Bush DAC90A Repairs

 
 

"On the Bench"

 

23rd March 2004.

The Bush DAC90A is a development of the DAC90, using the more modern B8A "Rimlock" series of valves. It is a 4 valve plus rectifier, AC/DC, Long Wave and Medium Wave set, and apart from a few minor changes to suit the new valves, it shares its circuitry with the DAC90. A very similar circuit (with the addition of push-button preset tuning) is also used in the Bush DAC10.

The two main failures of the DAC90 (hand capacity affeccting tuning, and burning of the back) werecured in the DAC90A, the hand capacity by moving the tuning control to the front panel, away from the frame aerial, and the burning by using lower current valve heaters, which require a smaller dropper resistor, which is now positioned well away from the back.

The first picture shows the chassis out of the case, and you can see the space to work around the components. Above chassis, I've replaced one of the dial lamps, the mains filter capacitor (use a Class X capacitor, the are designed to be connected across the mains!), the tone correction capacitor mounted on the output transformer, and some of the wiring around the mains dropper resistor (the green thing to the rear right of the chassis). I've also given the chassis a good clean, and lubricated the tuning drive pulleys.

Underneath, I've replaced all the waxed paper capacitors with new polypropylene units (you can see the new yellow units). The main smoothing capacitors were ok, so they have been left alone. I've also cleaned the switches with a contact cleaning solution.

When the set was powered up, the sound was weak and rattly. This was traced to a faulty loudspeaker, where the voice coil suspension had come away from the speaker frame. I didn't have a spare, so I repaired the original by removing the inner dust cover, and using strips of paper to centre the voice coil around the magnets pole piece (see picture).

The suspension was then glued to the frame (use something like Copydex or Evostik). Once the glue had set, the speaker was tested, and then the dust cover was re-fitted to the centre of the cone. The repair has worked well, and the speaker has no rattles.

 

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Copyright J.Beacon & M.Wroe-Parker 2003