This was a nice treat for a change. A Mid 70s Rotel amplifier. Took me right back to my teenage years, when I so wanted something like this.
Report was that the amplifier was crackling and popping and doing some odd things. So, connected up to headphones to start with and a signal into the tuner input. Certainly all the signs of scratchy pots and user controls. First things first, a good clean up and injection of cleaning fluid into all controls. I was hoping the volume control, which was bad, would respond to a good clean. This would be hard to replace, given it’s a 4 gang unit. With everything cleaned, all the controls were okay, but a small solder crack found under the PCB where the volume control is soldered to.
Connecting up to some speakers for a propper test, we still had a random noise and rumble on one channel. Using dummy loads on each channel and waving a scope over the output, it seemed that we had random DC fluctuations. This was supported by varying power consumption from the mains. Always look suspicious at the bias pots when you see this activity. Luckily, that was the issue. A clean of the pots and set the bias up, all was great.
With some big Celestion DL10s wired up and some Earl Bostic fed into the inputs, the amp sounded so nice. Clean, sharp and warm at the same time. Love the tone control contour settings. And, love the input sockets at the rear that face upward, so you don’t have to tip the amp up on it’s end when plugging something in.
Customer had obviously been filling his head with Google juice and started asking and “RE CAPPING” A term I hate. I had done a frequency sweep and both channels were balanced and the response was flat and true from the floor boards to the ceiling. Why go to the expense of fixing something when it’s working so well? Owner agreed to leave as is. Jolly good. If I were a lesser honourable chap, I would have said to the customer the amp was beyond economic repair and would take it off his hands as a gesture of good will!