JVC HR-D750 FF Trips: Requires Reel Motor Repair?

[Takapuna]

Am I alone in buying a unit for spares and then deciding it's good enough to fix? The VCR in question is a HR-D750EK and after bringing it back to life and servicing it all modes worked very well except for Fast-Forward which trips after a second or two and starts to rewind.

Long story short but I decided the tape start sensor was a likely cause. It appeared to behave OK after connecting a meter across it; blinded by the presence of either insulating tape or a part-wound cassette there's around 5V across it falling to near-zero when exposed to light as expected. However, I decided to 'scope the sensor output as I had no better ideas. The 'scope shows brief negative-going pulses which on inspection are synchronised to commutation noise from the reel motor. I'm assuming the reel motor (PU59926V) is a standard dc motor with commutator and brushes. At the times when the motor accelerates to increase the speed the pulses increase in magnitude and this is when the system trips and starts to rewind. Similar pulses can be seen across the tape start sensor but do not cause trips.

As power rail de-coupling and system grounding via the various leads and underside PCB grounding screws have all been checked I'm about to condemn the reel motor. Has anyone experienced anything similar or got any other ideas please? It is almost as if the phototransistor has been biased into its linear range and is amplifying the motor noise. However the supply and ground connections are all solid (no electrical noise) and I can't see any mechanism for signal pick-up or cross-talk. The sensor illumination is not being modulated!

Anyone attempted to disassemble one of these motors? I appreciate that the motor would be a replacement part back in the day but I have little to loose by taking it apart.

Thanks for any help and guidance.

Phil
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[Refugee]

From my brief VCR repair days I had quite a few that had shipping damage.
The bit you are looking at sounds like the tape end sensor.
There is also a take up spool sensor under the hub. This used to get damaged some times and gave rise to intermittent shutting down.
It could also be an open circuit cap on the power supply to the mechanism control circuit.

[No.1 Labrat]

I would be looking at the reel rotation sensors underneath the take-up and supply reels.
Were quite a problem in the past. More so on JVC than other brands.

Also, could be dirty or depending on sensor style, dirty reflective wheel on the under side of the reels.

Wayne.

[its ur aerial]

Although I am not over familiar with the last of the JVC models, I would agree with the last posting, before diving in too deep, check the spool sensors, and the tape counter operation .

Ken G6HZG.

[Takapuna]

Thank you all for the suggestions so far.

I'm convinced the signal causing the trip is from the end sensor circuit for two reasons: (1) The mode changes to rewind when FF fails and (2) disconnecting the sensor allows the signal line to remain high and everything works OK...except tape-end detection of course! I have tried a different sensor and get the same result.

Hence I'm suspecting a noisy reel motor as the source of interference and wondered if anyone has experienced similar or attempted motor disassembly/repair?

Thanks,
Phil
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[jayceebee]

I don’t know this model but I’ve probably serviced many of its Ferguson clones. The PU59926V reel motor was used in many models and quite a common failure but I can’t say I’ve had it cause an end sensor trip like this.

Easy to tell if the motor is faulty, they seem to suffer from a partial short of the windings. Remove the motor then give the rotor a good spin with your fingers and let go. A good motor will spin for several rotations before stopping, a bad motor will give barely one or two rotations before it stops and you can feel a resistance to rotate fast. Never opened one but if it is faulty then nothing to lose.

John.

[Takapuna]

Thanks John. Very interesting. I'll give it a spin, so to speak.
I'm aware of shorted turns on the capstan motor causing issues and blowing the circuit protector but not the reel motor.
Will report my findings.

Phil
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[G6ONEDave]

Without looking at the service manual IMO if the tape end sensor was at fault then the tape mechanism would go straight into rewind as soon as the cassette is inserted. The fact that the deck only goes into auto rewind when FF is selected suggests that the supply spool motion sensor is the problem. If it's an optical system then the reflector ring is dirty or the sensor has lost some emmission, either way not enough reflected signal is being returned. Hence the machine thinks that end of tape or a very tightly wound tape has been inserted, with a lack of correct signal the auto response is to go into rewind.

If it's a magnetic sensor then check that the pick-up head is clean and that the gap is not too wide between it and the rotating magnet. Also check that the sensor is not open circuit.

Dave

[dj_fivos_sak]

Looking at the manual, I don't see a supply spool sensor. Only the take-up spool has a sensor for monitoring its rotation. All sensors in these machines are optical, not the magnetic/hall-effect type.

Fivos

[jayceebee]

I've just found a manual online for this machine and this has confirmed a suspicion I had that this model is equipped with VISS Video Indexed Search System, It's possible to record marker pulses on the tape to stop at certain points in FF or REW or to replay a section of tape repeatedly.

It may be the tape you are using has marker pulses already recorded and the machine is rewinding when it sees one. Check the slide switches behind the door on the front panel. A faulty reel sensor either take up or supply if fitted will not cause a machine to rewind but stop the tape to prevent damage

John.

[Takapuna]

I’ve spent far too long on this but here’s an update.
Reading my original post once more I mistakenly stated tape start sensor when I meant tape end sensor. Sorry for the confusion.

I’m no further forward but have a few more strange and baffling clues. Your comments and suggestions have been helpful and interesting as always. Thank you.

It is only when FF or REW is selected that the tape end sensor signal goes noisy, hence the VCR does not start to rewind immediately a partially wound tape is inserted but only after FF mode fails or when tape end is reached. With a clear service cassette inserted FF mode should sustain as it sees pulses from the photo-interrupter below the reel table but REW mode invokes STOP after a few seconds if the supply spool doesn’t rotate.

As stated in post #5 the negative-going spikes appearing across the end sensor transistor are phase-locked to the edges of the low-amplitude square wave appearing across the reel motor. Thus, as the motor speed increases the spikes across the sensor increase in frequency too. At each stepped increase in wind speed the spike amplitude increases momentarily presumably as the motor draws more current. At these times a trip is more likely. Disconnecting the tape end sensor phototransistor “fixes” the problem (but please see below); the status line staying at a solid 5V or thereabouts.
The first picture shows noise across end-sense phototransistor vs. reel motor power connection and the second noise across end-sense phototransistor vs. noise across start-sense phototransistor. Scope is set to 1.0V/div for end-sense phototransistor top trace and 100mV/for reel motor power and start-sense phototransistor bottom trace @1mS/div horizontal.

Having convinced myself that the reel motor is the source of the problem I’ve removed, disassembled, cleaned, reassembled and replaced it, all to no avail. I’ve posted a couple of pictures. Not content with this, I’ve also substituted a PU58636W motor from a HR-D310 which is the same motor but with the lead outs clocked 90degrees or so. Still no significant change. I guess the reel motor is not the direct cause of the problem!

Whilst testing recently, circuit protector CP804 went open-circuit. It’s a N25 1A device. I wasn’t dangling crocodile clips or using meter prods at the time, just pressing buttons, so I’m not sure why. The CP is in the 7.7V feed to the “UNREG 7V” to the audio board. The same supply is also input to a section of the hybrid regulator providing the SW 5V output. Not related to motor supplies though. As a temporary measure, after checking for shorts, I’ve fitted a F800mA fuse in position and everything has come back to life.
For a few moments I thought that everything was fixed. Well, fancy that, a faulty circuit protector causing all those symptoms and problems. Difficult to explain though…but NO!

Using the clear test cassette at first, I noticed that FF was maintained with no trips. The joy was short-lived though after changing to a half-wound E180 when all the usual trips returned. Back on the ‘scope I can see that only small pulses are created with the clear test cassette. These become very slightly bigger when the take-up spool is loaded but never large enough to trigger the STOP/REW event. Behaviour beyond this has become somewhat random:

LAST NIGHT a fully rewound quality E180 cassette would FF until around 35-45min of tape had spooled then stop/REW. This was repeatable across 2 ,3, 4 cassettes chosen at random. Trying to wind further from this time point (35-45 minutes) FF mode trips almost instantly. Looking on the ‘scope, the behaviour is that noise spikes across the end-sense phototransistor start at a low amplitude and as the tape spools their amplitude slowly increases until at the stated time they cause a trip, accompanied by automatic switching to rewind. The service cassette in FF mode will sustain (nearly) forever, although I have seen the very occasional trip. Two Philips-Branded E120 cassettes FF for only a second or two if at all. This is in spite of spooling them end to end on another deck. They are not damaged in any way and do not appear to be tight. To add to the mystery, REW became susceptible to trips too.

TODAY: Even the quality E180s refuse to FF very far if at all. REW however will run the full tape.
As a control experiment, I’ve removed the connection to the end-sense phototransistor once more and both Philips cassettes will FF to the end as did a “quality” E180. One other “quality” E180 got to 2H 55Min then tripped. Never give demos…. Reconnect the end-sense phototransistor and both “quality” E180s wind to 39minutes then rewind. The two Philips E120s will not FF at all once more.

At this point I’m running out of ideas.
The fault still feels like a grounding/noisy power supply/signal pick-up fault but both end sensors see noise and they have separate connection and ground paths.
The power rails are to all intents and purposes noise free. I have changed a couple of electrolytics around the motor supplies but my experience is that with very few exceptions any capacitor I’ve checked has measured OK when removed. The ESR meter shows others to be OK. I’ve flexed PCBs and pulled and tugged on cables and connectors, all to no avail. I haven’t tried freezing anything as I don’t know what to aim at! Grounding appears solid and the PCB earthing screws under the deck have star washers on them. Following my obsession with the reel motor I’d be tempted to try a replacement M54647L driver chip if I had one, but for no scientific reason…

Any thoughts or comments very welcome.

Thanks,
Phil
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[Chris55000]

Have you got a bench power supply you can feed to the mechanical control circuitry in place of the 12V from the STK5490 to see if that helps?

There isn't a proper data sheet for this online, all there is is a block diagram cut and pasted from a service manual circuit with no electrical characteristics whatsoever!

However I found a website that states each section of the '5490 is rated at 1.1A nominal maximum.

It might be worth feeding a bench supply to the reel motor supply and watch the current metering of the PSU as you try FF and rewind in turn – if the current decreases as the motor grounds to a halt the motor or the motor driver IC is af fault, where a rise in current as the speed reduces points to mechanical problems!

If FF and REW work better off a bench supply it's possible there's an internal defect in the '5490 causing a high impedance in the motor 12V supply – there are genuine ones supplied in JVC packaging for about £5 to £8 on eBay!

If the reel and supply motors are mechanically interchangeable, try swapping them over to see if the Reel/FF motor will rewind a tape and the supply/REW motor will fast forward one!

If this test proves the motors to be sound, your best bet is a parts mule for another "03 Board" assembly, I'd keep an eye round your local car boot sales in case another similar type of machine with the same motors comes up – Members who've done VCR repairs regularly will be better clued up on parts interchangeability than me!

You've not got yakky electrolytics decoupling the reel drive circuits by any chance?

"Free Service Manuals" has by far the best quality manual, beautifully scanned in the original colours with crystal clear PCB drawings and photos!

Chris Williams

[Takapuna]

Chris, thanks for the suggestions.

I have a bench PSU and the genuine JVC service manual.
The reel motor driver IC is supplied from the UNREG 15V line which is a nominal 16.3V rail direct from the power supply assembly. The STK5490 is not in this line. I'll try sourcing this rail from the bench supply first. I've replaced C606 which is the local smoothing cap for good measure.

I will see if supplying a MOTOR 12V rail from the bench supply makes any difference too and also the UNSW 12V from which the 5V supply for the micro processor is derived locally on-board. Might be worth removing the series-pass transistor and sourcing an independent 5V as the pull-up resistors for the end sensors are referenced to this supply too.

The deck has a single reel motor with a swinging idler to switch between FF and REW. However, in all cases of "failure" I'm certain the motor is being commanded OFF by the system rather than stalling. None of the other motors are operating during FF/REW, so this rules out interference from them.

Will report my findings.

Regards,

Phil
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[Takapuna]

I don’t like admitting defeat or giving up but a few weeks (and one Wisdom tooth extraction!) later I’m pretty well at the bottom of my bucket of ideas for the fault on this VCR.
Yet another quick summary:
The VCR works reliably other than generating noise across the tape end sensor and, to a lesser extent, the start sensor. The noise on both sensors is synchronised to the commutation noise on the reel motor and becomes of sufficient amplitude to generate a tape-end signal causing the controller IC to command stop and then rewind. I’m convinced that a stop signal is being generated by the controller rather than the motor running out of torque or stalling etc.

So far, I’ve tried the following:
1). Changed the reel motor.
2). Replaced the M54647L reel motor driver IC with one taken from another VCR.
3). Hooked-up a 15V supply to replace the “unreg 17V” rail from which the reel motor IC is powered.
4). Removed the series pass transistor Q601 and sourced its 5V output from a bench supply. This is the 5V supply for the controller IC and also the rail which supplies the end sensor pull-up resistors etc.
5). Re-checked grounding paths for the deck signals, noting that the earthing screw on the so-called relay board to which the end sensor is connected is essential for correct operation. This is in spite of the manual suggesting that a secondary ground connection is carried through from CN601 pin 1.
6). Synchronised application of a bench 12V supply connected directly to the reel motor to selection of FF using the front panel controls. Low amplitude spikes can still be seen across the end sensor but they are bi-directional. They appear to be reasonably regularly spaced but not so regular as to allow the scope to trigger and freeze them. There is a clue here somewhere but I'm not sure what it's telling me.

I have not replaced the STK 5490 regulator chip and probably have nothing to lose by doing so. It’s just that under fault conditions the only motor being powered is the reel motor and its power is the “unreg 17V” directly from the PSU. Similarly, 5V for the controller IC and sensor conditioning is from a series pass transistor which I’ve sourced independently already.

I bought this VCR in a non-working condition and it soon came back to life after replacing the 1uF start-up capacitor on the PSU board and replacing a couple of other electrolytics. I don’t know if these symptoms were present previously or if my repair effort including removal cleaning re-lubricating and re-belting of the deck mechanism has provoked them.
I’ll put feelers out for a replacement M3 (main) board from a scrap machine including a request in the Wanted column. At the few car-boot sales I’ve attended I’ve only seen one VCR and it goes without saying that it wasn’t a JVC HR-D 750!

Again, any thoughts or comments very welcome.
Thanks,
Phil
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[ben]

Random idea: have you lubricated the spool carriers? Could it be that friction or static here is upsetting things?

[Takapuna]

Nice idea Ben.

I remember removing the take-up spool and cleaning the opto-sensor then applying a drop of oil. The other spool has remained in place because there's the tension band and other items in the way. I released the brakes and checked that it was free spinning and then cheated by applying a single drop of oil from the top. However, there is something that makes the noisy signal amplitude increase causing a trip around the 40-min in mark as certain tapes are wound. This is repeatable as is the inability to wind two Philips tapes at all. Applying load to the take-up spool with the service cassette loaded does not appreciably increase the noise signal.

Phil
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[G6ONEDave]

Just a thought, is the tape tension band too tight or possibly snatching on the supply spool? I'll dig out the manual in the next couple of days and have a look for other possible causes.

Dave

[G6ONEDave]

According to the Technical manual
"Auto Stop Mode" is as follows; The mechanism enters Auto stop mode in the following conditions.
a) IN REW and REV Search modes, the tape travel reaches the beginning of the tape. When the leader tape is detected, the tape is forwarded in the normal direction until it's leader portion is not detected, and then the mechanism enters the Stop mode.
b) In the case of the Pause/Still mode continues for more than 5 min and 25 sec.
c) In the Play, REC, Search and FF/REW modes, the take-up spool stops rotation for 3 secs and more.
d) In the Play, REC, Pause, Still and Search modes,the drum motor stops rotation for2.5 secs and more.

"3 Auto-rewinding mode" is as follows; a) The mechanism enters Auto-rewinding mode when the tape travel reaches it's end in the Play, REC, Search, FWD and FF modes.

The machine sounds like it is reacting to a problem with the tape transport system, hence my previous comment about the back tension band.

You could try slightly pushing the tension arm back by 0.5mm to see if the machine will get to the end of the 3hour tape. I suggest that you use a sacrificial tape for this in case any damage is done to the tape. I would also check that the supply spool rotates freely when there is no tape in the machine.

Dave

[Takapuna]

Dave,

Thanks for looking into this.
Originally, I'd checked back tension and take-up torque on this machine and "completed" the repair. I started to use it (without its cover) and all appeared OK until I found it couldn't FF a tape that was part wound. This thread details what happened since.

I'll check the supply spool rotation and tension band once more as you suggest.

However, from your descriptions it is obvious that the VCR thinks it's reached the end of the tape and enters "Auto-rewinding mode". The noise across the opto-sensor is the cause, generating a tape-end signal even though it has not been exposed to light. I've been on a wild goose chase because of the synchronisation of reel motor noise and sensor noise spikes. It isn’t the reel motor nor the M54647L driver chip. However, I do believe this to be an electronic fault rather than a mechanical one. BUT why or how it is sensitive to the amount of tape spooled I cannot think of a mechanism. I think that if the small noise spikes during normal operation were not there then they would not be “amplified” by the opto-transistor to reach trip-magnitude. The supply sensor exhibits these spikes too, albeit to a lesser degree.

Thanks,

Phil
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[G6ONEDave]

Phil, those overshoots in your oscilloscope pic might be normal but without comparison to a known working machine will be difficult to confirm. Do bear in mind that if the motor is working harder and hence drawing a higher current, that that will affect the spikes being displayed in the oscillogram.

Apart from the control system described in post 18, there is also the possibility that the motor supply circuit might have an excessive current section that sends a message to the syscon via the 4bit data lines to get the machine to do the auto function.

It won't be easy but you might get a better idea of what is happening if you can look at the 4 bit data lines to see what level they are at, either +5v or 0v. Page 3.10 Mechanism Control and Remote Pause Adapt schematic diagrams has a truth table for the M54647L reel motor drive. If you have multi inputs on your scope this would be easy to do otherwise you would need 4 dc voltage meters to check it. You are looking for a change in conditions.

Another thought is if the supply is dipping when being loaded due to a bad electrolytic capacitor. Check that with your scope as it will react quicker than your testmeter.

You could also try some 0.01uf ceramic caps across the sensors where you think there are interference spikes, they might help to cut down the interference. This is not a cure but it may help to prove a point. If it does work then you would need to investigate where the spikes are coming from. It could be due to an o/c filter cap that is involved with the motor. Perhaps C604 1uf 50vdc might be worth checking while you are in there.

Dave