[Racal] RA-6790 problem

[Robert Nickels]

First, thanks for the ideas and suggestions - I've spent several 
educational hours on the 6790 since then, and have found and fixed a 
couple of things - not at all what I expected to find.  Here's the box 
score:

1.  All supply voltages looked fine with a meter but a scope showed 
oscillation on the 5V rail, which measured 20 ohms to ground with power 
off.  I started isolating boards, suspecting a shorted tantalum cap was 
pulling the supply down, causing it to act like a relaxation oscillator.

2.  The problem was quickly isolated on the A8 module, and it turned out 
to be a 74LS74 IC, not a cap.   I applied 5 volts from a current-limited 
bench supply and found that something was drawing 600ma, then used one 
of the little IR thermometers to find the hot spot.   (Much easier on 
the fingers than the old way!)

3.  With the repaired board back in the receiver the power supply still 
oscillated.   Using a resistive load, I  found the 5 volt regulator was 
stable with a 10 ohm (1/2A) load but not with a 5 ohm (1A) load.  Since 
excessive ripple was observed on the input side, the 12,500uf filter 
capacitor was replaced and that cured this problem.

4.  With two "hard" failures out of the way I thought I was in for some 
good luck, but I still got BITE error with a failure code of 20 - again 
indicating a problem on the A8 board.   Working through the circuitry, I 
found a stuck logic line on another IC - and guess what type it is?  
Yes, another 74LS74 with an internal ohmic short, and it too is a 
Fairchild with the identical Indonesia 8146 datecode as the one 
previously replaced.  (Anyone smell a lot problem?)

5.  Having replaced that IC I am still getting BITE error 20, pointing 
to one of the two PLLs on the A8 board.  I've walked through each 
quickly looking for gross problems, so now will need to sit down with 
the manual a bit before proceeding.  I'm inclined to think it's the BFO 
synth - both had RF output but evidently one isn't locked.

At least the CPU is working again and I can get into BITE so that's 
progress.   Degradation of the p/s filter is understandable, but I'm 
really surprised by the two IC package failures.

73, Bob W9RAN