Searching service manual of Turntable Fisher MT-6350 > Quarz Lock Problem

TasoTaso

Member
2018-10-01 10:30 pm
Hi, i have a Fisher MT-6350.
The LED indicating the perfect speed remains off. Yesterday it worked ... today no. :(
The LP now turns slightly slower both at 33 rpm and at 45 rpm despite the "quarz lock" button is on. If with the hand I accelerate the rpm a bit ... the LED turns on ... I think there is some trimmer to be adjusted.....


Both 33 and 45 rpm pitch adjustments work normally.

Can you help me?
Many thanks
 
It seems very unlikely that the unit would have gone out of alignment from one day to the next. Besides, the very point of quartz locking is (ideally) having no adjustments... perhaps a coarse VCO adjustment, but that's it.

The symptoms indicate insufficient loop gain, which may indicate a defective opamp or transistor, or possibly just a dried-out electrolytic cap in the signal path that's too far gone now. The control circuit has to be some kind of PLL that compares the output of the platter position sensors (e.g. Hall sensors) with a divided-down clock signal from a quartz reference. Pulling in but not locking can be a lot of things: Weak signal from detector, weak signal from quartz reference, insufficient gain after phase detector.
 

TasoTaso

Member
2018-10-01 10:30 pm
...and...today while I was listening to a LP.. the speed changed for 3..4 seconds ... keeping the song ... then it was back to normal. of course I'm using it without the quartz lock button on ... because when I turn on the quartz lock..the turntable goes about 31rpm ..