[rec.video] RCA ColorTrak TV tuning problem SOLVED

wales@maui.cs.ucla.edu (10/06/88)

A few weeks ago, I reported an intermittent tuner failure in my RCA
FKC2023 ColorTrak (26-inch TV/monitor).  It appears that I have finally
found and solved the problem.  Thanks to all who responded to my query.

Since some of the net readership (including, particularly, those who
theorized on the cause of the problem and/or suggested diagnostic pro-
cedures) may be interested in how I went about troubleshooting the set,
here is a description of what happened.

The problem, in brief, seems to have been caused by poor solder joints
between a 10-pin "header" connector and the Tuner Control board.  This
"header" mates to a female connector on the Tuner board.  The Tuner and
Tuner Control (each in a separate shielded box) are mounted right next
to each other in the lower left-hand corner of the set (as you look at
the open set from the back; the VHF and UHF cable inputs on the back of
the set go directly into the Tuner module).

The solder joints in question worked OK when the set was new, but over
the years thermal expansion and flexing of the boards and/or their
shield boxes apparently stressed the joints until they started failing
earlier this year (after about three years of set operation).

In my attempts to troubleshoot the set, I opened up the Tuner Control
board's shield box (top and bottom); reconnected the board (with the
shield off); and let the set warm up.  When the TV started misbehaving,
I used a "cold spray" on various parts of the board until it worked
again.  I assumed I would find a temperature-sensitive component, which
I could then replace.

Unfortunately, while the "cold spray" did relieve the problem, I could
not isolate the misbehavior to any single component.  The best I was
able to do was narrow it down to a region near one end of the board
(right next to the above-mentioned 10-pin header -- though at the time I
did not yet realize the significance of this fact).  Spraying *any* com-
ponent in this area -- or spraying the board itself -- made the problem
go away for a few minutes.

There was a miniature trimmer pot on the Tuner Control board.  Thinking
that this might be dirty and/or need tweaking, I turned it back and
forth several times (returning it as close as possible to the original
setting).  Strangely, when the set started having problems, I could get
it to work again by pushing on this trimmer pot with an insulated probe
(even without changing its setting) -- but as soon as I stopped pushing,
the problem returned at once.

I then discovered that, if I jiggled the TV set -- or even tapped on the
Tuner Control board -- I could cause the problem to recur.  It was at
this point that I decided to take a closer look at the solder joints on
the Tuner Control board.  Sure enough, when I examined the 10-pin header
joints under a magnifying glass, about half of them had holes and/or
cracks in them.

I resoldered all ten of the joints and inspected them under the magni-
fier to make sure they were all good.  The tuning problem went away
after I did this, as far as I can tell.

It would appear that the holes on the circuit board, into which the pin
ends of the 10-pin header went, were somewhat larger than they needed to
be.  This made it harder to do a good, reliable soldering job on these
pins.  Assuming that the solder joints on the header pins were made by a
"flow solder" machine, rather than by hand, it is very easy to see how
the bad joints resulted.  The joints were, to be sure, good enough to
work initially -- but they were not mechanically sound, and as thermal
expansion and flexing took its toll over the years, at least one (and
maybe more) of the joints eventually gave way.

Anyway, the set seems to be fixed now, works very nicely, and I am very
happy with it.  Thanks again to everyone who offered me advice.

-- Rich Wales // UCLA Computer Science Dept // +1 (213) 825-5683
   3531 Boelter Hall // Los Angeles, California 90024-1596 // USA
   wales@CS.UCLA.EDU    ...!{uunet,rutgers,ucbvax}!cs.ucla.edu!wales
   "Zounds!  A Gorkon death station appears!  Evasive action!"