[Premium-Rx] RA1792 Problems. Help Please!

[Dan Rae]

David Schofield wrote:

> Hi Stuart,
>
> The short answer to your question is YES. There are very well known 
> faults on the RA1792. Basically suspect "every" tantalum capacitor in 
> the unit.
> The easy way to fix the unit is to replace all the tants. 


Hold on there!

It isn't easy; there are a LOT of them.  I did it once and it took a 
week of intermittent work to complete.   I forget the count, but it is 
up in the fifty plus range, and in particular the ones on the double 
thickness A4 board are a pig to remove since the plated through holes 
have no heat relief.

I will not disagree that in some of them, but not all, the tantalums are 
prone to failure, but the symptoms are sudden death, after being 
powered  off.  I only did the one I referred to above because it had a 
total of five dead ones when I got it, and I thought the message was 
clear enough and I could spare the time!

It isn't a bad idea however to replace all the aluminium electrolytics, 
as in any 25 plus year old radio.

In Stuart's case the symptoms he describes, a gradual failure, is almost 
certainly down to battery damage to the A6A2 card.   In both designs of 
cpu board the components that link the cpu board to the rest of the 
radio are adjacent to the battery and any leakage will give the sort of 
data bus problems he describes. 

"What we have here is a failure to communicate!"

Exactly what damage has been done varies.  I have often had to remove IC 
sockets where fitted, since the corrosion destroys the contacts inside.  
Also the VIAs on the board in that area are prone to go open circuit, 
since the alkali from the nicad gets under the resist layer and destroys 
the trace at the edge of the hole.  I have had to remove the 34 way edge 
connector a couple of times as well.  

And a plea to any 1792 or 6790/GM owner who has not already done so: 
change the battery for a new NiMH one!

73

Dan
ac6ao / g3ncr