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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 17th May 2018, 10:28 am   #1
sherman51
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Join Date: May 2018
Location: Stoke On Trent, Staffordshire, UK.
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Default Thorens TD 126 Mk 3 - problem with Speed Switches/lamps

Hi, i'm new to this forum, hope someone can Help with my weired issue ..
I have owned the deck for over 40 years, but it's spent the last 20 or so boxed.
It's in great condition just needed a good looking over. So the turntable works and maintain's speed well,
The motor nice and quiet but I had some issues with a bulb not working at first in one of the speed switch's
so I have replaced all the bulbs in the switch's, but I have this strange problem
If there is a bulb in place then the switch dosent hold contact ..
Example, there are 4 switch's (Buttons) 78, 45, 33 and stop, if I take all the bulbs out then they all work fine,
I press the button and release and the turntable gets up to speed and everything is ok. But If I put a bulb in any of the positions,
then when I press the button, it lights and the motor starts but stops when I let go ...
Another example, I take the speed switch bulbs out (all 3) I select one of the speed buttons (works the same for all) I press it briefly
and the turntable spins up to speed ... great .. but then if I place a bulb in the circuit board (for the speed I have running)
then the stop switch lights and the motor stops.
i'd be very grateful for any Help.
Cheers
Paul
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Old 17th May 2018, 11:25 am   #2
John Caswell
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Location: Wokingham, Berkshire, UK.
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Default Re: Thorens TD 126 Mk 3 - problem with Speed Switches/lamps

Hi Paul,
First of all check the power supply voltages as often one half of the power transformer secondary or primary fails due to it being run on 240V.
I have also had quite a few problems with this T/T caused by the switch pins on the PCB, which are silver plated, getting oxidised (sulphided) and turning black.
The only solution I have found is to dismantle the PCB, unsolder and remove the pins and suspend in "Goddards Silver Clean" for about 5 mins. Then refit them with a smear of contact lube of some sort. The contacts in the push buttons may themselves need a clean with IPA or the like.

John
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Old 17th May 2018, 11:45 am   #3
sherman51
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Location: Stoke On Trent, Staffordshire, UK.
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Default Re: Thorens TD 126 Mk 3 - problem with Speed Switches/lamps

Thanks for your reply john ...
I have in fact cleaned the switch pins which as you said had blackened, but I feel the problem is not there as the switch works well enough, just not when there is a bulb in the circuit ..
Seems that with the bulbs in the "latching" part of the electronics is not working .. (that's well over my head) I don't know how that is achieved in the circuit, is that done by the chip ?
I'll have a look at the power supply as you said ... I think I can at least manage that.
Thanks again
Paul
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Old 17th May 2018, 3:05 pm   #4
julie_m
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Location: Derby, UK.
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Default Re: Thorens TD 126 Mk 3 - problem with Speed Switches/lamps

It could be a power supply problem; if there is not quite enough juice in the PSU to light a bulb and maintain the logic state, it will switch off.

Alternatively, if incorrect bulbs have been fitted which draw too much current, this would also stop it from working.

As a first quick test, does it work properly if you put the original bulbs back? And do you have, or can you borrow, a digital storage oscilloscope?
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Old 17th May 2018, 4:46 pm   #5
John Caswell
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Location: Wokingham, Berkshire, UK.
Posts: 437
Default Re: Thorens TD 126 Mk 3 - problem with Speed Switches/lamps

Hi Paul,
Then it look as though you need to turn your attention to the speed select chip (Z105 - SAS 670) as both the speed switching and lamp are controlled through this chip.
I would suggest downloading the schematic from this website's library, then checking the voltages around this chip. There should be at least ±15V available.
julie_m's idea is a good one - lamps should be 12V 0.03A (30mA).

John
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Old 17th May 2018, 5:42 pm   #6
sherman51
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Default Re: Thorens TD 126 Mk 3 - problem with Speed Switches/lamps

Thanks to both of you ... I have played around with old and new bulbs as I only needed one but purchased 6 and so have some of each to play with, and I'm convinced it's not a bulb issue ..
Thanks again, I'll post if I find anything
Best regards
Paul
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