Changed drum assembly on JVC HR-S9600U and have tracking issue

[Tokyo_DIY]

I have a Japanese equivalent of a JVC HR-S9600U (the model is Victor HR-VX200), which started already some time ago to give issues with the dynamic drum system: every time that the drum tilt is adjusted, the mechanism becomes very noisy, and sporadically the deck power just turns off. So I decided to buy another cheap unit to get the drum assembly, and I found the same model -not working- for the equivalent of 10USD.
I followed exactly the installation manual to exchange the drum assembly, and now the dynamic drum works perfectly and without strange noises, but I can see a tracking issue. In particular, SP auto-tracking result is acceptable (but not completely perfect) while EP auto-tracking leads to one noise bar in the lower part of the screen with most tapes.

I noticed that the service manual specifies that one should do the following adjustments after substitution of the drum assembly:
- PB switching point adjustment
- slow tracking adjustment
- compatibility adjustment

I could not carry out the above adjustments because I do not have the Jig RCU mentioned in the service manual.
I can do some limited correction of the tracking with the manual tracking, but still the result is not perfect. I am wondering if the switching point and other adjustments need to be applied or not.
Do you have any comments or advice about how to improve this situation?

[Tokyo_DIY]

To make a bit clearer the situation with a EP tape and auto-tracking, this is the result with most tapes: one line of noise in the lower part of the image, as in the attached photo.

[latreche34]

From the picture looks like the exit guide needs to be tweaked a little bit, Normally if you replace the whole head assembly you shouldn't need to do that, only if you replace half the drum. Possible causes that the previous DD system was messed up and someone re-adjusted the P guides to compensate for that, now you get a perfectly working DD system (so far) so you need to re-adjust the P guide back to its original position.

I normally hate to mess with the factory alignment but if you don't know the history of this VCR just assume that it was tempered with and proceed with the P guide adjustment, Whatever you do make sure to mark your initial position.

[Tokyo_DIY]

Quote:

Originally Posted by latreche34

From the picture looks like the exit guide needs to be tweaked a little bit, Normally if you replace the whole head assembly you shouldn't need to do that, only if you replace half the drum. Possible causes that the previous DD system was messed up and someone re-adjusted the P guides to compensate for that, now you get a perfectly working DD system (so far) so you need to re-adjust the P guide back to its original position.

I normally hate to mess with the factory alignment but if you don't know the history of this VCR just assume that it was tempered with and proceed with the P guide adjustment, Whatever you do make sure to mark your initial position.

You are right. 1/4 of a turn clockwise and now it is basically perfect in both EP and SP. Thank you so much!

(The full story is that before starting this thread I had already tried to do the right guide adjustment, but I was not successful. The reason seems to be that I tried that in SP mode only. Today on the other hand I started from adjusting the guide in EP mode. Then I made a minor re-adjustment in SP and then back to EP. For some reason only this sequence seems to lead to a correct adjustment. Now also HiFi audio tracking stability seems to have improved a lot. Beyond expectations!)

[latreche34]

Keep in mind that the perfect adjustment is done with an oscilloscope using an alignment tape while observing the wave envelope for a a straight band, But since you don't have that you may need to test with more tapes to make sure all of them are playing perfectly.

[Tokyo_DIY]

Quote:

Originally Posted by latreche34

Keep in mind that the perfect adjustment is done with an oscilloscope using an alignment tape while observing the wave envelope for a a straight band, But since you don't have that you may need to test with more tapes to make sure all of them are playing perfectly.

Do you know what are the minimum specs (sampling rate etc.) for an oscilloscope when adjusting S-VHS vcrs?
I am asking because I see that e.g. on Amazon there are nowadays incredibly cheap USB oscilloscopes, but it seems the frequency range is very limited for these models.

[latreche34]

Frequency matters when zooming in samples, most modern scopes can do 100MHz which is more than enough for video work.

[hodgey]

In the SM for the betamax deck I'm currently tinkering with it mentions that the scope should be minimum 10 Mhz, I didn't see any specs noted in a quick look at VHS manuals though both formats use pretty similar frequency ranges on tape. That includes rf frequency response adjustment though, where being able to capture the full bandwidth is needed, and which is not adjustable on newer JVC decks for standard VHS (which admittedly is a bit annoying). for tape path alignment you may be fine with even less as you what you are looking for is the signal level to be mostly flat over the whole sweep of a video head.

Additionally the dynamic drum system does have a pre-set default "tilt" as noted in the service manual that can be adjusted, but as noted that requires the special service remotes because JVC decided to be annoying. Whether that is something that would change enough to require adjustment after swapping assembly I don't know.

I don't think the rf switching point needs to be all that precise, so unless it's moved a lot it's likely fine as is. If it's very off (i.e the distortion at the bottom of the image is high up or low/missing when viewing the full image without overscan) it's possible to tweak it manually by slightly rotating the drum motor slightly (just loosening the screws may give enough wiggle room, if it's extremely off one would have to take off the drum and adjust the brass nut but I woudln't expect that to be needed.)

[latreche34]

I would not mess with the head switch adjustment in this case (based on the sample provided) especially the DD system, Not to mention that some recorded tapes have high, some have low switch point, and test tapes are not reliable anymore even if you manage to find one.

[Tokyo_DIY]

Quote:

Originally Posted by hodgey

In the SM for the betamax deck I'm currently tinkering with it mentions that the scope should be minimum 10 Mhz, I didn't see any specs noted in a quick look at VHS manuals though both formats use pretty similar frequency ranges on tape. That includes rf frequency response adjustment though, where being able to capture the full bandwidth is needed, and which is not adjustable on newer JVC decks for standard VHS (which admittedly is a bit annoying). for tape path alignment you may be fine with even less as you what you are looking for is the signal level to be mostly flat over the whole sweep of a video head.

Yes, I would guess that at least 10MHz BW would be necessary.
The truly cheap oscilloscopes have a BW or merely a few hundred kHz (e.g. https://www.amazon.co.jp/dp/B07RT7D8DT#customerReviews ).