Improving a turntable (motor controller circuit) ;)

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Hi,

this is an unusual request, as I have found nothing about it on the internet.
I want to improve/modify a turntable motor regulation circuit.

I know I could buy good circuits and motors, but the player I want to rescue/improve is something special:

- it's my first player,
- it's as old as me, every part inside was built in the year I was born. ;)

The ugly properties I'm dealing with: a Technics SL-2000, an actual cr@p turntable... :(

I know I could buy 'contemporary' Technics gear for the sake of sticking to direct drives while having an easier start. But hey, it's a DIY project... ;)

Anyway, the SL-2000 has to be improved, if only for the learning experience.

Sorry about the rather long introduction, now to the description of my first problem: the motor control circuit.

I've reverse-engineered the control circuitry up to the point where I'm lacking further documentation that can't even be sourced from Panasonic any more!

About the working principle: the motor appears to be a 'brushless' three-phase 'DC' unit with a built in tacho generator (Matsushita 'MKL-15SI-T'). This unit is enclosed in a servo regulation, based on an integrated motor controller IC 'AN620' (Matsushita). BTW, can anyone supply information on this one, perhaps a modern substitution (does not have to be pin-compatible, but better ;))? I've added a picture of the motor internals, you can see three layers of drive windings and three small pairs of tacho 'pickups'.

The problem with it's current control unit: speed sagging under load, bad regulation and too low torque (even the needle slows the platter...).

Some constraints regarding the final result:
- I want to use lightweight platters (because I want to keep the look-and-feel and can't add infinite torque to the motor).
- And I want to keep it being a direct drive unit with it's oil bearing, of course, as otherwise I could just throw it away and buy better parts...

Some things go without saying:
- it will earn a stiff power supply (the old one sags and humms under load);
- a new base (heavy and unsuspended, because it's easier for now);
- someday a better tonearm, etc.

But how should I approach the improvement of the motor controller?

I have some ideas of course:
- PLL regulation (actually easy to build, but difficult to integrate into the old regulator),
- separate amplifier output stages (to drive with higher power), etc.

Are you still reading? Very good. Read on in the next post, where I'll continue with the controller circuit description.
 

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I've grouped the interconnects into four connectors (cable colors matching the original setup):

J1 - power supply, 18V DC (upper left conrner);
J2 - speed selection and pitch setting (on the very right);
J3 - 'S' windings, supposedly the speed meter (left);
J4 - 'A' windings, supposedly the actuator (lower right);

In the center of the drawing sits the AN620, which I know nothing about regarding it's internal structure, as I couldn't find any documentation about it.

The circuitry on the upper right (connected between power supply and pitch regulation input) appears to be some filtering and referencing/biasing.

I haven't measured/read the resistors yet, as they are of no particular importance as long as I don't know what the AN620 does whith what value.
D4 (upper right) *might* be a Zener, but it's not marked or printed on.
D1..D3 are of yet unknown type (long glass body, nothing printed on), but definitely connected this way.
What the RC network R5/R10 and C13/C14 (top, between +18V and pin 16) is for? I can't tell without knowing what pin 16 is for...
There is a wire '?' along the 'S' connections, which has no component print and the actual purpose is not yet obvious.

I've studied the schematics of the Technics SP-10MK2 and the SL-1200/1210MK2 for comparison. It's clear how everything is approached (servo loop with frequency-to-voltage conversion, enclosed PLL with quartz reference and phase comparision, higher-torque motor with appropriate drive), but turning a mediocre controller circuit into a better-than-average is not easy for me...

Does anyone have an idea?

Is it possible to increase the output power (haven't measured the motor yet, but a little more should work easily)?
Is it possible to improve the regulation (haven't replaced the capacitors yet, but they appear to be allright)?

Or is the SL-2000 normally superior to what I describe and mine is broken? ;)

Any comments are appreaciated, thanks for reading! :cool:

Sebastian.
 

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From what you say it sounds like the motor and controller are not working properly - I would fix the basic problems first and address upgrades after you are sure the motor itself is ok. It is unlikely at this juncture that you will find an improved motor controller IC for this application that is pin for pin compatible, plus I am sure it can do the job well as long as it is undamaged. Far more likely at this time that some or many of the electrolytics are bad and need replacing. Pots and some resistors may be suspect too. The motor itself is not too likely to respond well to higher current drive, the windings were designed to dissipate only so much power. I would focus on making sure that the current circuit is actually working properly. One area where you might make a big improvement is in replacing the power transformer with a slightly larger one, add more capacitance to the supply, perhaps consider voltage regulation to the servo using something like the LT1085, set to 18V to this I would add hex freds or schottky (if piv is high enough) rectifiers and maybe 10000uF 35V nichicon muse electrolytic after the rectifiers.
 
Hi Kevin,

thanks for your reply ;)

Do you know the SL-2000?

kevinkr said:
From what you say it sounds like the motor and controller are not working properly - I would fix the basic problems first and address upgrades after you are sure the motor itself is ok.

The behaviour didn't change (worsen) since many years. And the motor seems okay. But I plan on progressing slowly and only where I'm sure what I'm doing. So I plan to use the current circuit as long as I don't have a better solution. I could indeed try to 'repair' the old one and hope for a little improvement until the new controller is finished... ;)

It is unlikely at this juncture that you will find an improved motor controller IC for this application that is pin for pin compatible, plus I am sure it can do the job well as long as it is undamaged.

Well, as I wrote, I'm not lookin for a pin compatible replacement IC (I explicitely said so, because I actually don't expect any pin compatible IC to exist).

But a 3-phase 'DC' control is something very basic, there sure are controllers which implement a 'tighter' servo loop (and have a better reference), but are also suitable for said motor...

Far more likely at this time that some or many of the electrolytics are bad and need replacing. Pots and some resistors may be suspect too. The motor itself is not too likely to respond well to higher current drive, the windings were designed to dissipate only so much power.

Yep, I suspect that too (as always with old electrolytics). But the motor can handle higher power, because it's "underpowered" due to the poor power supply at the moment. I'not talking about an order of magnitude or so, just about an (as little as neccessary) higher torque for short durations.

I would focus on making sure that the current circuit is actually working properly.

I honestly don't expect the current circuit to be able to perform good enough, even if I could restore it's original performance. It's a really cheap player, due to the cheap power supply and simplified controller. It's one of the cheapest Technics players ever, known for it's mediocre performance... But the motor is capable of more, as it is used in better players, too.

One area where you might make a big improvement is in replacing the power transformer with a slightly larger one, add more capacitance to the supply, perhaps consider voltage regulation to the servo using something like the LT1085, set to 18V to this I would add hex freds or schottky (if piv is high enough) rectifiers and maybe 10000uF 35V nichicon muse electrolytic after the rectifiers.

Yep, I agree again, but this is something I'm already planning (as said in the former posts). I actually consider it neccessary, as I wouldn't expect an improvement in performance from the same transformer.

Again, can anyone else recommend an approach to improve or replace the old regulation?

Thanks again,
Sebastian. ;)
 
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Hey sebastian,
Sounds like you have things pretty under control.. <grin> Here is a thought - you mentioned that this motor was used in better technics tables as well, perhaps you can or already have identified one of those models and from there perhaps you can find a better controller implementation.. (Even salvage one from a damaged TT?)
Philips and National both had lines of motor controller ics at one time that may suit the application. I kind of wonder if anyone is doing much with these servo motor controls these days as the market for these cheap controls has almost dried up with the smallish cheap TT and obsolete cassette deck market.. Might also try JRC and some of the other japanese vendors.

I had an SL-23 25 + yrs ago and thought about getting an SL-2000 to replace it, but ended up going another route to a considerably better TT. As I recall the SL-23 would not win awards for its speed stability either, but the worst I ever owned was a Pink Triangle, (NOT cheap.. lol) even with a new motor and controller it drifted so badly I could not even listen to a single side without having to readjust. (As I recall it used a technics belt drive servo motor with the worst designed controller circuit I have ever seen - it did not really work at all! I disconnected the tachometer and nothing happened, two boards exactly the same.. lol)

Power supply improvement may help a lot as there should be somewhat increased torque. Measure motor currents and make sure IR drops in controller pcb traces are not excessive, bolster with wire point to point as needed.
 
Hi,

It has still been liquid when I replaced it. ;)

And the replacement oil I applied (a machine bearing lubricant of about the same viscosity) is still in it's original (good) condition...

Would increased friction in the bearing really worsen the speed regulation? I can only imagine it to constantly load the platter (and increase regulation delay), but the motor does not keep nominal speed when only slightly loaded (i.e. by the needle, and I assume that means friction by the needle is much higher than overall friction when unloaded).

I could experiment with the bearing (i.e. find out if bearing fiction is angle-dependent or not constant over speed), but that still makes the original SL-2000 a mediocre player with a good motor... ;)

Thanks,
Sebastian.
 
Kevin ;)

how did you push your post in there?
I must have overlooked it, as I only answered on the follow-up by Havoc.
Sorry ;)

kevinkr said:
Hey sebastian,
Sounds like you have things pretty under control.. <grin>

Umm, pun? :cool:

Here is a thought - you mentioned that this motor was used in better technics tables as well

I've read about that fact on the net. I'm not sure in which models (and can't determine in which not)... unfortunately! There don't seem to be too many people who like to talk about direct drives on the forum, though...

perhaps you can or already have identified one of those models and from there perhaps you can find a better controller implementation.. (Even salvage one from a damaged TT?)

They all use outdated Matsushita/Panasonic ICs without adequate documentation. But...

Philips and National both had lines of motor controller ics at one time that may suit the application. I kind of wonder if anyone is doing much with these servo motor controls these days as the market for these cheap controls has almost dried up with the smallish cheap TT and obsolete cassette deck market.. Might also try JRC and some of the other japanese vendors.

Exactly. I have found several suppliers of servo control ICs for three-phase brushless DC motors in the meantime. The keyword here is: BLDC (brush less direct current).

There are modern solutions, and it looks as if there are possibilities to control the motor with just a single IC for servo control and three-phase drive and another IC for the high precision regulation (i.e. PLL). ST Micoelectronics has such ICs, TI makes one, National used to make one, etc.

I had an SL-23 25 + yrs ago and thought about getting an SL-2000 to replace it, but ended up going another route to a considerably better TT. As I recall the SL-23 would not win awards for its speed stability either, but the worst I ever owned was a Pink Triangle, (NOT cheap.. lol) even with a new motor and controller it drifted so badly I could not even listen to a single side without having to readjust. (As I recall it used a technics belt drive servo motor with the worst designed controller circuit I have ever seen - it did not really work at all! I disconnected the tachometer and nothing happened, two boards exactly the same.. lol)

Odd. Funny, actually. Yes, there are bad motors around, as well as bad Technics turntables. Unfortunately, for many people this means: Technics and DD can't mean anything good :(

At least you've got a good turntable after all ;)

Power supply improvement may help a lot as there should be somewhat increased torque. Measure motor currents and make sure IR drops in controller pcb traces are not excessive, bolster with wire point to point as needed.

Okay. I'll do excessive measurements before I continue. I have to gather more specific information on the motor before I select a controller driver. Let's see what I come up with ;)

Further input is much appreciated! I know some of you guys are well informed in the direct drive department, ürobably just don't want to come out with knowledge about it (as DD is a no-no for many on the forums)... ;)

Thanks,
Sebastian.
 
Technics turntable DIP IC?

I am working out the schematics for the SL-23FS and need help with the DIP IC. The only markings on it are "K650 C1003", and I cant find a reference to it anywhere.

I have the CCT boards mapped out and a CCT diagram drawn, just missing the DIP IC info - WHAT IS IT??????

My plan is to build an external power supply with remote mounted controls.

I have 3 if anyone wants spares
 
Hi, I just found this thread since I am getting ready to put my SL-2000 turntable back in use.
I am the original owner of this device, having bought it in about 1977, and I have to tell you it has always worked very well for me. I have experience none of the problems you report.
One observation, though: I believe the speed adjustment pots get dirty and noisy, just like bad scratchy volume controls of the same era. I have cleaned mine occasionally and it gets the speed back to a nice stable performance level - for a while. The speed control pots are 5K each, I'm thinking of replacing them with 10-turn trimmers since the old pots are probably beyond cleaning.

- Doug
 
any luck

I know this is along time ago for the reply. Any luck with the sl-2000 Technics turntable? I just bought a used one for $53 usd. to replace my Dual 1219. The 1219 gets honors for being a real piece of crap. All mechanical...the speed switch is stuck, the speed is very slow, the anti-skating and weight balance all failed by just letting the unit not being used in 8 years. The sl-2000 works perfect. I will do some of your suggestions. Such as clean and put in new oil bearing oil. Change pots to 5 or 10 turn. Replace all electrolytics. And beef up the power supply with bigger electrolytics and maybe more if possible I have not opened the unit yet...just got it today.
Mitchell
 
I second that, although I don't know this particular Dual model. ;)


Any luck with the sl-2000 Technics turntable?

Not really. I have to admit that the conditions were difficult to start with. The platter was excentric, the arm bearings were loose, the power suppy was crappy and the acrylic cover was damaged beyond repair. :(

I had stripped the motor assembly from the chassis and experimented with the combination of motor and platter. They could benefit from a hefty laboratory supply (with regulated 18V). My attempt would have been to replace the 33/45 switch and pitch control unit with something more convenient (button switch, single potentiometer) and the stroboscope with LEDs for a sharper image.

But then I got hold of an SL-1710MK2! :devilr:

There's no way back.

I currently use an SL-1200MK2 that I refurbished and repainted (white chassis, white LED illumination, improved power supply, renewed pitch assembly, recabled tonearm, etc.).

Don't get me wrong, I don't dislike the SL-2000 parts for what they are, it's just impossible to improve them beyond what they are capable of - at least not without spending more than for a used MK2.

Cheers,
Sebastian.

PS: I like to keep the SL-2000 parts in the drawer, though. We share the same year of birth. :) So in case you have any questions, don't hesitate.
 
We are working on something similar here:

"New Technics SP10 motor controller specification"
http://www.diyaudio.com/forums/showthread.php?s=&threadid=121934

Sek, your shematic shows no frequency reference (quartz crystal) so it will not be as accurate as designs like the SP10. (probably because they want pitch control, and pitch control with PLL is (was) not an easy or cheap circuit)

The motor also appears to have no true tacho coil - only the three phase detection windings. These CAN be used for tacho purposes as well, but are not very accurate (from the SP10 motor measuremnts I did, only about 6 pulses per revolution, as compared to 190 for the true tacho pickup). For PLL control you will need that sort of accuracy (190 pulses per rev). You could always add a photo-detector on the rim of the motor, and stick on a laser printed band of stripes for it to read.

To answer some of your questions, I'm guessing the following:
D1-D3 are germanium signal diodes, to demodulate the AM signal from the phase detector.
The white wire at S2 is likely the excitation voltage for the phase detection coils - probably at around 50kHz.
R10/R5 etc. MAY be the lead/lag compensation for the servo loop.

I imagine you could certainly increase the current drive to the motor. - after all, it' only the impulse current to get it up to speed, its normal runing current will be unchanged. It has a long thermal time constant and won't suffer badly from a higher start current.
 
Hi Steerpike,

I'm not the one who wants to improve it (anymore). ;)

I'm aware of the things you mention since I dissected a SL-1710MK2 and a couple of SL-1200/1210MK2 for comparison. The SL-2000 incorporates the technology that has also been used in the SL-1200 series (without MK2): no frequency generator, no crystal reference.

Oh, and I recognized your thread and followed it from the beginning. ;)

In other forums, I discussed the option of retrofitting the motor assembly with a frequency generator coil (i.e. from a superseded MK2 series player or a DIY one) and a reference generator circuit. But the amount of work is similar to your approach of a redesign...


D1-D3 are germanium signal diodes, to demodulate the AM signal from the phase detector. The white wire at S2 is likely the excitation voltage for the phase detection coils - probably at around 50kHz. R10/R5 etc. MAY be the lead/lag compensation for the servo loop.

Thanks for your description.

It's even more severe: I made a mishap in my circuit description and accidentally took the drive coil connections for the position detector connections and vice versa. It's the other way 'round. :rolleyes:

The circuit can be improved as you mention - by increasing drive current and adjusting the lead/lag network in the feedback loop to the servo. Measured frequency is lower than 50kHz, btw...

But as the controller IC is not documented anywhere, the risk of increasing the current through the IC is too high for me. At least, it incorporates a metal plate for mounting a heat sink. External driver transistors are a possible solution, but too complicated to fit into the original housing...

Cheers,
Sebastian.
 
Heres what I am going to do. I will upgrade the power supply. It is 18 volts. I will use a LM7818 T220 chip instead of the existing supply. The LM 7818 can supply 1 amp constant and 2.2 amps peak (sounds good). Will use existing heatsink. However I will need a bigger transformer, I will try and find one that just fits inside. I need one that is 24vac (thats what I measured, 20-22 volts is ok too). Capacitor after the bridge rectifier will be 700 t0 1200uf. Capacitor at output of regulator will be 330-500uf. The bridge rectifier will be changed to 1 amp.

This is a little overkill since fuse for existing power supply is 350ma, I might change fuse to 500ma if I get a bigger transformer only if fuse blows once in a while. Existing power supply has only one capacitor at 330 uf. Using a LM7818 is better than using discrete componets. Existing transformer looks like it is 6 watts, hope to replace it with a 10 watt. In reality I need a beefed up power supply that will supply 1/2 amp instantly:).
 
Is anyone still watching this thread?
A couple of weeks ago I got an wrecked Marantz TT, that upon disassembly proved to contain a Technics motor - not *quite* the same as the SL2000, but *very* similar. I'm still trying to figure out which Technics it comes from - the motor ID is PM2330009, with the ubiquitous AN620 IC in it. And no FG!

So I might look into upgrading this motor too - but the SP10 one comes first.
 
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