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Upgrades for a Thorens TD 126 MKIII? (vinyl newbie)

What do you mean by taking the snubber out? Did you replace the cap and resistor or just pulled it out? I'd swap both if it was me.
At this stage I'd swap out the rectifier bridge and the driver transistors. It costs peanuts. What's to loose? Did you get the 4.7V back?
 
Yes, put the 34 ohm R183 back in and got the 4.7V back and it's clean and stable as before. Pulled one end of the .1uf snubber up bypassing it and the resistor in the primary. First did all 3 new 470uf caps. Then put new 800's in the +/- 25. I can say that the +25V is cleaner than it ever was but still dips when the motor is catching up. I agree the driver transistors are suspect just because they see all the transients from the motor. Will get them ordered up next. Picked up a bunch of 1N4148 diodes. Haven't installed any yet. The supply store guy said the 1N4150's also cross over to 1N4148's.
 
dinger said:
First did all 3 new 470uf caps. Then put new 800's in the +/- 25. I can say that the +25V is cleaner than it ever was but still dips when the motor is catching up.
Click to expand...
Well, if that didn't fix the issue it sure can't hurt and is a good move for stability and peace of mind in the future. Not a waste of time.

The 1n4148 and 1n4150 don't look the same to me current wise. If they were - why would Thorens bother with both?
I'm not sure it's related to the issue, it looks more like it has to do with the regulated 15V, but maybe D118 and D121 are limiting the current to the motor when it tries to draw it on corrections. FWIW the 1n4150TAP diodes are still available on Mouser.
 
Thanks for the info Tom. I'll hold off on the diode work for now. I had a Radio Shack 1173 rectifier hanging around so I popped it in this morning - no change. It handles 8 amps VS the B250C800 which does just under 1 amp. Put the original back after the experiment. Did some calculating based on the max drop across the motor and figured its drawing about 200ma max when fully loaded - not too much I think. It still seems like the speed control is too sensitive, dropping the motor voltage to nothing when it gets too fast then over-ramping to recover. I need work on house stuff this afternoon, get back on it later.
 
IDK, there's a 150mA fuse on the PS so 200mA seems a little too much. You can easily measure the motor current draw. Just plug the meter in series with the motor at B4 and after that at B2.

Honestly, if it was me, I'd be ordering and replacing all semi-conductors including diodes, the T112 FET, T111 and the driver transistors T113/T114. After that I'd give up on fault finding and hack in a couple of 24V/1A regulators like MC7924/MC7824 with heat sinks hooked up pretty much the same way as in the 15V regulated circuit. That will keep a steady 24V regardless of load.
 
I've been thinking about that hack to regulate the +/- 25V for a while...
I'll check the motor current again in a more direct way like you suggested.
I've measured about 26ohms on the motor before and after surgery and at all rotational positions.
 
Well, I found another "knob" in the circuit. The current limiting resistor R205 (56 ohms) seems to be the torque knob as it limits the motor current. Popped a 140 ohm in it's place tonight and got an improvement in speed stability at a cost of needing to help it get going up to speed. It did hold speed though. Not perfect but definitely better. Backing up a little, I never had to help the platter start up until I operated on the motor. After some research pretty sure I should have left it well alone! Seems the magnets in the motor can loose a chunk of their strength when disturbed. I sure my rapping on the shaft with a small hammer didn't help anything either. Motors also loose their torque over time just from the armature demagnetizing the magnets as well. I think what I have is an age weakened motor to start with that doesn't have enough torque to respond quick enough to the servo circuit causing it to over-correct and so on. I've made it worse by tampering with it too. Right now, without the outer platter it will hold a steady 33rpm, add the outer platter (mass) and it doesn't have the torque to keep up with the corrections. I put the 56 ohm res back in for now. I think I need to re-focus from "repair" to "tuning" the servo circuit for what little torque I have left in the motor. I'd like to find the knob for reducing the voltage swing to the motor next. It currently varies from 0 to about 3 volts in 2-3ms segments.
 
I tried shunting some of the op amp circuit inputs last night to see if I could tone down the overcorrections - got some interesting results but nothing I'd call an improvement. I think I have already reached the conclusion the motor is a turd! BUT, I have a whacky idea on how to restore it's torque. First I want to take some measurements on where it's at by treating it like a generator and measuring the output (manually turning it and measuring the power returned on the inputs). Working theory is that the permanent magnets are weak due to age, demagnetization from the spinning coil, and recent abuse by yours truely. I'm not sure I could figure out how to re-magnetize them back to full strength but I think I might be able to stack additional magnets on the outside of the case that would boost them. Something like these here: https://www.apexmagnets.com/magnets/neodymium-arc-magnets-27-2mm-x-23-2mm-x-36mm-l-neodymium-magnet
I'm gonna pick up something cheap and crude for now just to see if I can make a change to the motor output.
 
Did some measurements, spinning the motor with a cordless screwdriver, and able to affect output level by placing magnets against the metal case. Spent time trying to figure out the polarity of the internal magnets with a compass. I wish I had taken pictures the first three times I had it apart to see how the magnets were arranged! After a lot of bargaining with the devil decided to rip it open one last time to be sure I knew exactly which way the magnets were orientated. Was expecting to find two magnets with opposing poles and found one continuous magnet!
DSC03544.JPG

Back to the compass and figured which position attracted the north and south poles and marked them on the outside of the case.
Picked up some magnets at ACE this afternoon and attached them in line with the polarity.
DSC03563.JPG

Will let it saturate in magnetic flux overnight and see what happens!
In the meantime, broke the feed wires to the commutator finally. They were definitely heat stressed and holding on by just a few strands. Attached new wires.
DSC03562.JPG

Don't worry, will clean this up prior to assembly.
Really oiled the crap out of it during the previous assembly and now I can see that probably didn't help anything.
 
First, I can't believe how lucky I was during my previous motor surgeries that the thing ever ran! I never even thought about the magnet polarity! In the video I spent a lot of time figuring out how to align the brush housing with the rest of the motor. Pretty sure I got it back to original torque - no issues starting up now. Not sure if my overnight flux bath did much good or not - I think I'm back to my starting point and maybe a slight improvement - nothing to proclaim "this is how you fix a TD126 MKIII". I gotta call an end to the triple overtime on this and get on with life!!! Yeah, it still bugs me when I see a video of a new Technics direct drive solid as granite on the strobe.

 
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Gentlemen, I see that somebody has the "latest" version of TD 126 Mk III
schematic (see video in post 61) that I can find nowhere. Please tell me and
other people where you found it or even download to this site - thank you.
 
Thank you for reply, yes I know Vinylengine and this late edition of the schematic is not there - otherwise why should I ask.
 
PS to the video, it is easy to see the rotational phenomenon has a 2 seconds period,
so exactly related to the platter speed (not motor speed) and hence is a platter and
bearing problem.
...
Tried to decipher the browser tab inscription in said video to find where the source of
this schematic may be, but no success, a lamp is spot on ..
...
All dd turntables I know, not only Technics, have "WOW and Flutter" or other problems,
thats why they arrive in my workshop.
 
Update : Thorens TD 126 Mk III motor does not spin

Motor without any drive voltage : Integrated Circuit Z104 LM3900 had to be replaced.

Yes, this late schematic is way downstairs in the service manual after the regular one.
 
At Vinyl Engine I found : "ve_thorens_td126_mk3_service_de.pdf" to be the only file with very good (sharp) schematics. All info regarding the modified version (serial 47276 and above) starts on page 28.
 
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