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Technics SP-10 mkII speed adjustment question

Hi,

I'm on my way to complete my Technics SP-10 mkII project. Actually, a friend of mine, a professionnal audio technician, is working to upgrade the PSU, which is done but a small adjustment on the speed must be done and he need some cue on this issue.

We already asked Bill Thalmann, Artisan Fidelity and Oswald Mill audio. Plus, I'll post on DIY Audio today. We'd like to get the answer as quickly as possible to finalized this for the week-end. Hope someone on Audiogon can help.

Here's the message from my technician:

"Hello,

I'm an electronic technician and I do repair for audio equipments, vintage, hifi pro and more. I have a client here that brought me his turntable Technics Sp-10 MKII to fixed. I have a little question about it and he gave me your email because he pretended that you have some experience with this kind of materiel. So, hope that you can response my technical question.

I replaced all capacitors in the power supply and a big solder job. I checked for defect solders or capacitors on the circuit boards inside the turntable and I tied to do the adjustments . Everything seem good right now, the turntable work fine. I tried do do the period adjustment with the VR101 and VR102 potentiometers like in the service manual ( see attachment, Period adjustment method). When I looked the stroboscope at the front of the turntable, It's pretty stable but I can see a tiny rumble at 33 1/2 and 78 speed. 45 is the more stable speed for the stroboscope. So, I fixed the phase reference with T1 at 18us of period and I try to do the period adjustment at the point test T and S on the board with the O point for reference. When I put my scope probe on the T point, I can observe the stroboscope running. It is not stable at all. If I pull off my probe, the stroboscope is stable again. So When I have the 2 probes at point S an T at the same time to do the adjustment, it's impossible to fixed the wave T because it going right to the left on my scope. When I turned the VR101, the T wave going faster or slower but never stable. I tried to ground lift my scope, plug it into the same power bar and try to pull off the reference at the O point. I can't have a setup that I can see a stable T wave in my scope with the one that I can do the right adjustment. Why? Is there a problem with the turntable or maybe it's a incorrect probe or ground setup? Please let me know what you think.

Best regards"

Thanks for help,

Sébastien
128x128sebastienl
lbelchev
64 posts
 
I would guess that the Mk2 and Mk3 are just so completely different that it was just fortuitous fall-out that resulted in the Mk3 having lower rumble.

Lewm, Out of question the Mk2 and Mk3 are completely different. Some of the differences are due to the different materials - respectively different technical data. This is from Technics SP-10MK3 brochure: "Also contributing to lower noise is the sophisticated cabinet construction featuring diecast zinc, diecast aluminum, and Technics original acoustic damping material TNRC (Technics Non- Resonant Compound) in the lower base. - 92 dB rumble means real silence; it's a difference you can hear from the moment you lower a stylus into the grooves." :-)

Moreover, somewhere around that time, the method for measuring TT rumble was changed such that the newer numbers were all about 3 db better than before.

Yes, This is the reason that I indicate the SP-10 mk2A rumble -86dB (IEC 98A) and the SP-10 mk3 rumble -92dB (IEC 98A). The SP-10 mk2 have different method for measuring -73dB (DIN 45539B).
lewm
13,009 posts
 
Lbelchev, When they wrote "sophisticated cabinet construction", it sounds like they were talking about their plinth, which I believe was supplied as an accessory to the Mk3. But is it possible, or do you think, they were referring to the chassis proper of the Mk3? If the latter, very interesting to me.
pryso
1,928 posts
 
Technics SP-10 series plinths:

From what I've been able to find, Technics offered as many as five separate plinths (bases) for the various SP-10 motor units.

The one SP-10 I've seen (not quartz locked and no outboard PS) had a traditional wood box frame plinth. Quite lightweight and I don't recall seeing a model number.

The SH-10B3 was a 12 kg layered combination of Obsidian, rubber, and wood and offered for the SP-10 Mk II motor unit.

The Vintage Technics site lists a SH-10B4 at 5.2 kg and a lower price. I've not seen any other reference to it.

The SH-10B5 was 19 kg and all Obsidian. It was introduced along with the SP-10 Mk 3 motor unit. Technics engineers obviously felt a more massive unit was needed for the increased torque motor.

The last one offered that I found info for was the SH-10B7. It was also all Obsidian but weight was listed at 17.5 kg. I've wondered why this was slightly less than the previous model? This plinth was offered during latter production of the SP-10 Mk 3 and Mk 2A.

The dates for these plinths ranged from the mid-70s to early 80s. Obviously Technics engineers were attempting to make improvements through materials and mass as they revised plinth design. But that is not to say it is what would be designed today with 30 more years of engineering knowledge and material choices.

If there was a change with the SP-10 Mk 3 motor unit frame (or chassis as Lew calls it) I would be interested to see a reference to that. I've never see a Mk 3 but own a Mk 2A.
lewm
13,009 posts
 
Tim, the Mk3 "chassis", as you know, does not contain any onboard circuitry. It is merely a cradle for the motor and a mount for the on-off switch and the strobe. Ergo, the bit that hangs down below the square escutcheon (the "dog's balls", in Dover's parlance) is circular in aspect when viewed from below, and its diameter tapers toward the bottom. IOW, it encases the motor assembly, only. Thus it does not have a potentially resonant cavity (or at least the cavity is very much smaller and confined), compared to the Mk2 chassis. You cannot put a Mk2 into a Mk3 plinth, as the former would not fit, not only a square peg into a round hole, but also a square with a side slightly larger than the major diameter of the Mk3 plinth circle.
pryso
1,928 posts
 
Hi Lew,

Now I'm confused. What I'm asking about is the cast metal frame that surrounds the platter, and in the case for the Mk 2 and Mk 2A provides support for circuit board attachment from underneath. Since I've never seen a Mk 3 except in photos I'm not sure about it. But from the top it appears identical to the Mk 2 and Mk 2A. This is what I understood you to mean by chassis.

The Vintage Technics site, which of course is not infallible, states the SH-10B7 plinth was intended for both the Mk 3 and Mk 2A mounting. The opening in that plinth is indeed not round. See this from Vinyl Engine -

http://www.vinylengine.com/library/technics/sh-10b7-obsidian-plinth.shtml

What I'd like to know is if the cast metal frame for the Mk 3 is different from the Mk 2A? That is, were additional vibration damping methods applied?
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