Arfa Daily wrote:

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
Sure they do that, if for some reason the input voltage drops to fast
and still has output voltage on the reg, it can back feed and short at
times. It depends on what is happening on the input side, like maybe a
short circuit yanked it down real quick.

Normally a back feed diode is installed for those regs to stop that.

Of course, you can only verify this if pin 1 and pin 3 are ohmed
shorted. It could also be the common leg becoming detached inside. I've
seen that happen do to poor mechanical mounting practices and near by
vibration.

Jamie