Corrosion in a Mosley 6020 STL Receiver. Not Good!

I got called in the other night to pick up and fix a Mosley 6020 STL receiver that had no LO1 level on the meter. The AFC Lock was on but there was no indication of receive. All voltages, +-15 and 5 were good. I brought it back to the shop and as soon as I opened it I could smell that burnt board smell we all dread. I took the top off the RF Module section and didn’t see anything wrong but started checking a few voltages and noticed the +12 volts rail that supplied the section where the first LO was located was not present so I took the bottom off the RF module and this is what I found.

As you can see from the picture the board is burnt and there seems to be some sort of corrosion around whatever that piece is that is actually mounted on the other side. It is not a component and it is not named on the board layout picture either. It is just a brown block that is soldered to ground on the other side of the board also. That trace supplies 12V to half of the board. The next picture shows the part at the top center.

I cleaned up the area and ran a piece of wire wrap wire to the point further down the trace as well as a point a little further up. I measured the current on that trace and it was about 220mA, not excessive since it runs the oscillator and a bunch of other filters and things.

I locked down the wire with some conformal coating and let the unit run for 12 hours. After that I did a full alignment and everything was spot on.

The same corrosion seems to have started under the block on the opposite side of the board. I cleaned it off too and started looking around and found more of the stuff.

I cleaned the whole bottom of the board with isopropyl alcohol and some vinegar for good measure. Before I cleaned it I measured it for resistance and found it was about 10K/mm. I don’t think that would account for the burnt board and trace since 12V/10000Ohms=.012A even if the trace was 1mm away from the ground pad on the brown block thing which it is not. It is more like 2mm.

I also do not know how the corrosion even started. I know this 6020 is 33 years old but it has lived its entire life in a pretty nice environment.

Oh well. It will remain a mystery I guess but at least now the station has a functioning backup Studio Transmitter Link receiver and if you follow my YouTube channel and watched this video https://www.youtube.com/watch?v=VupFexWWMKo you will realize that backup STLs are a good thing.

On to the next mystery.