bush: DAC90A (DAC 90 A); Volume control problem fixed

 

Hello,

I have just restored/repaired a 1955 Bush DAC90A. This example exhibited the common problem with this model, namely the volume not shutting off at minimum setting, and this set was still playing quite loud at minimum setting. The beginning of the track on the 500K potentiometer often wears on this model which causes the problem and fortunately I was able to locate a new replacement dual pole switched potentiometer. This was marketed by RadioSpares in the UK in the 1960s and is called an 'Add-a-shaft' and was available with a range of spindles to suit different brands of radios.

I changed all the wax capacitors and the switched potentiometer and the set now works perfectly. If the sound quality from one of these sets is felt to be a little dull, I am informed that changing the 0.003 uF capacitor connected to the anode of the UBC41 to 470 pF will brighten up the sound.

If anyone would like one of these replacement potentiometers, please contact me.

Regards ........... Howard Craven, Surrey, England

EE March 25, 2014 fixed picture size to 600 pixel width.

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The initial problem was that the minimum volume was slightly off the ccw stop of the potentiometer.That happened at strong signals. As previously described that was found and proven a constructional fault, not a worn component.

There may have been also a worn track in the potentiometer, but before one starts to buy and replace it, it would be easier and time and money saving to check whether the missing little cap is the culprit.

Just rotate cw starting from the minimum stop and watch if the volume will immediately increase (caused by a worn track) or go to a minimum before it gets louder. The latter is caused by stray IF being fed into and demodulated by the audio stage. The blocking cap is missing. See threads below.

 

To thank the Author because you find the post helpful or well done.