Some questions about Sony WM-DC2 (center gear, head adjust, versions)

[alxwz]

This is a follow-up on this thread (http://www.tapeheads.net/showthread.php?p=200732).

In the auction linked in the other thread, the seller claimed the following:
The first thing to say about this DC2 is that, immediately I got it, I replaced the centre-gear, which was clicking. The replacement part is a genuine Sony spare, not a botched repair of the broken part, and as such should last a lifetime.
This is from the ensuing discussion:

Alex, as far as I know there are no original replacement parts for none of the Sony WM-DD series machines which includes the DC2 as well. The center gear is the weakest link of any DD machine as it turned out in the long run but there is a way to fix it.
If the eBay seller is selling a metal gear with claims of a "longer service life" keep in ind all the other gears inside are plastic and teflon. As you may or may not know, plastic compounds such as polypropylene (used for the gears in this case) have a very low friction coefficient. It is a different story with metal and given this is a mechanical gear system, this plays a vital part. Sure, the metal gear will last longer but it will damage the other components along the way. Much better option is to fix the existing gear. Once fixed, it won't break again because of reduced tension stress along the rim once a new piece of gear is inserted into the crack.
There may be no genuine replacement parts today, but I'd imagine there were some in the past. Were there?
If so, were there some genuine Sony gears that wouldn't crack? A later version, perhaps?

I understand that making a replacement wheel completely out of metal is not a good idea, but what about making one completely out of some kind of plastic?

A second point from the auction text:
I’ve put a lot of time and effort into optimising the tape path of this DC2, which has resulted in a very stable sound, not just in terms of speed stability, but also in terms of image stability. I used a Philips mirror cassette, blood, sweat and tears :-) to get the entire tape path carefully aligned, including the oft-overlooked head zenith.
Head azimuth is easy enough to optimise with test tapes and a software-based spectrum analyser (which I used in conjunction with my Audigy 2 24-bit, 96kHz sound card), but zenith, which is often not optimal on those Sony Walkmans with a single swing-arm-mounted head, is trickier. It is, however, a requirement for the best possible tape path.

Again, from the other thread:
Adjusting the head azimuth isn't difficult and is accomplished by rotating the screw that's located on the main chassis on the opposite side of the capstan in respect to the head.
I understand that azimuth adjustment may be easy (disregarding the questionable precicion of adjustment tapes themselves). But what about the zenith story? How important is that?

Note that earlier versions of DC2 have a hyperbolic head design so tuning the azimuth must be done precisely to achieve optimal frequency response.
How can one tell which version of DC2 one has?

Regards,
Alex

[R.Daneel]

Hi Alex!

The center gear in the Sony WM-DD series is a very soft compund and I cannot say for sure which one is it. It is most likely polyethylene. The gear itself is a combination of an inner metal disc and the gear rim. This is to reduce the mass of the gear while still alowing for it to have a solid metal base. The gear rim is factory installed onto the metal part and cannot be taken off without breaking it.

While it is possible the seller did in fact install an original replacementpart, they have become unavailable some 5 or 6 years ago and were rare even then. The last of the DD series, the DD33, was manufactured in 1992 which was 20 years ago and Sony, like most manufacturers, provide parts for a period of 7 years. This would mean the production ceased in 1999 (if there was any in the first place). That aside, even if the gear is new, it will be a decade old and most likely on it's way to breaking.

The rate of decay is not really a matter of use but rather a matter of environmental conditions such as humdity and temperature. Since the gear rim is plastic, it's mechanical properties (which in the case of plastic are always isotropic) are highly dependent on the temperature. At 50degrees C, plastic loses half of it's elasticity modul which inevitably means lower yielding point of the material. In contrast, at lower temperatures, plastic becomes stiff. This means the gear rim contracts and compresses against the center metal core. Being unable to compress the metal, tension stress along the rim increases and causes breaking. Stiffer materials break by cracking, not by yielding and this is the cause of the problem. I've tried to be as plain as possible without going too deep in the mechanics.

Plastic gear could be manufactured and few have alreadymade metal ones. The problem is not the design but the manufacturing process. One could use a 3-D scanner to scan a gear that's still intact, then convert the image to a raster and load it into AutoCAD to get the final drawing. This is then inserted into a drawing press for plastic and it's done. Problem number one is that we don't have a gear that's intact. I already said I'd be willing to do the CAD part. However, the actual manufacturing would be somewhat difficult to do. First of all, we're talking about a small quantity (anything less than 1000 or even 10000 pieces is insignificant). Hydraulic presses are complex machines and to manufacture the appropriate die and set the press up would cost a lot. It would make the entire project time consuming and very expensive.

I have a low serial number DC2 (below 50.000) with the hyperbolic head contour. The difference is quite obvious actually. Wheareas the newer models have typical head design like in most cassette decks. teh hyperbolic head a disticntly sharper look to it. If in good shape, a hyperbolic head will provide less low level distortion and as it turns out, a weighier bottom end.

Send me a PM with your DC2's serial number and I'll see what can be done. Assuming your head is the original one of course.

Cheers!
Antun

[LesX55]

Hi Antun,

Very interesting read. I had my DC2 gear replaced with one from a DD33 about one year or so ago and so far she is working perfectly.
Also just checked the serial number, and mine is 504044, the other DC2 ( in bits/just for parts ) is 503599.
The heads in both look the same, the head in the spare parts DC2 has these labels, 5H03 and PA263-3602.

Cheers
Les

[R.Daneel]

Okay, just got a word from Alex I wasn't clear enough on the "yielding stuff". Yielding point is often referred to as "yielding stress". A rather crude definition of stress is: Stress = Force / Area (area is the surface on which the force is applied).

Now imagine this: you have a steel rod you are trying to extend by applying tension force at one end and keeping the other end of the rod fixed. So you apply this force and the rod begins to extend. We want to see what happens next so we keep this force constant and the rod continues to extend. However, something is happening... the rate of extension is faster and faster. What's more, even if we would reduce the force, the rod would still be extending. This is because we have crossed the yielding point of the material which is steel in our case.

So basically, yielding point is a property where material starts to deform non-proportionally to the force. This is the most dangerous state of any material because soon after you reach it's ultimate point where the material breaks.

I tried to explain it as plainly as possible so I ignore a lot of things here. I hope it is satisfactory.

"Stiff materials break by cracking, not by yielding", means precisely what it says. Stiff materials such as some plastic compounds, glass and concrete virtually have no yielding point. It is almost the same as their ultimate state. These materials break immediately and without any obvious warning.

[R.Daneel]

Hi Antun,

Very interesting read. I had my DC2 gear replaced with one from a DD33 about one year or so ago and so far she is working perfectly.
Also just checked the serial number, and mine is 504044, the other DC2 ( in bits/just for parts ) is 503599.
The heads in both look the same, the head in the spare parts DC2 has these labels, 5H03 and PA263-3602.

Cheers
Les

Les, do you have a picture of your DC2 and it's head?

[Pacific Stereo]

IIRC the material is Delrin (or it may be nylon, I just can't remember), which has a tendency to shrink over time. The metal inner hub does NOT shrink, and the results are explained above.

[LesX55]

Hi,

Will post pics of working DC2. And try to get a close up of the head.
The other DC2 head will be easy as its dis-asembled

[Nakdoc]

Hah! Adjusted the head zenith? Oh you mean bend the head base?