 2 pin mains input connector, which is dangerous in the wrong hands! It is easy to leave a bare connection that can be touched by small fingers, also, remember, these sets have no mains transformer, the orientation of this plug determines whether the chassis is connected to live or neutral (the plug is reversible).

The front of the chassis shows the speaker and controls, the wires to the right connect to the frame aerial.

Under the chassis you can see the results of my repairs, all of the original wax paper coupling and de-coupling capacitors have been replaced with modern polypropolene types, and the mains filter capacitor has been replaced with a "class X" rataed unit, as it is connected directly to the mains. The electrolytic smoothing capacitors have also been changed, with the new ones being installed in the can of the old one (the can was cut in two and emptied, the clamp covers the saw cut). This work was carried out on both sets.

Both sets required new ouput valves. One set had its output valve (CL33) replaced with a 20P3, which is a TV line output valve, the bias had been changed to suite, and it seemef to work fairly well, but the set has now been returned to original.

The switches and variable resistor in both sets were cleaned with switch cleaner. The sets have not been re-aligned, the don't seem to need it!

The final repair to both sets was to patch a new piece in to replkace the burnt back panel

the patch can be seen in the upper right corner. The material was cut from a scrap back panel from a Cossor set, and glued in with Araldite, then re-coloured to hide it a bit! (after this picture was taken).

These are nice sets, and well worth the effort!

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Copyright J.Beacon & M.Wroe-Parker 2003