TD-125 with on-off thump...

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Hi friends

As the title says, my TD 125 (MK II) has quite a on-off thump. It of course gets amplified just as much as the tiny signals of a low-output MC and thus is a bit annoying (but not more).
What could be done, could anything be done?

Many thanks!
david
 
IF you wanted to modify the table, you could also look at rewiring the tonearm wires to a separate set of RCA jacks or straight into a phono cable going to the phono preamp.

There is no reason for those wires to be in the same electrical circuit as the motor control for the platter. Your cartridge delivers its output signal level straight into your phono preamp.
 
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I've used a TD-125 MKII since about 1980 and never had that issue. I still have it. There is zero need for modifications.

The motor is electronically regulated, so an oscillator and amplifier running off a DC supply. Turn it off and the drive waveform collapses instead of suddenly stopping as an AC motor would. In other words, no inductive kick. Maybe the power transformer, easy to fix and not normally an issue.

I suspect you've lost the chassis ground. If you are using the original arm (and it is far better than most replacements), great! It should be problem-free.

-Chris
 
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Thank you to all!

Here’s some more details: the deck, originating in the 7ties, was repaired by a professional—a friend of mine who is running a little hifi-business, working with british audio (ATC, naim, that kind of stuff), but not thorens, brought it back to life (!). Considering the rather complicated circuitry, I bow in respect that he got it working— this could be the entry point for this flaw.

OTOH, I recently took it apart for better adjustment, disconnected the mains, replaced the rca-wires, thus the probability of me having introduced a flaw somewhere is higher…

The arm (3009SII imp) is not tweaked, everything original. Yes I am considering a rewiring with a continuous cable to the preamp.

But I firstly want to see if I can tame that on/off thump as the proposed solutions seem easy. Will check the chassis ground first! (the chassis/arm-ground originally was tied to a channel’s cold, I changed this and use a „dedicated“ separate ground-wire connected to the ground-lug on the preamp (salas ufsp)…
@anatech , do you mean the TP-25 or the TP-16?
 
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Hi myleftear,
Mine has the TP16 on it. These tonearms are far better than you might think, and you would be very well served by installing the factory arm. I also have a TD-126 MKII in my main system with it's factory arm. That is one beautiful table! Powered arm lift at the end of the record, which is really nice. Low mass arm on this one.

Your arm and signal wiring is not the cause of your issues at all, not unless the wiring is really messed up. Direct wiring the wires isn't going to solve anything unless you correct a problem at the same time. I've been repairing Thorens tables since about 1978. I've rebuilt countless TD-125 MKII tables including servo problems caused by other techs.

The servo is pretty straight forward to service and repair. Just do not get creative and do not increase capacitor sizes! Just build it as it was using axial lead capacitors. When I see radial caps in place, I know the tech didn't bother to order the correct parts. I also know it is probably not set up properly, so I start from zero.

Yes, the separate chassis ground is normal and the only way it should be done. That is the factory connection. Make sure the tone arm is not connected to chassis ground and signal ground at the same time.
 
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No problem!

Just make sure either signal ground does not have a connection to the tone arm ground or chassis ground. The tone arm should be grounded to the chassis.

Do you have a meter? Measure between the chassis and each RCA ground skirt, then to to tone arm and chassis. No connection between the chassis and signal grounds. Of course the turntable is unplugged from everything for this test.
 
Hi friends

As the title says, my TD 125 (MK II) has quite a on-off thump. It of course gets amplified just as much as the tiny signals of a low-output MC and thus is a bit annoying (but not more).
What could be done, could anything be done?

Many thanks!
david
The TD 125 has spark suppressor/ snubber consisting of C1 and R1. Before doing anything else, check those components. Refer to the Linked discussion and the attached image.

https://www.vinylengine.com/turntable_forum/viewtopic.php?f=17&t=57656

The service manual is available for download on VE and electrotanya.

Regards
Screenshot_20231102-162702.png
 
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The spark suppressor is there to protect the switch contacts only. At worst this would cause a "click" or light "snap". Besides, you showed the TD-125, not the MKII. They are different.

Anyway, step #1 is to check the grounding as it is not the original tonearm. You do that first before removing parts to check them. Yes, you have to pull the part to properly test it. Step #1 is always understand how it should go together, then use your eyeballs to see if it looks right. Then use a meter to confirm basic connections are in fact made (never assume anything, prove it!).
 
There could be some circuit differences but is the Mk I or whatever version the op has , is it without the spark suppressor?

The thump during on/off is almost certainly due to a failed suppressor. I have also worked on several tts and have not found this issue to be caused by anything else. I also have the misfortune of the output transistors of a nice amp being taken out by one such failed suppressor.

Tonearm ground problems lead to hum or buzz not a thump during on/off and silance otherwise.

I have said whatever I wanted to , so am out of here.

All the best to the OP for a quick resolution without much heir pulling.

Regards,
 
Whatever C1's type is, it ought to be replaced with an X2 grade capacitor, which is intended for connection across the mains live and neutral.

Farnell 2429368 is just one example - there are lots out there. Choose one that will fit the original holes - and remember to order an X2 safety capacitor.
 
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I'm writing with a big smile on my face :)

thank you for the ongoing quest (a hunt in the fogs of missing pictures, isn't it?)
Analogadikt, thank you for the schematics!
And Anatech, you seem to remember my posts of quite some time ago? I'll have to verify but am quite convinced it is a MKII. I retrofitted it to be able to reuse the 3009 with its own lift instead of the (quite elegant) solution thorens provided, and replaced the front-plate (with the logo / model imprint). (It has a pressed in bearing axis for starters...)
Also, many thanks to sawyers, for the detail on the X2 safety cap!

The grounding scheme is correct, signals are not connected to chassis or tonearm ground (these are soldered to the 5th solderpoint on the sme connetor, signals have theirs, as in the pic...)
 

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