Pot giving me a headache

[dav305z]

No that kind of pot. I mean the potentiometer on my turntable.

I have a Marantz 6170 that is having speed stability issues. I replaced the pot, and it worked...for a while. After some time, it reverts back to losing speed. Several times, I've opened it up again and resoldered it. That too, produces a temporary fix, but never permanent.

So what's the deal here? Am I just a terrible solderer or is there a more serious underlying issue with the electronics?

[Gerard]

Hi Dav,
I don't own a Marantz 6170, so I can't say exactly what's the problem. However I do own a 30 year old Sansui TT. And my experience with that problem lies with not only the pot, but leaky capacitors. Seems the caps were too old and grown "weak". Having replaced all the caps, changed the pot, and don't forget to oil the spindle shaft, the speed's stable.
Rgds,
Gerard

[dav305z]

Not to be really dumb, but what are the capacitators, and where are they usually found in the tt?

[Gerard]

Hi Dav,
How old is your TT, and is it belt driven ? Assuming it's belt driven, can you check out the motor to determine if it's a d.c. motor. Seldom have I seen a.c. motor in a TT with bullt in inverter for speed control. Assuming it's a d.c. motor, trace the wires back to the pcb. This is usually a variable d.c. voltage regulator with a pot so that the speed can be adjusted with reference to a strobe. d.c. voltage regulator circuits have capacitors to stabilise the output voltage, that the load, in this the d.c. motor, may run very constantly.

Capacitors come in various shapes and sizes. Many types as well. Electrolytic capacitors are the subject of our observation here. Usually packaged in round metal cans. Rubycon, Elna, Panasonic, Nichicon are the more commonly brands used. On the skin, the values are printed, eg; 470 uF, 35V, 85C. This means the storage capacity is 470 micro Farad, applied working voltage should never exceed 35 Volts d.c., working ambient temp. max. 85C. It is best to measure the capacitors out of circuit. A simple analog multimeter can be used to check charge and hold of that capacitor. If that capacitor leaks, it won't hold any charge. Alternatively, a more accurate method is to use a capacitance meter and note if the displayed value on the capacitance meter matches the printed value on that capacitor.

A faster method of trouble shooting, is to run a "live" check. This is also dangerous as high a.c. voltages from the mains is present as this method requires the TT to be switched on. Measure the d.c. voltage upon initial power up, and compare this reading later on, to, when your speed has slowed down. If there's a drop in voltage, it can be due to either the p.c.b. or the motor. My personal experience with a d.c. motor which had a hidden p.c.b. incorporated into the housing was the cause of my tape deck exhibiting the same symptoms as your TT. To confirm the condition of the motor, I used a separate d.c. source to power up the motor. The symptoms appeared. It's the motor.

To confirm the p.c.b. you need a known reliable load. This is not so straight forward, as you need to calculate the rating / power consumption of your motor, and then finding a suitable load resistor to load up the p.c.b. Then use the multimeter to measure the voltage across the load resistor, first upon initial power up, then later on, and compare the readings. If the voltage drops, then it's the p.c.b. Depending on how complex your p.c.b. is, nevertheless, if you are willing to spend time on it, I'm willing to help you out components wise. You might get lucky and get your TT running again.

Rgds,
Gerard