Unusual issue after changing video head

[IanNVJ35]

Never had this behaviour before.

I changed the video head on my Ferguson FV-21 VCR because the old one was damaged and after trying out the VCR all seemed well. It:

Played old tapes without needing any tracking adjust
Played all tapes without noise on the screen
Recorded and played it's own recordings perfect

However when recording a tape on the Ferguson and playing it back on other machines THEIR head switch point is pushed up the screen so as to be visible.

I just can't work out what in the recording stage can do this? Anyone come across this phenomenon?

Thanks!

[Sanbeta]

Ian

You will have to adjust manually the head switching point on the one you changed the heads on.
It's always when playing back on other machines you will notice if it's correct or not, especially as they usually are.

Good luck...............Kev

[IanNVJ35]

Thanks Kev - I've ordered the service manual to see what adjustments I can make. You know what - I didn't know that head switch point adjust affected recording and playback so thats given me a starting point.

[Sanbeta]

Ian

How I know that is someone had turned woefully out of spec one for recording on a Sanyo 5150 I overhauled.
The switching point was at the TOP of the screen!
It was more noticeable on a CRT TV than a LCD.

Good luck....................Kev

[IanNVJ35]

I've been trying to work out the physics of this and ended up drawing myself a diagram. Why the switch point changed with new heads that were the same as the original and installed in exactly the same place I dont know but I broke it down as follows - correct me if I'm wrong.

DURING RECORD

Heads at some point must switch to the next frame so something is regulating this - it cannot be the ACE head though. Some 'timing' circuit linked (?) to the signal being recorded to the ACE head is also timing the signal to the video head chips.

If the signal starts too early then the head switching will be too far up the tape.

But why this only shows on other VCR's and not playing its own recording is a mystery indeed - unless such errors are summed.

Still so much to learn!

[pichacker]

The switching point is normally controlled by a hall sensor on the head motor assembly. As the motor roates a small magnetic pip passes the sensor and the heads are switched.

If the new heads are not exactly in the same place as the originals, due to manufacturing tollerances, then the point at which the switching occurs will be different. This is recorded onto the tape.

Ideally you need a dual beam scope and locate the drum FF (Flip Flop) squarewave. Trigger one channel of your scope on this.

Then connect the other channel to the composite video out and then adjust the switching point whilst playing an alignment tape to be in the correct position. I cannot remember exactly what it should be as it's been a long while. In my mind something says 6.5 lines.

Whilst you are at it you could scope the FM signal and check the tape path guide alignment.

Good luck.

Remeber this post in relation to the switching point? https://www.vintage-radio.net/forum/...ad.php?t=61558

[IanNVJ35]

Thanks for the input. I've just been given a dual trace oscilloscope which will help with this but I cannot find an alignment tape anywhere. Are they still available from any source or could I use a home recorded tape made on a known good VCR?

Once I get the service manual through I can see what test points are available.

[HamishBoxer]

All very odd i never used to have to make adjustments to head switching on fitting new heads.Fitted many on all makes.

Also i checked them with a test tape.

[IanNVJ35]

Oddest thing is playing its own recordings (or recordings from other VCR's) the head switching is absolutely bob on - only when playing one of its own recordings on another VCR does the head switching show on that VCR. Talk about brain ache...

[HamishBoxer]

This isn't something silly like not enough height or different screen format on the other televisions?

[julie_m]

If the same head drum is being used for recording and playback, the location of the switching point won't matter; it could be halfway up the picture and you would not notice it, because the geometry of the tape path would be exactly the same and thus the next track would always be exactly positioned.

The problem playing back tapes recorded on this machine on other machines is odd. I never heard of a VCR doing any head-switching during recording. Usually, both heads are driven together, and one of them is simply trying to record onto thin air for almost half of each revolution. Not only is that less bother electronically, but it also builds in a bit of tolerance for playback (since there is now a moment when both heads are picking up the same signal, one from the head that was about to leave the tape and the other from the head that had just entered, so there will be no artefacts on switching).

Which brings us back to tape path geometry. Is everything OK mechanically?

[IanNVJ35]

I now have it fixed to my satisfaction. What I wanted to do was see exactly what change the switch point pot made, so after marking the original set point I turned it bit by bit (viewing the image on a 100% TV). What I found is that it was actually set too low on PB compared to known 'good' machines. The setting is like a see-saw so too low on PB means too high on record (and vice versa). There was (is) an optimum point where the same level of switch noise was visible on its own recordings and PB of other tapes and also when playing its tapes in other VCR's. Trying a selection of other tapes gave good results with varying levels of noise present which matched other VCR's. I understand why a test tape is needed now as the setting being out on the recorder being used to make the tape will then not allow the correct adjustment to be made with a scope.

The correct Ferguson calibration tape specified in 1988 had a 'stairstep' signal recorded on it - what would that look like on screen?

[8 Tracker]

Ian,

The best way to describe "stairstep" or "staircase" as it is otherwise known is like colour bars, but in black and white. Can't remember which way round they are, but there is white at one side of the screen and black at the other, with varying shades of grey in between.

Mark.

[Sanbeta]

It's not so much the head switching that is affected but the height this will be shown onscreen.
The head switching part is usually obscured to just below vision on a CRT tv.
You should 'just' be able to see the white switching point at the very bottom of the CRT screen.
LCDs seem to not show this at all.Mind you I am talking Beta here, but I would think VHS is just the same.
The way I adjusted the one I had was play back a few good other recordings on said vcr for reference of the correct point,and then slowly adjust the pot ,during recording until you had the exact switching point height for it's own recordings on a good quality blank tape (no creases).This was also confirmed by playing back on other vcr's.
It also helps if you set the screen ratio to 4.3 as it shows even more this point.
This is set to how the manufacturer set's these up as if you go too high it shows by raising this 'point' about half a centimetre up the bottom of the screen, (or even to the top as this one had been moved too).You can also get it to not be visible at all at the bottom, but I like to set things how they were intended.
Also the luminance of the picture can effect too how much is visible, so it's good to make sure the images you use for reference and recording are similar.

.......Kevin

[IanNVJ35]

Thanks Kev. I have got a test tape on the way and since I have the service manual I can set it exactly 'to spec'. I do note exactly as you say - when it is correct you should just barely see it at the bottom of the screen - except on vary dark footage where it is more visible. I use a 4:3 CRT television so some overscan is built in.