Got two similiar late 80's JVCs, both are 4-head mono and basically have
the exact same problem. The bottom half or so of the picture is just snow,
the top is normal but it looks like the bottom of the picture pushed up to
the top of the screen. Now here's the interesting thing: every time the VCR
is powered, the amount of snow changes. Next time, there might be 75% of a
picture, or 25%. It seems to be completely random. Tracking adjustment makes
no differnence, and the tape path is aligned properly.
Tried swapping out the power supply and head preamp boards, but this made
no difference. Perhaps the replacements were bad too?
Anything else I could check, without a service manual? These are both
really nice machines (one is a commercial unit in a white case) and I hate
to just junk 'em.
Thanks for any advice.

Check for a bad surface mount electrolytic on the drum motor board.I forgot the
value I think it was 4.7uf? Also check and make sure the guide posts are not
loose and out of adjustment. Also check and make sure the brass guide locators
are intact and that the guides properly seat in the V-Blocks in play mode. Try
the cap first!
Lew

Chris F:
The repair suggestions in the reply post from Lew are right on...... all of
what he suggested should be looked into.
The drum motor smd cap is a 3.3 uf unit.... and is a common failure.....
located on the bottom of the drum motor.... a regular radial lead
electrolytic can be substituted.. The brass retainer pins on the roller
guides are just a press-fit and can get loose and fall out..... a drop of
super glue will help keep them tight when you press them back in..... and
don't forget the plastic pins on the roller guide assembly that connects the
pivot arms, verify that they are fully seated.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
----------------------------


"Lew" wrote in message
...
Check for a bad surface mount electrolytic on the drum motor board.I
forgot the
value I think it was 4.7uf? Also check and make sure the guide posts are
not
loose and out of adjustment. Also check and make sure the brass guide
locators
are intact and that the guides properly seat in the V-Blocks in play mode.
Try
the cap first!
Lew

"Chris F." bravely wrote to "All" (13 Dec 03 00:11:47)
--- on the heady topic of "Old JVC VCRs - half of picture missing"

Tracking has no effect indicates that the transport servo is locking
randomly on noise and not on the proper head drum pulse. The cause is a
loss in value of the motor signal electrolytic cap on the head drum pcb
under the chassis. Check the electros for high esr or low capacitance.
After this is fixed make sure the switch point is correct. It is a
very common JVC problem.

CF From: "Chris F."

CF Got two similiar late 80's JVCs, both are 4-head mono and basically
CF have the exact same problem. The bottom half or so of the picture is
CF just snow, the top is normal but it looks like the bottom of the
CF picture pushed up to the top of the screen. Now here's the interesting
CF thing: every time the VCR is powered, the amount of snow changes. Next
CF time, there might be 75% of a picture, or 25%. It seems to be
CF completely random. Tracking adjustment makes no differnence, and the
CF tape path is aligned properly. Tried swapping out the power supply
CF and head preamp boards, but this made no difference. Perhaps the
CF replacements were bad too? Anything else I could check, without a
CF service manual? These are both really nice machines (one is a
CF commercial unit in a white case) and I hate to just junk 'em.
CF Thanks for any advice.

How do I check the switch point?

"Asimov" wrote in message
...
"Chris F." bravely wrote to "All" (13 Dec 03 00:11:47)
--- on the heady topic of "Old JVC VCRs - half of picture missing"

Tracking has no effect indicates that the transport servo is locking
randomly on noise and not on the proper head drum pulse. The cause is a
loss in value of the motor signal electrolytic cap on the head drum pcb
under the chassis. Check the electros for high esr or low capacitance.
After this is fixed make sure the switch point is correct. It is a
very common JVC problem.

CF From: "Chris F."

CF Got two similiar late 80's JVCs, both are 4-head mono and basically
CF have the exact same problem. The bottom half or so of the picture is
CF just snow, the top is normal but it looks like the bottom of the
CF picture pushed up to the top of the screen. Now here's the interesting
CF thing: every time the VCR is powered, the amount of snow changes. Next
CF time, there might be 75% of a picture, or 25%. It seems to be
CF completely random. Tracking adjustment makes no differnence, and the
CF tape path is aligned properly. Tried swapping out the power supply
CF and head preamp boards, but this made no difference. Perhaps the
CF replacements were bad too? Anything else I could check, without a
CF service manual? These are both really nice machines (one is a
CF commercial unit in a white case) and I hate to just junk 'em.
CF Thanks for any advice.

"Chris F." wrote in message
...
Got two similiar late 80's JVCs, both are 4-head mono and basically have
the exact same problem. The bottom half or so of the picture is just snow,
the top is normal but it looks like the bottom of the picture pushed up to
the top of the screen. Now here's the interesting thing: every time the
VCR
is powered, the amount of snow changes. Next time, there might be 75% of a
picture, or 25%. It seems to be completely random. Tracking adjustment
makes
no differnence, and the tape path is aligned properly.
Tried swapping out the power supply and head preamp boards, but this
made
no difference. Perhaps the replacements were bad too?
Anything else I could check, without a service manual? These are both
really nice machines (one is a commercial unit in a white case) and I hate
to just junk 'em.
Thanks for any advice.

Looks like one of the heads is at fault. Try some cleaning.

Dimitrij

Just replace that cap - the guys who suggested it are correct without any
doubt.

The switch point is checked by using a dual-trace 'scope and comparing the
SW30 signal to the video signal found at the VIDEO OUT jack. Trigger on the
SW30, and adjust the sweep to show the relative position of the leading edge
vs. vertical sync. Spec is 6.5 lines prior to vertical sync, + or - 1.5
lines.

Mark Z.

--
Please reply only to Group. I regret this is necessary. Viruses and spam
have rendered my regular e-mail address useless.


"Chris F." wrote in message
...
How do I check the switch point?

"Asimov" wrote in message
...
"Chris F." bravely wrote to "All" (13 Dec 03 00:11:47)
--- on the heady topic of "Old JVC VCRs - half of picture missing"

Tracking has no effect indicates that the transport servo is locking
randomly on noise and not on the proper head drum pulse. The cause is a
loss in value of the motor signal electrolytic cap on the head drum pcb
under the chassis. Check the electros for high esr or low capacitance.
After this is fixed make sure the switch point is correct. It is a
very common JVC problem.

CF From: "Chris F."

CF Got two similiar late 80's JVCs, both are 4-head mono and basically
CF have the exact same problem. The bottom half or so of the picture is
CF just snow, the top is normal but it looks like the bottom of the
CF picture pushed up to the top of the screen. Now here's the
interesting
CF thing: every time the VCR is powered, the amount of snow changes.
Next
CF time, there might be 75% of a picture, or 25%. It seems to be
CF completely random. Tracking adjustment makes no differnence, and the
CF tape path is aligned properly. Tried swapping out the power supply
CF and head preamp boards, but this made no difference. Perhaps the
CF replacements were bad too? Anything else I could check, without a
CF service manual? These are both really nice machines (one is a
CF commercial unit in a white case) and I hate to just junk 'em.
CF Thanks for any advice.

Well after replacing the cap and correcting the guide posts, I got a full
pic on one of the units. However there were some very small streaks trailing
to the right of bright highlights (such as white lettering) on the image.
Cleaning the heads made no difference. I had some other video heads and I
tried subbing with them, but they were all bad (snow through whole pic, or
just snow). I think at this point a new head is needed.
I also seem to have created another minor problem - I forgot to power down
the VCR while removing/replacing the drum motor/head preamp assembly, and
now for some reason the drum slowly spins when there is no tape in, but
shuts off when the unit is powered down. Operates normally in play mode.
Both of these units had been worked on by another "tech", who among other
things re-installed both video heads backwards after checking them. I think
I'll be lucky just to get one going....
Thanks for your advice.
"Chris F." wrote in message
...
Got two similiar late 80's JVCs, both are 4-head mono and basically have
the exact same problem. The bottom half or so of the picture is just snow,
the top is normal but it looks like the bottom of the picture pushed up to
the top of the screen. Now here's the interesting thing: every time the
VCR
is powered, the amount of snow changes. Next time, there might be 75% of a
picture, or 25%. It seems to be completely random. Tracking adjustment
makes
no differnence, and the tape path is aligned properly.
Tried swapping out the power supply and head preamp boards, but this
made
no difference. Perhaps the replacements were bad too?
Anything else I could check, without a service manual? These are both
really nice machines (one is a commercial unit in a white case) and I hate
to just junk 'em.
Thanks for any advice.

"Chris F." wrote in message ...
Well after replacing the cap and correcting the guide posts, I got a full
pic on one of the units. However there were some very small streaks trailing
to the right of bright highlights (such as white lettering) on the image.
Cleaning the heads made no difference. I had some other video heads and I
tried subbing with them, but they were all bad (snow through whole pic, or
just snow). I think at this point a new head is needed.
I also seem to have created another minor problem - I forgot to power down
the VCR while removing/replacing the drum motor/head preamp assembly, and
now for some reason the drum slowly spins when there is no tape in, but
shuts off when the unit is powered down. Operates normally in play mode.
Both of these units had been worked on by another "tech", who among other
things re-installed both video heads backwards after checking them. I think
I'll be lucky just to get one going....

You probably shorted out a transistor on the voltage drive to the drum motor.

-A

Thanks for your advice.
"Chris F." wrote in message
...
Got two similiar late 80's JVCs, both are 4-head mono and basically have
the exact same problem. The bottom half or so of the picture is just snow,
the top is normal but it looks like the bottom of the picture pushed up to
the top of the screen. Now here's the interesting thing: every time the
VCR
is powered, the amount of snow changes. Next time, there might be 75% of a
picture, or 25%. It seems to be completely random. Tracking adjustment
makes
no differnence, and the tape path is aligned properly.
Tried swapping out the power supply and head preamp boards, but this
made
no difference. Perhaps the replacements were bad too?
Anything else I could check, without a service manual? These are both
really nice machines (one is a commercial unit in a white case) and I hate
to just junk 'em.
Thanks for any advice.