5. AC10 repair.
I received an enquiry from a customer with an AC10 that needed some TLC. He reported some hum issues, and poor tone. (The amp had been in storage in his garage for some time).
The amp had at some point previously had the mains transformer replaced, and the valve rectifier removed and replaced with silicon rectifiers.
Both of the original loudspeakers had also been replaced with Eminence speakers. (Date codes on the speakers suggest 1967)
There was also an interesting addition to the amplifier. A booster unit using an EF86 had been fitted inside the amplifier.
Voltage and signal checks on the amplifier revealed that it was only giving about 6.7 watts. The output voltage from the RS mains transformer was not high enough, also the transformer was suffering from loose laminations, and was generating considerable hum. A new vintage specification transformer was ordered.
There were many components on the tag strips that had been changed at some point with incorrect values, and the mains switch and voltage selector were also both faulty.
The mains transformer was replaced with one of the correct vintage specifications, and additionally the valve rectifier was re-fitted at the request of the customer. (Limiting resistors were fitted in the supply to the rectifier as per Mullard specifications). All power supply filter capacitors were replaced, as was the faulty mains switch. The voltage selector was bypassed as it was faulty, and the amp was wired for UK mains voltage.
Correct value components were fitted to the tag strip, and the booster unit was removed at the request of the customer. I fitted a hum reduction potentiometer on the heater supply to get the usual hum down to reasonable levels.
After the repairs, the amp was tested and found to be giving just over fifteen watts, not bad for an AC10! It was running a little warm, so the cathode bias resistor value was increased to help the amp run a little cooler.
The amp sounds great now, no wonder these amps are quite sought after!
6. 1964 AC30 chassis repair and restoration