THORENS TD 126 MKIII speed problem

henry11106 · 27th July 2011, 09:46 AM

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

analog_sa · 27th July 2011, 12:17 PM

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.

henry11106 · 27th July 2011, 04:11 PM

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!

analog_sa · 27th July 2011, 05:43 PM

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.

henry11106 · 28th July 2011, 01:19 AM

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.

27th July 2011, 09:46 AM	#1
henry11106 diyAudio Member Join Date: Jul 2011	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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
thorens td 180 in ....high speed	sakis	Analogue Source	0	7th October 2008 08:29 PM
Thorens TD 125 speed problems	singinghairdo	Analogue Source	22	27th February 2008 08:11 PM
Thorens TD 125MKII speed inconsistent?	600LPs	Analogue Source	0	26th October 2007 05:32 PM
Thorens TD 150 problem	johnsmills	Analogue Source	3	11th September 2006 03:04 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

27th July 2011, 12:17 PM	#2
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	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.

27th July 2011, 04:11 PM	#3
henry11106 diyAudio Member Join Date: Jul 2011	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!

27th July 2011, 05:43 PM	#4
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	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.

28th July 2011, 01:19 AM	#5
henry11106 diyAudio Member Join Date: Jul 2011	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.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)