VCR RF Output works but Video Out Does Not

[VCR Gymnast]

Strange problem: I have two VCRs that work perfectly when the RF output
is used, but there is no video output from the RCA Video OUT (the RCA
Audio OUT works fine). Since the RF out shows the video, the problem
must be after the signal splits out to each output. The only elements
in the path are a PNP transistor (tested good) with its collector
output in series with a resistor and capacitor before going to the
75-ohm RCA Video OUT. There are no switches in the circuit. Any ideas??

 

[mc]

Strange problem: I have two VCRs that work perfectly when the RF output
is used, but there is no video output from the RCA Video OUT (the RCA
Audio OUT works fine). Since the RF out shows the video, the problem
must be after the signal splits out to each output. The only elements
in the path are a PNP transistor (tested good) with its collector
output in series with a resistor and capacitor before going to the
75-ohm RCA Video OUT. There are no switches in the circuit. Any ideas??

Signal-trace the video. It should be present at either the collector or the
emitter of the PNP transistor, maybe both.

Broken connector?

 

[Bob AZ]

<< There are no switches in the circuit. Any ideas?? >>

I have had this occur at times. It is simply that some VCRs will not
output video when there is a load on the RF and vice versa.
Bob AZ

 

[Dave Plowman (News)]

I have had this occur at times. It is simply that some VCRs will not
output video when there is a load on the RF and vice versa.

But surely the normal setup for RF is Aerial > VCR > TV so the TV always
presents a load regardless of what the VCR is doing?

 

[VCR Gymnast]

Thanks MC. I traced the video signal and here is what I found:

Q062 (Video OUT) is a PNP transistor (2SA1048-GR) whose emitter
provides the base input (via a 470 ohm resistor) to Buffer Q063 (NPN,
2SC634SP) for the RF video output and (via a 100 ohm resistor) to
Buffer Q081 (PNP, 2SA1048-GR) for the RCA video OUT.

It looks like the emitter output of Q062 should have the Video Output,
but it does not (could this be the problem?). Instead, its collector
has a weak video output.

The emitter output of Q063 is fed to the RF unit, and works fine with a
nice strong video output. However, the emitter output of Q081 has no
video signal.

As a temporary workaround, I have tied the emitter output of Q063 to
the pin that supplies the RCA video OUT. This works (albeit not an
elegant solution), and a good video signal appears at both the RCA and
RF video outputs.

This leads to three questions:
1. Is it most likely that Q081 is bad?
2. Could Q062 be bad even though it feeds the base of Q063, whose
output works?
3. Is what I have done (splitting the output of Q063 to both video
outs) a safe long-term solution?

Thanks!

 

[Bob AZ]

Dave

<<But surely the normal setup for RF is Aerial > VCR > TV so the TV
always
presents a load regardless of what the VCR is doing?>>

I agree. We tested some at work one day and there are some VCRs that
will not do both RF and Video at the same time.
If I had a correct schematic then I could be of further help. But for
openers I would put a dummy load on each output, get out my scope and
have a look.

Bob AZ

 

[Franc Zabkar]

Thanks MC. I traced the video signal and here is what I found:

Q062 (Video OUT) is a PNP transistor (2SA1048-GR) whose emitter
provides the base input (via a 470 ohm resistor) to Buffer Q063 (NPN,
2SC634SP) for the RF video output and (via a 100 ohm resistor) to
Buffer Q081 (PNP, 2SA1048-GR) for the RCA video OUT.

It looks like the emitter output of Q062 should have the Video Output,
but it does not (could this be the problem?). Instead, its collector
has a weak video output.

The emitter output of Q063 is fed to the RF unit, and works fine with a
nice strong video output. However, the emitter output of Q081 has no
video signal.

As a temporary workaround, I have tied the emitter output of Q063 to
the pin that supplies the RCA video OUT. This works (albeit not an
elegant solution), and a good video signal appears at both the RCA and
RF video outputs.

This leads to three questions:
1. Is it most likely that Q081 is bad?
2. Could Q062 be bad even though it feeds the base of Q063, whose
output works?
3. Is what I have done (splitting the output of Q063 to both video
outs) a safe long-term solution?

Thanks!

IMO Q081 needs to be replaced. For troubleshooting purposes you could
always interchange Q062 and Q081.

- Franc Zabkar

 

[Dave Plowman (News)]

I agree. We tested some at work one day and there are some VCRs that
will not do both RF and Video at the same time. If I had a correct
schematic then I could be of further help. But for openers I would put a
dummy load on each output, get out my scope and have a look.

Right. I think. ;-)

But how does it know which output - RF or video - you want to use? VCRs
are normally left plugged up? Or does it sense a video connection and
default to that?