Racal RA6790/GM

[simba15]

So my repairs continue down the line!

I have a Racal 6790 that is not in good shape, the dreaded backup battery leaked!

It power on and has a real fit!~  All displays light, then the freq display goes blank.

I cleaned some corrosion off the board and tried again.

This time, messing with the power switch ( on-off a few times) the freq display will work and even change with the main knob, but only for 500ms, after this the Fault light comes on.

Power rails all test ok, I suspect the fault is with the A6A2 Board.

The tantalum caps seem ok (C13,C14)

C17 on the other hand seems iffy, Across reads ~40ohm, but once removed, Q7 Drain is still ~40ohm to gnd (this is the reset line to the CPU)

I noticed that Q7 and Q6 are listed as TIS74 but on my board they are 2N4392. Can I replace with TIS74?

Any ideas if these values are OK, or should I start replacing Q7, caps, ect.

[w3jn]

Before you go any further, check the filter caps.  I had a 6790 that acted really wonky; all the filter capacitors were pretty much open and the regulators weren't able to deal with the ripple.  Just poke at each power supply output with your meter on AC; the ripple will show its ugly head if the caps are bad.