Toshiba-TV channel-lock-in (AFC) problem

[Gene Gardner]

Need suggestions on Toshiba TV problem: (Model CF2750K, chassis # TAC-9011, Mf'd Nov 1991)
It works pretty well much of the time, but the Automatic Frequency Contol seems to
be "straining" to lock on to a channel after changing the channel. It usually takes a
second or two for it to "pull in" and lock. Sometimes it does not succeed, and seems to
lock in between two adjacent channels....sound from one, and weak video from the other.
My intuition is that there may be a single Capacitor-trimmer, or Potentiometer that
might "re-center" the "capture-window" and provide more certain "lock-in".
Can anyone suggest a specific adjustment, or an approximate location based on a similar
model? I do not have a schematic diagram. Thanks in advance!
(I was fortunate enough to get excellent advice recently which cured my Vertcal Linearity
problems!)

 

[Jerry G.]

I cannot tell you specifics about your set. But, there is a chance that
a component in the AFC detector, or one of its stages that has become
off value over time. The preference is to find the defective part, and
change it.

The AFC detector should be located in the IF amp section. There is a
phase detector coil for this operation. If you look on the circuit
board, of the IF amp section, you may see something marked AFT to
indicate the related area.

If you call the parts dealer for Toshiba, you may be able to purchase a
service manual, if the model is not too old.

At least this little contribution here may be a good start for you.


Jerry G.
========

 

[Jerry G.]

I cannot tell you specifics about your set. But, there is a chance that
a component in the AFC detector, or one of its stages that has become
off value over time. The preference is to find the defective part, and
change it.

The AFC detector should be located in the IF amp section. There is a
phase detector coil for this operation. If you look on the circuit
board, of the IF amp section, you may see something marked AFT to
indicate the related area.

If you call the parts dealer for Toshiba, you may be able to purchase a
service manual, if the model is not too old.

At least this little contribution here may be a good start for you.


Jerry G.
========

 

[Jeff Rigby]

Need suggestions on Toshiba TV problem: (Model CF2750K, chassis # TAC-9011, Mf'd Nov 1991)
It works pretty well much of the time, but the Automatic Frequency Contol seems to
be "straining" to lock on to a channel after changing the channel. It usually takes a
second or two for it to "pull in" and lock. Sometimes it does not succeed, and seems to
lock in between two adjacent channels....sound from one, and weak video from the other.
My intuition is that there may be a single Capacitor-trimmer, or Potentiometer that
might "re-center" the "capture-window" and provide more certain "lock-in".
Can anyone suggest a specific adjustment, or an approximate location based on a similar
model? I do not have a schematic diagram. Thanks in advance!
(I was fortunate enough to get excellent advice recently which cured my Vertcal Linearity
problems!)

Your guess is probably correct BUT the "Q" of that circuit might be too

low for it to work properly. Mold, dust, moisture, breakdown of insulation
can cause the Q of a tuned circuit to decrease. The fix would be to
PROBABLY replace the coil. I don't think they would be available 13 years
after mfg.