THORENS TD 126 MKIII speed problem - diyAudio
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Old 27th July 2011, 09:46 AM   #1
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Default THORENS TD 126 MKIII speed problem

Hi all,

after a lot of looking around google etc I have found this issue to come up now and then but always without a definite answer, so here goes:

I have a thorens td126 MKIII - SN 481xx? - that I want to use, but something is wrong with the speed control as it runs REALLY fast at any setting, and selecting speed makes no difference at all, just goes as fast at it can go on about 12.5v dc.

I figured it could be the tachometer on the motor, but it shows about 0.2 to 0.4v ac when running at 33-78 rpm (powering the motor with a benchtop power supply @ 1.8 to 3.3v dc, no connection with the turntable)


I can't find any obviously damaged electronic parts, any transistor I checked was ok, nothing burnt etc. but I don't know exactly what to look for at this point.

tonearm lift is good and the buttons all appear to work, belt is good, strobe works, connectors seem fine. might be an IC?




I do have the german service manual with the circuit drawings for up to SN 47275 and after SN 47276 (which matches mine) too bad my German is no good!



any ideas? I'm fairly stumped at the moment.


Thanks,
Henry
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Old 27th July 2011, 12:17 PM   #2
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Hi Henry

From your description i would suspect the speed controller IC. Unless it's something silly like the cap shunting the tachometer coil. I wonder if these ICs are still available new. An old German cassette deck may be a possible donor.
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Old 27th July 2011, 04:11 PM   #3
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cassette deck! neat idea. I didn't thingk ICs would be so readily interchangeable.

well, I pulled the main board and did some poking around... turns out the problem was really dumb. somehow, one of the pins on the "fine speed adjustment" pot was shorted to ground! I don't know how I missed it, as I looked many times in the last while.

I found it by plugging the removed board in and wiggling things while the TT is running. short the pot to ground and it runs wide open. wow.

should have occurred to me sooner, since this one has had it rough. certainly was bashed around a bit in its life yet still turns perfectly.


Thanks!
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Old 27th July 2011, 05:43 PM   #4
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Congratulations for the happy ending. Owned once a 126 Mk3 and a 226 with the same dc motor. No idea why they didn't stay with the ac drive.
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Old 28th July 2011, 01:19 AM   #5
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I imagine they went with DC to reduce hum, though DC motors can cause a lot of signal noise... it's probably just a marketing point. digital/solid-state/electronic speed control means cool

just imagine if they said it can play CD's.


I'm happy regardless! it is a nice unit, and mechanically pretty tough.
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Old 3rd February 2014, 02:23 PM   #6
fred78 is offline fred78  France
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Hi,

I have almost similar problem on the same turntable , from the same serial number or the model with 4 LEDs for the strobe .
If I plug the turntable, I select a speed, turnatable normally operates at the selected speed ...
if I want to stop the deck by pressing the right button , the motor races and running at full speed !
If I turn off the turntable, that I start by choosing a speed , everything returns to normal until I press again the Stop button !
I replaced all the capacitors ...
I have checked the polarity of the capacitors replaced ...
I replaced the diode bridge ...
I all transistors replaced one by one with those of another identical plate ...
still the same problem !
I did not invert the potentiometer Strobe ...
I have not replaced the resistors or diodes !
If I replace the printed one of my other turntable loop , everything works perfectly ...
If I ascend printed on the other deck circuit, the problem becomes identical ...
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