Mitsubishi VCR HS-U748 rewind/fast forward problem

[davexnet]

Hi all,
when I either FF or RW a tape from one end to the other, the transport abuptly stops with about 5 minutes of tape
remaining. Usually after two or three seconds it attempts to continue and moves the tape at a snails pace. Sometimes it makes
it to the end after about 5 minutes, other times it goes into another mode and engages something that seems like playback
with fast search for the last bit of tape left.

Another symptom, if you try to RW/FF starting from somewhere in the middle of the tape, it takes much longer to get up to speed.
Other times you can hear the motor running but the tape doesn't move at all (Usually does this with older heavier tapes)
Slipping clutch ?

I've had the mechanism out, only one motor to drive the transport, using a gear system. I've cleaned the optical sensors under the reels
and another one at the mode position wheel. Hasn't affected the problem.

Any idea's appreciated.

[JVRaines]

Yup, I would check the clutch. Also replace any any dried, gunky grease and relube bearing points for anything that spins.

[johns0]

Check the clutch manually to see if there is the proper tension,if it feels very loose in that it spins without driving the gears it's linked to then it either needs to be replaced or you can try and tighten the clutch assembly.

[davexnet]

Thanks for the tips. Here's a couple of images showing the mechanism (I have a PDF of the service manual)
It's mostly disassembly/re-assembley instructions. It does mention to be careful
that grease does not contaminate certain parts. Perhaps if I open it, something may be obvious.

I am not a VCR/TV tech, just picking up stuff as I go along. I got quite a few tips from "12voltvids" youtube channel
and similar. Even a book from the library; they have at least four books on vcr repair in the Los Angeles system -
should keep me busy for a while.

[johns0]

Look at the slip adjuster and make sure there's enough pressure that it lets the rewind/ff pulley get a good grip.I worked in vcr repair shops years ago.

[davexnet]

Thanks Johns0

[orsetto]

Just a supplemental note: your 748 is from the final series of Mitsu VCRs circa 1998-2000. These featured super-high-speed FF and REW: when working properly it should take just over 60 seconds to spin a T120 from one end to the other. The turbine sound emitted by the VCR while it does this can be quite disturbing until you realize it is normal. It is also normal for the sound and wind-speed to shift abruptly near the middle of a T120, and slow down dramatically as the end of the reel approaches. During the course of a normal end-to-end FF/REW cycle, highest speed will sustain from full-reel to mid-reel, slow down a bit from mid-reel to quarter-reel, then slow noticeably from approx eighth-reel to leader-end. This is done to create even tape pack tension and avoid snapping the tape off the reel hubs, while providing the convenience of high speed wind. (If you invoke STOP/FF/REW while the tape is partially played, the speed may vary considerably.) JVC used a similar approach in many of their later VCRs.

[davexnet]

Hi orsetto, there's another weird thing it does. I bought it used and I thought it was a fault. I even spoke to a repair guy
who has a shop in Culver City; he said it's caused by a command from the micro controller but he couldn't explain why it did it.
The only thing I can think of is that it tightens the tape on the take up spool during high speed movement, perhaps to reduce
the slack even more..But it sounds kind of weird, could it be a design fault and it shouldn't actually be doing it?

I found a video on youtube that shows this; it's not the 748, but looking at what I can see it's an almost identical chassis.
At 20 and 26 seconds you can hear a brief sound as the mode wheel (upper right) just momentarily engages the brake
and then backs off again.

When it was working properly it moved the whole tape from one side to the other in under 50 seconds. And when it
reached the end, you could always hear the mechanism move to its default position (as it should)

Here's the video showing the anomaly
https://www.youtube.com/watch?v=eHD-R8lUCDU