Thorens TD104 runs fast

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This turntable runs too fast in 33 or 45 and the speed control pots have no effect. The motor has an integral tacho-generator which is supposed to regulate the speed thru an op-amp/BJT control circuit. The two wires from the tacho-generator connect to the +/- inputs of the first op-amp stage and it's output should be a 9Vp-p square wave - I'm seeing no output at all on a scope. All control pcb test points look OK except the motor voltage which is about 2X What it should be (7VDC vs. 3+VDC).

Does anyone know if the tacho-generator's wires should have a measurable resistance between them? I'm getting infinite resistance when disconnected from the control circuit.
 
I don't own a TD104 but offering my $0.02

From the schematic, the tachometer looks like a pickup coil and it should have continuity between it's leads, and no reading to ground.
I'd put my multimeter on continuity (beep) and wiggle a few things to find out where the open circuit is. Otherwise, take it apart and do surgery.

I've seen tachometer coils open-circuit when the potting compound cracks and breaks the fine magnet-wire. Or the leads were yanked or bad solder joints because of magnet-wire varnish. Let us know what you find.
 
Thanks for responding. I was also thinking it was probably is a broken wire but I wasn't sure it should be continuous between it's leads , and I have been reluctant to attempt to disassemble the motor for fear of permanently destroying it (as if it's not useless as-is). BTW What schematic are you looking at that shows a coil ? I only have the TD104/105 service manual which shows a circle with a "G" in it to represent the tach. The potting compound around each tach terminal on the back of the motor is cracked as you suggest, Since I don't know which side could be open I've been afraid to dig around them. I will try the wiggle test and let you know.
 
Hall effect or optical tachs are 3-wire. The service manual mentions 100mV AC output from the sensor and the op-amp circuit is a typical variable-reluctance pre-amp, so I infer a coil is what's used.

I would guess the tach wire break is in the potting compound cracks. For that work, I've used a heat gun to soften potting compound and peel it off. Otherwise you need a solvent to dissolve it that won't attack the mag-wire varnish. Or just rip it apart and rewind it from scratch.

The same motor is used on TD 104/105/110/115/126III. Might be for sale here: Original Thorens Motor TD 104/105/110/115/126III Ausverkauf - die nadel - Ihr Onlineshop fuer HiFi-Zubehoer und mehr
 
It appears the motor terminals are not held by potting compound but they're embedded in molded plastic. I guess I might try to disassemble the motor housing without damaging the motor/tach assy further or maybe replace the motor/tach if it's available at reasonable cost.

Thanks again prairie for your knowledge & help.
 
Hi folks, apologies for reviving a dead topic but i have the exact same problem as the OP, but unfortunately none of the technical smarts that any of you seem to display. Any chance any of you could let me know where to start with locating the problem and then fixing it in simple terms? Thanks in anticipation.
Dand
 
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