I have Technics SL-D2 turntable that has an interesting problem that led me to this forum to try and understand the circuit controlling speed.
I replaced all of the electrolytic capacitors thinking my problem was there with no effect. All pots/switches are clean. I re-flowed solder to many of the joints that even remotely looked suspect (such as the stator coil). No effect. I also replaced 2SC1846Q based on advice from an audio forum. That had no effect either. I did not replace any of the film caps.

The turntable will start up perfectly and lock speed as indicated by the strobe:

https://www.dropbox.com/s/kclugyl3wa2c4b0/20190123_173439.mp4?dl=0

After 20 minutes or so speed lock is lost as the video shows. Strangely, I can thumb the platter as a brake and speed lock is restored temporarily. Or shut off the turntable and turn it quickly back on and speed lock resumes - but only for 10 or so seconds. Only a long time (overnight) can I run the unit for 20 minutes again.

One other observation I just observed - when I took the bottom off the unit so I could see the circuit board while the platter was spinning, I could hear a very slight pulsating thump/pop that only occurred when the speed lock was lost. I'm trying to find out what part (capacitor?) could produce the sound (it is very very faint, but does occur and only when the platter loses speed lock.

Below is a schematic to the motor control. I am trying to understand which parts could take up to 20 minutes (heat up) before losing spec, making that rythmic tut-tut sound (which speeds up as the platter speeds up) and be responsible for the turntable going 'open loop' (platter speeds up and nothing controls it). Perhaps the IC (AN630U) - in which case there is nothing I can do to fix this board.

 

My approach would be try using Rapid freeze or similar product, & carefuly cool each individual part when its in the fault mode. If the fault is heat related it should isolate the faulty component..

 

A small (silent) fan might be your best option.

 

My approach would be try using Rapid freeze or similar product, & carefuly cool each individual part when its in the fault mode. If the fault is heat related it should isolate the faulty component..

I wish I could do that - but unfortunately - the parts face upward towards the spinning platter. There is no way for me to spray rapid freeze on individual parts. This is why I am trying to understand the circuit and how it works - what parts are involved in locking the speed and most important why does it take almost exactly 20 minutes after run time before failure? non-electrolyte caps? any of the transistors?

 

... How did you clean the variable pots? A frequent problem is moisture or humidity infiltrating into certain dials, controls, and maybe the VR 1, 2, and particularly VR3 resistors. Try using a spray can of electrical contact cleaner on VR3, assuming the speed lock problem is present on both of the optional speed settings. This procedure will take some particular care, since moving VR3 in order to clean it will necessitate getting it back to its original position in order for things to work correctly. ... making a locating mark or dot may be a good idea.

 

... How did you clean the variable pots? A frequent problem is moisture or humidity infiltrating into certain dials, controls, and maybe the VR 1, 2, and particularly VR3 resistors. Try using a spray can of electrical contact cleaner on VR3, assuming the speed lock problem is present on both of the optional speed settings. This procedure will take some particular care, since moving VR3 in order to clean it will necessitate getting it back to its original position in order for things to work correctly. ... making a locating mark or dot may be a good idea.

I thoroughly cleaned VR1,2 and 3 using first an electronic cleaner spray follwed by De-oxit lubricating solution injected into each of the moving parts. I worked each piece many times and they all are very fluid and "new" like in their feel. Unless they are genuinely faulty some how where after 20 minutes run time, they lose their resistance properties - I don't think this is the problem. It's why I posted on this forum. What is it - in the circuit that when parts warm up over 20 minutes, they can lose their spec and lead to the an open loop (no speed control). Could it be C9?

 

... ok ... has to be something else.