Technics Direct Drive - Running too fast

[MadCarburetor]

Hi guys,

I recently acquired a "non-functional" Technics SL-D1 turntable, that I want to fix up.

First thing I did was spraying some contact cleaner on the potentiometers. While cleaning up the internals, I noticed that one of the capacitors(C1) appeared to look suspect (bloated appearance) so I replaced it.

The problem now is that the turntable is spinning way too fast at both 33 and 45 settings, no matter how I adjust VR1, VR2, and VR3(pitch control dial).

Even with all potentiometers all the way to the "-" position, the speed is too fast.

As I did not really try adjusting the speed before changing C1, I'm not sure if that was the cause of the problem.

Would appreciate any suggestions on how to go about troubleshooting the issue..

Here is the schematic:


And the PCB diagram:

 

[JonSnell Electronic]

A basic voltage check is where I would start; regulator etc and then check the waveforms shown in the service manual.
C1 is only the main smoothing. I doubt that was the cause.

 

[MadCarburetor]

A basic voltage check is where I would start; regulator etc and then check the waveforms shown in the service manual.
C1 is only the main smoothing. I doubt that was the cause.

Won't switching on the turntable to check voltages and waveforms without the platter on cause any damage to the IC, since the motor's rotor/top part is a part of the platter on the Technics direct drives? Heard that these motors should never be powered without the platter present, hence I was hesitant to do any checks with the power on

 

[bullpeters]

check voltage at J1. Mine will run fast (or backwards) if the voltage is lower than 21.5 ish.
I always took of the platter and powered mine up. Never seemed an issue

 

[MadCarburetor]

I finally had some time to work on the Technics and checked the voltages as per the service manual.

I was not sure if the tests had to be done with S1 closed or open, but there were no voltages showing up with it open so the test was done with S1 closed (N.O terminal connected to C)

I have attached a copy of the schematic with the values I got annotated. The original schematic and IC block diagram have also been attached for comparison.

One thing that struck me as odd was that I was getting high AC voltages (Discovered this when I accidentally used the multimeter AC Setting) as well as the DC voltages marked on the schematic. Not sure if this is normal or a error due to how the meter was connected?)

I am beginning to think the control IC is at fault. What do you guys think?

 

[MarcelvdG]

One thing that struck me as odd was that I was getting high AC voltages (Discovered this when I accidentally used the multimeter AC Setting) as well as the DC voltages marked on the schematic. Not sure if this is normal or a error due to how the meter was connected?)

With respect to what node did you measure those voltages?

I first thought it might be due to your meter; for example, it might use a half-wave rectifying circuit and multiply the result by pi/sqrt(2), which only gives the correct result on perfect sine waves with 0 DC level. That can't be it, though, because the ratios of AC to DC would then be the same everywhere, which they are not.

For your information, some direct-drive turntables become unstable without platter, their control loops are simply not designed to be stable with much reduced inertia. I have no experience with Technics turntables, but I wasted quite some time figuring out why my Marantz TT 4000 didn't lock until I found out it was simply the missing platter. This doesn't explain why your turntable runs too fast with platter, of course, it is just something to keep in mind when measuring direct-drive turntables circuits.

 

[MadCarburetor]

There were some errors in the annotations, here's a corrected version:

I tried checking the waveforms on the IC pins with my cheap diy-kit oscilloscope. Could not get anything except the DC offsets on Pins 1, 2, 3, 5, 6. There were some low amplitude sine-waves on some of the other pins shown on the schematic. Here is the album link: scope readings - Album on Imgur

check voltage at J1. Mine will run fast (or backwards) if the voltage is lower than 21.5 ish.
I always took of the platter and powered mine up. Never seemed an issue

I am getting 20.8V DC at j1. All the voltages around Q1 and Q2 are a bit lower, but is it enough to cause an issue?

With respect to what node did you measure those voltages?

I first thought it might be due to your meter; for example, it might use a half-wave rectifying circuit and multiply the result by pi/sqrt(2), which only gives the correct result on perfect sine waves with 0 DC level. That can't be it, though, because the ratios of AC to DC would then be the same everywhere, which they are not.

For your information, some direct-drive turntables become unstable without platter, their control loops are simply not designed to be stable with much reduced inertia. I have no experience with Technics turntables, but I wasted quite some time figuring out why my Marantz TT 4000 didn't lock until I found out it was simply the missing platter. This doesn't explain why your turntable runs too fast with platter, of course, it is just something to keep in mind when measuring direct-drive turntables circuits.

I measured with reference to the - terminal of C1 which is connected to the ground lines. (As shown in the PCB diagram in my first post)

 

[bullpeters]

This is very un scientific
rewired tone arm and painted base. re assembled 1200 turntable.
running very fast and both 33 and 45 lights are on. took it apart, wiggled wires on the on switch and speed selector
disabled the pitch selector (easy to do) and all is good, working turntable.
I would love to know if you got yours running and what was wrong

 

[MadCarburetor]

Hi guys,

I recently acquired a "non-functional" Technics SL-D1 turntable, that I want to fix up.

First thing I did was spraying some contact cleaner on the potentiometers. While cleaning up the internals, I noticed that one of the capacitors(C1) appeared to look suspect (bloated appearance) so I replaced it.

The problem now is that the turntable is spinning way too fast at both 33 and 45 settings, no matter how I adjust VR1, VR2, and VR3(pitch control dial).

Even with all potentiometers all the way to the "-" position, the speed is too fast.

As I did not really try adjusting the speed before changing C1, I'm not sure if that was the cause of the problem.

Would appreciate any suggestions on how to go about troubleshooting the issue..

Here is the schematic:
View attachment 807611

And the PCB diagram:
View attachment 807613
View attachment 807614

I changed all the electrolytic capacitors and the AN630U IC. The turntable worked fine when I test-fit everything, but when I reassembled to apply thermal paste and replace the speed selector switch (the old one was wonky), the turntable started acting up again.

This time the speed keeps jumping around, as in in momentarily speeding up and slowing down again..

Checking continuity on the board between various points, I found that pins 18 and 19 are shorted, thus Pin 18 which is connected to the position sense coil is also connected to B+ (emitter of Q1). Removing C16 doesn't make a difference.
Seems to me that pin 18 should not be directly connected to B+? Or is it normal?

 

[HAF_IN_RIO]

Hi,
Very important is the DC power supply voltage, it must be very stable, abou 1mV or better in variation. Any changes will affect the platform speed. Check too the pin 2 of the IC, it must be stable, with any AC mains power voltage variation, the DC voltage must be steady!
Verify in red the critical voltage measure points.

 

[RickRay]

The AC after the diodes and especially after the regulator is an issue. Try a different voltmeter to verify. If it is still that high with AC, where could it come from? You more than likely have a shorted or leaking D1 or D2.

 

[HAF_IN_RIO]

Was tested with laboratory precision voltmeter as HP 3456A.
The problem was with AN630U, the internal zener diode (pin 2) not working.
Was replaced with good one IC, all speed problem gone. All DC voltage steady, with variation about 1mV. Tested and compared with others two Technics turntable who using the same AN630U ( SL-3200 & SL-D202 ) all running perfectly stable.
Thank you.