Customer brought this vintage 80’s bit of kit to see if could get working as had sentimental value, they felt that the preamp was dead. Plugged in and there was no light on the preamp and indeed seemingly no output.
However the preamp is powered from the power amp via a special cable, and could hear some bits inside the power amp case, so went about opening to find this stuff coming out!
On further investigation it looks like there was some corrosion in the case, perhaps left in a damp area or something along those lines?
Then the main cause of the failure was found – the 24V regulator that supplies power to the pre-amp – had corroded completely through and disconnected the unregulated input pin, suggesting that electrolysis was in effect to speed up the corrosion process – damp and powered up?!
I had a spare 7824 in stock (the last one so more ordered!) and after preparing the area cleaning up and mounted on a new heat transfer insulating pad, the new regulator was soldered in place to the wiring, fitted using new rubber sleeves and the original cable tie put back where it belongs.
The unit now powered up, 23.939V from the regulator, good enough! Initial testing was fine, both channels working as expected, although the volume control was a little scratchy, unsurprisingly, as was the balance control – both were treated with some Deoxit F5.
After checking and resetting idle current to the specified 7.2mV at measurement points for each channel, attention was moved to the condition of the cases, which were quite – erm – mildewy? A good scrubbing with some “Flash with bleach” followed by general cleaning and some 303 Aerospace Protectant improved the situation no end:
Inside the preamp, there’s the 323 phono input boards. These were removed and a little Deoxit D100L applied to the contacts to ensure the future trouble free connection.
The power knob was misaligned too, so after releasing the deep-inside grub screw, was able to position correctly and tighten up so could see the power on led through the little hole!
All sorted and customer looking forward to many hours of listening through this desirable little bit of kit!