[R-390] Saturday's Line & Local Report

[Charles Steinmetz]

Craig wrote:

>Sometime today the IF output of both receivers are going to be checked.
>Money says I'll order some new caps and see what happens.
>    *   *   *
>all but two caps have been replaced (C612,C601) C601 looks like a Vitamin Q,
>    *   *   *
>I'm tired of swapping modules. Where I can pick a point and check voltages,
>watch the O'scope, etc; much easier.
>
>If [C603] should bypass all audio to ground, we have a suspect.

Bob wrote:

>I might say that the harness is suspect, as you have found the 
>jumper(s) and components touching a jumper.

Tisha wrote:

>Your problem with an excessively high audio level should follow the bad
>module. If you have an assortment of known-good modules you should be able
>to swap your way into isolating the problem to a bad module.
>    *   *   *
>My concern is that by undergoing a wholesale swap-out of capacitors really
>does not put you any closer to solving the problem. While you may
>eventually luck-out in replacing the bad part it may get pretty expensive
>and frustrating until you reach that point.

Craig wrote:

>What I can't remember doing is putting both IF & AF modules from the
>Motorola into the Amelco, "at the same time"
>    *   *   *
>Some how, did I get a R-390/A with
>several modules with problems that no one was going to fix?

Like Bob and Tisha, I am concerned that you are not proceeding in an 
orderly, logical way and are not making use of all the data that you 
are collecting as you go.  This can only lead to frustration and 
wasted effort.  Module swapping is a way to EXCLUDE modules as the 
source of a problem as much as it is a way to identify bad 
modules.  Some of the data you have collected seems to indicate that 
the problem is NOT IN THE AMELCO IF OR AF MODULES AT ALL.  Thus the 
suggestion to swap modules again, to be sure.  Note that it is just 
as important to swap the questionable modules into the known good 
radio as it is to swap the known good modules into the radio with the 
problem.

If swapping the Moto AF module into the Amelco radio (which you say 
you have done) does not fix the problem (which you say it doesn't), 
it strongly suggests that the Amelco AF module is not the source of 
the problem, because if the problem is in a module, we expect it to 
follow that module.  You would confirm the "Amelco AF module is not 
the source of the problem" hypothesis by installing it in the Moto 
radio (which you do not say you have done).  If you do, and the Moto 
radio works fine with the Amelco AF module, then THE PROBLEM IS NOT 
INTERNAL TO THE AMELCO AF MODULE and you can stop looking for it there.

The same applies to IF modules.  You say putting the Moto IF module 
into the Amelco radio does not fix the problem in the Amelco 
radio.  You would confirm this by putting the Amelco IF module into 
the Moto radio.  If it works fine there, THE PROBLEM IS NOT INTERNAL 
TO THE AMELCO IF MODULE and you can stop looking for it there.

Until you do the experiment and find otherwise, it appears from what 
you have said so far this would indeed be the case.  So, the path to 
greatest information for the least work is:  (1) swap the AF modules 
BOTH WAYS (Moto AF into Amelco radio, Amelco AF into Moto radio).  If 
the Amelco radio still has the problem and the Moto radio still works 
fine, leave the AF modules where they are and (2) swap the IF modules 
both ways (Moto IF into Amelco radio, Amelco IF into Moto radio).  If 
the Amelco radio still has the problem and the Moto radio still works 
fine, THE PROBLEM IS NOT INTERNAL TO THE AMELCO AF OR IF 
MODULES.  Again, from what you have said so far (Moto AF module in 
the Amelco radio does not cure the Amelco radio, Moto IF module in 
the Amelco radio does not cure the Amelco radio), we expect that this 
will be the case.

That may be a fair amount of work, but if you do it and the results 
are as above, you can QUIT LOOKING FOR THE PROBLEM INSIDE THE AMELCO 
AF AND IF MODULES.  Think how much work that would save you.  (If you 
had done that first, and those were the results, you wouldn't have 
had to do any of the work you have done these past several years and 
could have concentrated your efforts on looking elsewhere for the problem.)

SO, if the problem is not internal to the Amelco AF or IF modules, 
where can it be??  The next most likely suspects would be grounding 
and the wiring harness.  For example, if you have audio at the 
junction of of R606 and the AF B+ choke (L603), yet the capacitors in 
C603 appear to be good, it could be because the negative end of C603 
is not properly grounded due to a faulty ground to the main 
chassis.  Since you have already found a number of issues with the 
harness, further problems in that area should not be a surprise.

Best regards,

Charles