Sony KV-25XBR: no channel 3

[Andy Cuffe]

I'm working on this 80's Sony TV with a very strange tuning problem.
The picture looks excellent on all channels (cable) except channel 3.
On ch 3 the picture looks overloaded, distorted and white text streaks
to the right. It looks a lot like a combination of the RF AGC being set
too high and the fine tuning being off. I've replaced the tuner and PLL
module and checked for bad soldering in the IF section. As a last
resort, I tried carefully tweeking the AFT coil. I've confirmed that
channel 3 looks fine on my cable. This TV has no manual fine tuning, or
CATV system switch. Does anyone have any ideas? Could the micro
controller be bad, telling the PLL thr wrong frequency for channel 3?

 

[Sofie]

Andy Cuffe:
Strange symptom for sure. As you probably already know, if something in
the tuner is faulty, it will usually show up in groups of channels..... i.e.
"low band" channels 2-6.
You might try a few experiments by hooking up a VCR with it's output set
for channel 3 connected with the RF connector via channel 3 on the TV.
Does it look OK when playing a tape? How about when the VCR tuner is set
for channel 3? or any other channel (leaving the television set on channel
3) And then set the VCR RF modulator out to channel 4 and the television
to channel 4...... when you tune all the different channels with the VCR
tuner, do they come in clearly.... including channel 3 ??
Immediately, by analyzing the results, you will be able to confirm several
things........ most obvious would be:
1) that your television will properly tune in channel 3 (from the VCR
modulator)....
2) and that the cable is sending out a good channel 3 that your VCR can
tune in clearly.
Note: Have you tried another television on the exact same cable outlet that
your Sony TV is hooked up to?

 

[Leonard Caillouet]

Are you using a signal generator for a reference or your cable? How have
you confirmed it?

Leonard Caillouet

 

[Andy Cuffe]

Andy Cuffe:
Strange symptom for sure. As you probably already know, if something in
the tuner is faulty, it will usually show up in groups of channels..... i.e.
"low band" channels 2-6.
You might try a few experiments by hooking up a VCR with it's output set
for channel 3 connected with the RF connector via channel 3 on the TV.
Does it look OK when playing a tape? How about when the VCR tuner is set
for channel 3? or any other channel (leaving the television set on channel
3) And then set the VCR RF modulator out to channel 4 and the television
to channel 4...... when you tune all the different channels with the VCR
tuner, do they come in clearly.... including channel 3 ??
Immediately, by analyzing the results, you will be able to confirm several
things........ most obvious would be:
1) that your television will properly tune in channel 3 (from the VCR
modulator)....
2) and that the cable is sending out a good channel 3 that your VCR can
tune in clearly.
Note: Have you tried another television on the exact same cable outlet that
your Sony TV is hooked up to?

I tried it witha CATV box with a ch 3 output and it looked bad on all
channels. Ch3 and all other channels tuned though a VCR with ch 4
output looks good.

I've tried the sony on 2 different CATV lines that I know are good.

 

[Andy Cuffe]

Are you using a signal generator for a reference or your cable? How have
you confirmed it?

I'm using the CATV which looks good on other TVs, including a similar
Sony. The only thing I can think to check for is some kind of high
frequency oscillation in the area of the tuner, PLL, or IF at the
frequency of ch 3. When on ch 3 the AGC voltage maxes out even though
ch 3 is strong. Reducing the AGC control does bring the AGC voltage
down, but only results in a snowy distorted picture. AGC is working
properly on all other channels (at least CATV 2-72, 95 and 99). The
picture looks like the AFT is locking onto the wrong signal, but the
fact that it won't work with a VCR with ch3 output rules out any weird
problems with ch 3 on the CATV.

I tried an experiment manually adjusting the tuning voltage by
connecting a resistor from the tuning voltage input of the tuner to
ground. When set to ch 3, Vt=4.56v. If I connect a 1.6k resistor to
ground Vt changes to 4.29v and ch 3 looks perfect. Obviously the tuner
and everything else is capable of receiving channel 3, it's just tuning
a little too high. It behaves like an old preset tuner with a
misadjusted preset.

 

[eddumweer]

Probable like other Sony ktv's here on the dutch market, try to resolder the
tuner-pcb.

Greetings Peter

 

[Andy Cuffe]

Probable like other Sony ktv's here on the dutch market, try to resolder the
tuner-pcb.

I already replaced the tuner with a spare from a junk set.

 

[JURB6006]

Just an idea, have you tried bypassing the RF switcher ? Doesn't sound all that
likely but now we are in the realm of the wierd, you seem to eliminated just
about everything else.

JURB

 

[Andy Cuffe]

Just an idea, have you tried bypassing the RF switcher ? Doesn't sound all that
likely but now we are in the realm of the wierd, you seem to eliminated just
about everything else.

JURB

Well, I had to call it quits on it. I ruled out just about everything
that could cause this kind of problem. I'm just going to sell it
cheaply to someone who uses these as RGB video game monitors.