Alan Stevens wrote:
I would appriciate any advice I could get on my next step with
troubleshooting a Sansui TVM2500C "Version A" 25-inch television set. I
should declare right now that I'm a two-way radio guy, not a TV repair
man. I have good general troubleshooting skills but I don't have the
equipment or knowledge to work on circuits specific to the picture tube,
flyback transformer, etc. This repair is being done as a favor to a
friend.

The unit was brought to me in the following condition, so I have no
information on its behaviour prior to this point. When first powered up,
the screen lights up with normal empty-channel snow even though a
station should be present on that channel, there is no volume, the
channel up/down buttons work properly (the numbers change on the
on-screen display, anyway) but the volume up/down buttons do not - one
volume button causes the channel to change and the other doesn't do
anything.

after the TV has been running for a while, a popping and hissing
starts to come from the flyback transformer (at least it sounds like
it's coming from there) and a TV station starts to appear erratically on
the screen for whatever channel It's tuned to. This behaviour goes on
for a while, a pop or click whenever the TV station appears or
disappears, and when the TV reception is flickering rapidly the
transformer is hissing. Eventually the signal settles down, the noise
stops and the station is displayed but there is no color. The volume
buttons continue to operate improperly as described above.

I have checked most of the electrolytics and they're fine. Although I
do not know the exact voltages that should be present during operation,
the levels I see seem reasonable for the rating of each of the
capacitors I've checked. Oddly enough, I do not detect any voltage
fluctuations on the circuit board during the unstable period of
operation. Whether there's video on the screen or not, everything I
measure seems to be the stable. I'm only checking the low and medium
voltage points of the board at the base of each capacitor so I don't
accidentally encounter any high voltage related to the picture-tube, and
I'm also checking certain points in the power-supply section that I feel
are safe for me to meter.

I've only been able to find data on two of the chips on the circuit
board, so checking ICs for voltage and logic levels according to the
published data isn't going too well.

Due to the noise coming from the flyback transformer during the
erratic period of operation, I suspect this component. There are two
low-voltage points silk-screened on the bottom of the transformer, a +25
volt and a lower one I can't recall right now. At no time do I see any
voltage whatsoever at either of these points, whether the channel is
being displayed on the screen or not.

Basically, I want to know if there's anything else I should be looking
at or if I should be telling the owner that the flyback transformer is
shot and it's out of my hands. It just seems odd to me that the flyback
transformer should be affecting the user controls and that I can't find
any unstable voltages. I keep thinking there's something I've missed.


At this point I wouldn't worry too much about the noise from the
flyback.
I would check for bad connections to the micro or the crystal. Also,
given that symptoms are worst when cold, and seem to stablise, I would
replace the secondary electrolytics in the power supply to be sure.

From there, another area worth pursuing is the IF section (esp. its
electrolytics), your description of the problems with reception sound
like this area may be of importance here.

In one case, I was working on a Thomson chassis and there were similar
symptoms to yours (poor tuning, jumpy picture, loss of reception, snow
etc,) coupled with intermittent user controls. Inspection showed that
data and clock lines went round the tuner on the pcb, these traces had
been cracked due to someone forcing a plug into the antenna jack
(integrated in the tuner.) Due to the tuner being soldered directly
onto the mainboard, there was not enough support for this heavy
component and the board got cracked, lifting the tuner earth points and
the data and clock lines. A few moments with a sodering iron effected
a repair.
good luck, Ben