I never cease to be amazed ...

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

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Arfa Daily
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I've not seen a 78/79xx go dead short either, but it happens often enough on other series pass power supplies.

It all circles back to "check the power supply". The bad -5 volt rail in my bench supply where I assumed the completely missing zener was just an option left out at the factory was pretty quickly solved when I did finally check all voltages in the service guide.

Not checking that first was almost inexcusable, the power supply had two indentical, isolated outputs 1 and 2 and the the voltages at the connector going to the control board behind the bezel were clearly marked with what voltage should be present.

Hats off to the designers at TTI in the UK for doing their part right.

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Cydrome Leader

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