Guy that I know brought me a Polytone Mini Brute to look at earlier this week. He reckoned that it hummed, but otherwise worked ok. I slung it up on the bench today, and he was right. It had a noticeable raspy sort of background noise that was controlled by the master volume. Otherwise, it worked ok. I also discovered that the level of the noise could be altered a little by flexing the preamp board. When I got it out to look at, it had previously had some pretty poor work done, so I first tidied that up. You could tap the board pretty much anywhere, and the level of the noise altered, but never went away. I did a blanket solder up of the most sensitive area, and the noise was then at a fixed level. Not sensitive to tapping any more.
The preamp has its own simple dual rail power supply with a bridge, a couple of filter caps, and a pair of 78 / 79 15 volt regs. Scoping the positive rail, there was a fair bit of ripple on it - certainly more than on the negative rail. This was still there checked at an op amp, which I thought was odd, as those 3 terminal regs are usually pretty good at ripple rejection. I then checked the actual voltages at the IC.
-15 volts at pin 4 ...
... and +28 volts at pin 8 !!!!
So the poor little TL072 ICs on there had 43 volts across them ! A quick check back at the 7815 showed 28 volts in and 28 volts out. A new regulator restored the voltage to +15 volts, and the whole amp became almost totally quiet.
I don't think that in over 40 years of being in this game, I can ever remember a 78xx series device going short between its input and output pins - and it was a measurable dead short. I was also amazed that the poor old '072s had survived having all those volts across them, absolute max rails being quoted at + / - 18 volts ...
Arfa