Improving a turntable (motor controller circuit) ;)
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.
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:
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.
thanks for your reply ;)
Do you know the SL-2000?
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...
Again, can anyone else recommend an approach to improve or replace the old regulation?
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.
You do mention an oil bearing. Are you sure, the oil has not become something approaching sticky glue?
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... ;)
how did you push your post in there?
I must have overlooked it, as I only answered on the follow-up by Havoc.
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.
At least you've got a good turntable after all ;)
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)... ;)
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.
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