Loose Capstan Belt = Creased Cassette Tape?

[knobtwiddler]

I haven't had a chance to get the spare machine on the bench and take photos, but it did occur to me that I might get some clues from the thread I linked to earlier HERE

If you zoom in on This Photo, you can definitely see the same wear pattern on the right-hand guide nearest the front capstan / roller. I think there are also visual clues to suggest that the erase guide has the same wear pattern as mine as well.

The question in my mind now is whether I use and enjoy the machine as-is, but being paranoid and having to inspect the tape after every play; or whether I try and get new guides machined (probably from brass and then nickel plated).

Obviously, I haven't inspected the spare 9900 yet. If that has a wear pattern, then either Technics didn't align them straight from the factory, or it's an inherent flaw in the design. But the link above shows 0.019 W+F, which is what you expect from a reel machine, not a cassette. A heck of a good figure for something with an inherent flaw in the transport!

[knobtwiddler]

The plot thickens:

Out of interest, I decided to FF to a clean, unused section of the D120 tape and then play it in my Sony machine, to see if it creased tape in any way. I then noticed the same crease appearing on the edge furthest away from the front face - In an identical fashion to what was happening with the Technics...

I am now wondering is this is a plausible scenario: the wear in the Technics guides is due to alignment / hard (less rubber than later, same OD) rollers. Having replaced said rollers, it's not creasing a mirror tape currently, nor any other tape (bar the aforementioned D120).

Maybe the creasing in the D120 was mutually exclusive from the guide wear, if I'm getting the same effect in the Sony? And if W+F is good (0.03-4 RMS) are the worn guides having an effect?

[knobtwiddler]

Finally tested Take up Torque and Back Tension last night (I needed to order a new gauge, as old one only went to 10 g/cm on BT scale).

Technics suggests 60 g/cm for TuT, but I am seeing ~75 g/cm

It’s generally felt that 5-10 g/cm is decent for BT. The RS-9900 is giving somewhere around 12-15 g/cm…

My feeling is that the original creasing was due to roller condition more than anything and although related, the creasing and guide wear are separate issues. If I hadn’t used a macro lens when I did the rollers, I probably wouldn’t have twigged about the guides – as the mirror tape was looking fine.

As Paul suggested, the wear in the guides has likely been happening for many years, most likely not helped by such high tension along the tape path. From studying the schematics, there is no obvious way to adjust Tut or BT. The power going to the reel motors is via a CC scheme, so unless anyone has a better idea, I suspect I might have to bodge in a couple of trimmers and monitor torque in real-time. But if it’s not creasing tapes and has been like this for 45 years, is it really worth it? It probably is stretching tape as it is… W+F is excellent. Will it remain so if I modify the CC scheme for the reel motors?

Technics must have known about the high BT. Or maybe the current-setting Rs have drifted out of tolerance? But for both TuT and BT?

Thanks to all so far who’ve advised!

[TIMTAPE]

Take up tension of 75 sounds excessive. High take up tension is a recipe for creased/chewed tapes, especially at the beginning of the tape when take up tension is at its highest. As a comparison, a range of late model Tascam cassette machines advise 25- 65 g-cm. If it's adjustable I generally aim for the middle of that range, ie: in this case 45 g-cm on the average reading.

On some decks with two separate reel motors or one dedicated reel drive motor, there is still a felt clutch built into the take up platter which sets a limit on the torque. Felt clutches have a tendency to "gum up" and increase tension. Does your 9900 have this felt clutch on the take up side?

My priority would be to identify exactly where in the tape path the tape is skewing, then having identified that, identify why it skews. For me, a machine which damages tapes is reduced to a display piece only.

[knobtwiddler]

Hi Tim,

It hasn't creased any tapes since I changed the rollers. The mirror tape suggests no creasing, and W+F is better than OEM spec (I found out that the D120 was being creased in an identical way by another machine running with 45g /c m TuT, so concluded it was a red herring).

My feeling is that the worn guides are a separate issue, albeit one that helped crease tapes before I changed rollers.

The 9900 has separate motors for both reels, driving each reel via a belt. There are no mechanical tensioners from what I can see in the exploded diagram. The 9900 is - to my knowledge - the only cassette machine where the rec / repro electronics are in separate boxes. It weighs around 25 KG in total. It was meant to be Technics' cassette machine to end all others. I haven't put enough thought into the drive circuit to work out how to do it exactly, but I am 90% sure that tension will be adjusted electronically.

One idea could be to replace the resistors in the drive circuit with modern 1% types, simply as it's a quick and easy job. But I am not optimistic that it'll do anything. I think some circuit analysis is needed, followed by very careful experimentation (maybe fitting a dummy load in place of motor, tweaking current and working out how to do it precisely without risking anything in the path). If anyone knows better, I'd love to be corrected as it seems some hard work is needed!!!

edit - service manual here: https://www.hifiengine.com/manual_li...s-9900us.shtml