Super fast REW or FF on VCR, problem, Sharp VC MH711

[N_Cook]

Will rewind 90 percent of a 4 hour tape in 40 seconds or so and 20s for the
remainder.
What would allow the motor to run at full tilt ? clunking loudly to a stop.
If it was not for the "crumple zone" of tape pulled into the gap between
tape roll and spool disc then the tape would have snapped at the end of
tape.
What to look for in the way of a problem ?
System presumably monitors ratio of supply and take up rotation rate and
direction of wind and unlaces tape from drum and over 3 stepped increases ,
sets at maximum revs until the ratio goes over a preset value and drops by
stages to a slow rate until end of tape sensor acts.

 

[David Farber]

Will rewind 90 percent of a 4 hour tape in 40 seconds or so and 20s for
the
remainder.
What would allow the motor to run at full tilt ? clunking loudly to a
stop.
If it was not for the "crumple zone" of tape pulled into the gap between
tape roll and spool disc then the tape would have snapped at the end of
tape.
What to look for in the way of a problem ?
System presumably monitors ratio of supply and take up rotation rate and
direction of wind and unlaces tape from drum and over 3 stepped increases
,
sets at maximum revs until the ratio goes over a preset value and drops by
stages to a slow rate until end of tape sensor acts.

I'm not exactly sure what you're saying is the fault. Is it that the motor
is going too fast and/or it is making a clunking noise near the end of the
tape? Newer VCR's do bump up the rewind speed after several seconds to but I
think you know that already. Is the machine damaging the tape?

 

[N_Cook]

I'm not exactly sure what you're saying is the fault. Is it that the motor
is going too fast and/or it is making a clunking noise near the end of the
tape? Newer VCR's do bump up the rewind speed after several seconds to but I
think you know that already. Is the machine damaging the tape?

It should recognize getting close to the end of tape and start reducing the
power / revs of the motor but it once continued at full revs to the end of
tape. Been monitored by human since it happened. It does not speed up if
human stops the motor close to end, goes normal speed to the normal clear
tape auto stop if REW is engaged again ofter pre-emptive stop.

I'm thinking a break in the line from the fast , take up spool sensor, so
less pulses coming from it and false interpretation.

 

[David Farber]

It should recognize getting close to the end of tape and start
reducing the power / revs of the motor but it once continued at full
revs to the end of tape. Been monitored by human since it happened.
It does not speed up if human stops the motor close to end, goes
normal speed to the normal clear tape auto stop if REW is engaged
again ofter pre-emptive stop.

I'm thinking a break in the line from the fast , take up spool
sensor, so less pulses coming from it and false interpretation.

Does the same thing happen when you fast forward to the end of the tape?

 

[N_Cook]

Does the same thing happen when you fast forward to the end of the tape?

Taking the cover off and visually monitoring the amount of tape left, the
VCR works fine FF or REW . Going up to express rate and back down , in
either direction, as long as there is plenty of tape run..
It just once , so far, spectacularly failed.

 

[b]

Will rewind 90 percent of a 4 hour tape in 40 seconds or so and 20s for the
remainder.
What would allow the motor to run at full tilt ? clunking loudly to a stop.

this is almost certainly something to do with the A/C head. I suspect
the deck reads the timecode pulses off the control track and stops or
slows down according to the tape position as deduced from this (many
vcr s have a 'goto' function which locates a point in time on the tape
you can enter in hours/ mins).

chances are, the tape in question may have had edge damage, or there
was dirt on the Ac head which messed up the signal.
B

 

[jango2]

this is almost certainly something to do with the A/C head. ......

Very unlikely, a blank tape has no control signal. The OP ought to
scope T and S reel signals all the way to the micom.Dirty optical
pattern on the reels maybe?.

 

[Jeroni Paul]

It should recognize getting close to the end of tape and start reducing the

power / revs of the motor but it once continued at full revs to the end of
tape. Been monitored by human since it happened. It does not speed up if
human stops the motor close to end, goes normal speed to the normal clear
tape auto stop if REW is engaged again ofter pre-emptive stop.

I'm thinking a break in the line from the fast , take up spool sensor, so
less pulses coming from it and false interpretation.

I've seen that with these short length tapes that have big reels. The
VCR always thinks they are in the middle and FF and REW full speed the
entire tape.
Philips VCRs trigger their brake when the end sensor detects end of
tape, avoiding a possible tape break.

 

[N_Cook]

stop.

this is almost certainly something to do with the A/C head. I suspect
the deck reads the timecode pulses off the control track and stops or
slows down according to the tape position as deduced from this (many
vcr s have a 'goto' function which locates a point in time on the tape
you can enter in hours/ mins).

chances are, the tape in question may have had edge damage, or there
was dirt on the Ac head which messed up the signal.
B

Not played up since, perhaps 20 FF REW runs.
Including using a new tape so no timecoding. Perhaps just a spec of dust to
jump into an opto gap at the wrong moment is all that is needed.