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The DAC 70 as received. The case was basically unmarked. The dial assembly was intact but dirty. When the back was removed a thick coat of dust covered the valves and chassis a very good sign that no one had worked on the set recently. Also there were no obvious signs of repair. (Note: Click on each image for full size).
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View of the chassis rear after removal from the case. Everything is in place and no signs of any repair. Note the dial lamp. Many DAC 70's have been scrapped due to the lamp melting the plastic scale. |
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The dust build up can be seen clearly on the valves. Valve lineup is UCH81 UF89 UBC81 UL84 and UY85. |
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With the Dial backing plate removed. Note the dreaded Hunts capacitors. |
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All the Hunts Decoupling
Capacitors have been replaced (new capacitors are yellow).
C24 across the mains input was replaced with a modern type and a new mains cable fitted.
The smoothing capacitor (3 capacitors in one) was put through a reform cycle but one capacitor failed so it was a matter of removing the insides of the old one and inserting the three new capacitors into its case (it can be seen in the picture) with a patch of white tape holding the top on while I powered it up.
On cleaning the wavechange switch (S1a and b) it was discovered that someone had forced it over the stops resulting in the switch blade displacing the contacts. The switch was removed from the set and the contacts and stops adjusted thankfully without anything breaking. The switch was then refitted.
After completing a clean of the chassis and the valve pins the speaker was reconnected, Dial lamp replaced and ready for power up.
Before applying power a resistance reading was taken accross the mains input, all ok.
On applying power with a lamp in series to limit the current in case a short circuit developed. After a few minutes the valve heaters lit and the ht had had peaked at about 175 volts. It was left like this for an hour, no signs of distress were seen so time to remove the series limiter.
Power was again applied and the ht checked, it was well up at around 230v dc, but no sound except a background hum from the speaker?.
Applied a 1kHz tone to pin 2 of valve 3, lots of sound, so applied the tone to either side of C14 nothing. Volume Control was obviously faulty so removed it and stripped it down. The carbon track was open circuit!.
Replaced the volume control (RV1) and we had sound, distorted but it was there, so swapped speakers and the distortion disappeared. Replaced the speaker with another from a DAC70 with a smashed case.
Volume was now good but weak on stations so checked the tuning, it was out. Set was retuned to the instructions.
The set was reassembled and all the rubber mounting grommets replaced. The case was polished and it is currently in use every day. |
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The circuit for the DAC 70 |
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