Author Topic: TD125 restoration  (Read 264 times)

Offline fredeb

TD125 restoration
« on: June 28, 2020, 07:43:53 PM »
About 3 years ago , I ordered all the electrolytic , polyester and ceramic capacitors and resistors to replace the old ones on my Thorens TD125 MkI motor speed controller board . So I finally got round to doing it . The little mirror in front , for the stroboscope had also unglued itself , so I had to dismantle the front controls too , to re-glue that .



The TD125 already had had some work done by someone else , and all the small transistors appear to be new-ish . They are modern components , but I checked and compared the datasheets and they appear to be good equivalents with similar ratings to the old  original components . The AD149's are the same as originals .



The original transformer was replaced with a toroidal with much higher voltage , which appeared to run quite hot , so I replaced that with an EI core version of lower voltage and higher current ( x4 current rating ) that I had lying here . For the record , the correct transformer's output on TD125 mk1 revision 2 is 12-0-12VAC , the earlier pcb uses a transformer with 2x 12-0 VAC taps . I'll post the 2 versions of the TD125 mk1 too .     

Before the strobe would sway a bit when speed was set , and after a couple of hours of operation the motor would slow , and eventually just stop . The little toroidal would get toasty hot . Apparently core saturation makes voltage droop .



Here's the one that was there , I'll use it in a phonostage .



There are 3 primary potentiometers to set initial approximate voltages for 16 , 33 and 45 rpm . Then there are another 3 for setting speed dead-on right with pitch control pot in centre position .The controller mechanism is a Weinbridge oscillator . It's quite amazing . Once set and I apply pressure to the motor spindle , the voltage stays completely stable . I raised the plinthe with 4 cans of baked beans so-as to adjust the voltages and frequency for 16 , 33 and 45 rpm respectively , and "0" DC on output



Quite a pleasure having pulled it off successfully .  ;D

I'm running the motor till tomorrow morning to see how everything holds up , then I'll start setting up suspension , etc.

Here are the 2 different mk1 version schematics :

original mk1 pcb



revised mk1 pcb



I could spot one error in the revised schematic . Look at the bottom right-hand corner of the schematic , parallel to C9 is 42k R24 . This is not indicated on the PCB image , and should be fitted to the right directly , of the lamp .



« Last Edit: June 28, 2020, 07:46:20 PM by fredeb »
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline fredeb

Re: TD125 restoration
« Reply #1 on: June 28, 2020, 10:38:22 PM »
I cleaned up and straightened the images from the manual for the revised pcb , hopefully it helps someone one day .

Right-click to enlarge .



The motor has been running for 6 hours now and is still rock solid dead stable on speed  :Whoohoo:


...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline mygoggie

Re: TD125 restoration
« Reply #2 on: June 28, 2020, 10:48:46 PM »
I do not see R24 on your revision? Maybe put a yellow circle around it?
Herman

~ To heal the soul is to embark on a new journey. ~

Offline fredeb

Re: TD125 restoration
« Reply #3 on: June 28, 2020, 11:48:45 PM »
I do not see R24 on your revision? Maybe put a yellow circle around it?

Hi @mygoggie . I've put a red circle around it on the schematic , and I've drawn it in it's correct position on the pcb . If you download the enlarged image it's easier to view in pc image viewer .

« Last Edit: June 28, 2020, 11:50:49 PM by fredeb »
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline johan.pretorius

Re: TD125 restoration
« Reply #4 on: June 29, 2020, 07:29:14 AM »
Awesome, thank you for sharing the details.
There is no better idle time than Thorens time.

Offline mygoggie

Re: TD125 restoration
« Reply #5 on: June 29, 2020, 08:10:50 AM »
Quote from: fredeb link=topic=87733.msg999704#msg999704 date=1593380925

[img width=657 height=800
https://i.postimg.cc/sjqr3WHB/new-pcb-r24.jpg[/img]


Thanks @fredeb now it shows in the PCB image!
Herman

~ To heal the soul is to embark on a new journey. ~

Offline fredeb

Re: TD125 restoration
« Reply #6 on: June 29, 2020, 10:11:51 AM »
Pleasure guys , I'm glad if it can be of use .

I'm happy to report that after running for 18 hours straight , the speed remains stable , and the transformer is just warm enough that you know it's on . :) 
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Qualityten

Re: TD125 restoration
« Reply #7 on: June 29, 2020, 10:03:45 PM »
Nice work. I refreshed my MkII a couple of years ago, but you've inspired me to take another look under the hood. After doing that work, I had to set the rorary pot quite far to the right instead of centre. I'd like to adjust that.

Offline Hi-Phibian

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Re: TD125 restoration
« Reply #8 on: June 29, 2020, 10:11:28 PM »
Nice work. I refreshed my MkII a couple of years ago, but you've inspired me to take another look under the hood. After doing that work, I had to set the rorary pot quite far to the right instead of centre. I'd like to adjust that.


Thatís odd.  One should do all adjustments with the speed adjust control central.  I actually keep it in place with masking tape while adjusting. 


 
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Offline fdlsys

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Re: TD125 restoration
« Reply #9 on: June 30, 2020, 09:13:01 AM »

Thatís odd.  One should do all adjustments with the speed adjust control central.  I actually keep it in place with masking tape while adjusting.
Yep, it's clear in the service manual.
If you find that you cannot calibrate the correct voltages and/or DC offset, then it's time for recapping, usually. I found the resistors on these boards to hold to the orignal values splendidly so fault finding In order of likeliness is: caps, semiconductors, motor.

Ari's case was unique because his 125 had a non orignal transformer that was an extremely poor choice for the use case. Keeping a small toroid transformer under stress with constant draw that was over the VA rating results in fast heating of the windings without anything that helps cooling. The hotter the windings get, the less of the current secondary is able to supply - vicious circle. Once he replaced the transformer with higher VA rating, all weird issues went away.
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Offline Qualityten

Re: TD125 restoration
« Reply #10 on: June 30, 2020, 11:38:50 AM »
I recapped it a couple of years ago, and the speed is stable. I'd just like to get the pot centred.