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Yamaha PX-3 troubles, help requested

T

therocket290

Member
Short version: The tonearm will only move to the right (away from center of platter). I know it is not a slipping belt, because the motor is not moving.

Longer version: I did have a problem with a slipping belt on my PX-3. The tonearm needed a little help to get moving at the beginning of play, but would move once I gave it a little push. The problem was that "indentations" in each side of the tonearm belt had formed, which were probably caused from sitting unused for so long. I swapped sides of the belt and flipped it over and my PX-3 was back up and running. It seemed to work fine for a couple of months until a new problem came up. The tonearm would stop moving partway through playing a side of the record. My first guess was that the tonearm was getting stuck at the indentations which were now in a different location relative to the tonearm. But I was incorrect. The record would play for a bit and then the tonearm would stop moving to the left. The angle of the tonearm would increase and the tonearm motor was definitely not spinning. Sometimes the angle of the tonearm would increase and then the tonearm would move a bunch to catch up, but usually it would just start to skip. I experimented a bit rotating the tonearm back and forth and it moved to the right (away from center) when I rotated the tonearm counterclockwise, but the tonearm motor does nothing when I rotate the tonearm clockwise.

After some internet research, I decided to take it back apart and test some voltages. To my disappointment, the tonearm motor will now never spin to make the tonearm move to the left. If I push a play button, the light turns on, but the tonearm motor does not spin. If I push the tonearm toward the center, the platter spins. Moreover, the "move right" button works (both slow and fast). The voltage at the test pin is 1.2v at "move right slow" and 3v at "move right fast." Also, if I rotate the tonearm counterclockwise, it still will move to the right. But the "move left" button does not work at all (in terms of activating the motor or producing any voltage at the test pin). Also, when I rotate the tonearm clockwise nothing happens at the test pin or the motor. I tested all of the voltages that are marked on the circuit board and they all check out ok.

To summarize, the tonearm will motor will not move to push the tonearm to the left ever (whether trying to do it via "move left" button, either play button, or by rotating the tonearm when in the middle of the playing position. But the tonearm will move to the right via the "move right" button, the cut button, and by rotating the tonearm counterclockwise when playing.

I really like this PX-3 when it is working, but I am currently stuck. Anyone have any suggestions for something to check/try? I would really appreciate it.
 
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Besides the large, flat belt that the tonearm is moved by, there are 3 smaller belts. Have you checked that all 3 of these are in place and not stretched?

I'm not sure what you mean by "rotate the tonearm clockwise/counterclockwise?" Are you grabbing the headshell and then rotating it? Is that what you mean? Or, are you grabbing the tonearm and moving it left (clockwise) and right (counterclockwise)?

The base of the tonearm is intended to track with the arm, moving ever so slowly inward toward the spindle. This allows the stylus to remain in a preferred position in relation to the groove it is in. If the tonearm base stops moving, the tonearm will still try to track inward but will be halted by the stationary base and skipping (or remaining in place) will follow. I assume you have cleaned the rail that the base tracks on and the associated plastic wheels?

I'm not sure how much experience you have with this table and don't mean to insult your knowledge thereof. It can be very tricky. There are also some photo sensors that can get wonky and cause problems. You might do a search and see what you can come up with.....likely just do a search for PX-3 and you'll see names of others who have had issues as well as those who have knowledge that can help.
 
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Fishead said:
Besides the large, flat belt that the tonearm rides on, there are 3 smaller belts. Have you checked that all 3 of these are in place and not stretched?

I'm not sure what you mean by "rotate the tonearm clockwise/counterclockwise?" Are you grabbing the headshell and then rotating it? Is that what you mean? Or, are you grabbing the tonearm and moving it left (clockwise) and right (counterclockwise)?
Click to expand...

I replaced the three small belts when I had it apart last time. I had the tonearm assembly removed and the motor was definitely not spinning. The motor is not turning on at all to move the tonearm to the left.

By rotating the tonearm, I mean pushing the headshell left (for clockwise rotation) or right (for counterclockwise rotation). The tonearm motor is supposed to spin when the angle of the tonearm is larger than some threshold in order to reduce the angle.
 
therocket290 said:
The tonearm motor is supposed to spin when the angle of the tonearm is larger than some threshold in order to reduce the angle.
Click to expand...

Yep, that's what I thought and you are correct in this. Might be worth just hitting those sensors with some compressed air but I rather think it is another issue. I had a little sluggishness not long ago with my "30" button so I took it apart and removed enough stuff to get deoxit into it. Did all the rest while I had it apart and worked them repeatedly to help get rid of the oxidation.....no issues since.

But, help is on its way.....there are a couple other nice folks here that helped me out in the past that, when they see "PX-3" will likely be chiming in shortly (pfcs49 for one).
 
Fishead said:
Yep, that's what I thought and you are correct in this. Might be worth just hitting those sensors with some compressed air but I rather think it is another issue. I had a little sluggishness not long ago with my "30" button so I took it apart and removed enough stuff to get deoxit into it. Did all the rest while I had it apart and worked them repeatedly to help get rid of the oxidation.....no issues since.

But, help is on its way.....there are a couple other nice folks here that helped me out in the past that, when they see "PX-3" will likely be chiming in shortly (pfcs49 for one).
Click to expand...
Thanks for the reply. I couldn't tell where the sensors for tonearm angle are, but I might try to hit them with compressed air once I take it back apart. I'll wait to see if I get some other suggestions for things to check.
 
I would put money on it being something in the motor drive circuit. It sounds like something that sets the motor moving in the ‘left’ direction may have failed.

I’d need to load up the circuit diagram to try and see what it might be but this won’t happen for a bit I’m afraid!
 
Check TR8 & TR9 (2SC2320). Is DC-balance set to 0 ± 20mV? Adjust with VR1. Did you get the service manual?
 
Dr. Ear said:
Check TR8 & TR9 (2SC2320). Is DC-balance set to 0 ± 20mV? Adjust with VR1. Did you get the service manual?
Click to expand...
Thank you for the response.

Those transistors checked out ok.

But I do have some more information after checking it out this morning. When I disconnect the power from the tonearm motor, I get voltage at the test pin when I press 30, press the move left button, or rotate the tonearm clockwise (which I do not get when the power is connected to the tonearm motor).

The following readings are with the tonearm motor power disconnected:
  • When I press 30 to play, the voltage is -2.2V until I move the tonearm to the first photo interrupter. Then it is at 0.01-0.02V.
  • Clockwise rotation: -2.12V (in up or down position)
  • Counterclockwise rotation: 3.51V ( in up or down position)
  • Cut: 3.2V until I move tonearm back and it depresses the limit “switch”
  • MR slow: 1.25V
  • MR fast: 3.28V
  • ML slow: -1.3V
  • ML fast: -2.2V
When the tonearm motor power is connected, I get voltage for the stuff that would make the tonearm move right, but nothing that would make the tonearm move to the left (so none of the negative voltages shown above). Anyway, my knowledge of electronics is pretty limited (if you haven’t already figured that out) and I thought this might indicate a particular place to check.

Also, there are spots on the circuit board marked ML fast, ML slow, MR fast, MR slow. Those all show around 12V when the appropriate button is pushed. And this is true with the power to the tonearm connected and disconnected.
 
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Sorry to post another comment so quickly, but here is a summary of where I am at:

If I connect the leads of my multimeter directly to where the tonearm motor connects to the main circuit board (so the motor itself is not connected), then I get what I suspect are the correct voltages for both directions (shown in my previous comment). This is true for pushing any of the appropriate buttons or rotating the tonearm in either direction. The positive voltages would move the tonearm to the right and the negative voltages would move the tonearm to the left. Now when I connect the tonearm motor to the circuit board, I only get the positive voltages (i.e., only for actions which would move the tonearm to the right) and nothing that would make the tonearm move to the left produces voltage at the test pin (which appears to be connected to the tonearm motor positive terminal).

However I do get 12V at the spots on the circuit board marked ML slow, ML fast, MR slow, MR fast when the appropriate button is pushed (this is true with tonearm motor connected and disconnected).
 
Very weird. Did you try to remove the belt between the motor and the transmission? Just to check if the motor, without load reacts to any „left“ signal at least a little bit? You could also connect a 12V lamp instead of the motor.
 
If you get the voltages without the motor connected it could be a mechanical issue with the worm gear. Are you able to rotate the motor pulley by hand in both directions?
 
I'll try connecting a lamp and see what happens. I'll also try removing a belt.
I don't think it is a mechanical issue because if I switch the leads to the motor when the voltage is positive, the motor turns. Also, the motor turns easily in both directions by hand (with the belts connected).
It will show negative voltage (around -2.2V) when the motor is disconnected and the tonearm is rotated clockwise. Once I plug in the motor (keeping the multimeter connected), the voltage goes from around -2.2V to 0V. If I rotate the tonearm in the other direction, I get around +3.2V and it remains there after I connect the motor.
 
I just tested a bunch of the diodes and diode D12 tested bad. It is a W03B diode and it looks like I can use a 1N4003 diode as a replacement. I will order one and report back after I replace it.
 
Hey rocket, sounds like you're making some progress and good work! I'm a little surprised not more folks have responded.....there are quite a few here that own the PX-3 (or 2) so I would have thought more had chimed in by now but hopefully they will. On the other hand, maybe you have found your issue and a replacement for that diode fixes the problem! Those are great tables, but they also have a lot of electronics on board which, over 40 years, can experience failures.
 
Would like an explanation why, according to one of your previous posts, the voltages are within specs with the motor disconnected
 
Dr. Ear said:
Would like an explanation why, according to one of your previous posts, the voltages are within specs with the motor disconnected
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I would also like an explanation and I am not sure if replacing the diode will solve the problem. I still do not understand what is going on. The only lamp I had available was an led. I did not have the motor connected. When my multimeter showed positive voltage, the led turned on (with the appropriate lead on the positive terminal). When my multimeter showed negative voltage, the led would not light up no matter how I connected the leads.

I do not understand why my multimeter reports -2.2V and once I connect the motor, the voltage goes to 0V. Again, if the voltage is positive, then the motor will spin in both directions (if I switch the leads).

The only reason I checked the diodes was that I discovered the diode testing function on my multimeter. The replacement diode is already on its way from Radio Shack. If replacing that does not work, then hopefully someone else will chime in with some ideas.
 
Did you carefully check alle the solder joints with a good light source and a magnifying glass? I never had a problem with a broken diode in all the vintage stuff I came across. Especially not in devices running cool like turntables.
 
Dr. Ear said:
Did you carefully check alle the solder joints with a good light source and a magnifying glass? I never had a problem with a broken diode in all the vintage stuff I came across. Especially not in devices running cool like turntables.
Click to expand...

I will carefully inspect the solder joints when I get it back out. I think I will wait for the diode to arrive and remove the circuit board to get a close look. Thanks for your responses. In the meantime, I have an ST-70 driver board replacement to keep me busy.
 
Here are my hunches:
You should do the two servo bias adjustments. You may find that the voltages with the motor disconnected get closer to one another but that the L drive voltage goes away with the motor connected.
My guess would be that the left drive circuit has high resistance and can't deliver hardly any current when loaded with the motor (which is a simple DC motor and resistance would be the same no matter what the polarity)
Since the L & R is a matter of polarity, the diode sounds very juicy!
You might find it informative to check the voltages before and after the diode (AC & DC) while asking of L & R tracking. If the input side stays up and the output side flatlines with a loaded command, there ya' go!
(if it flatlines on both ends of the diode, then it's obviously caused upstream of the diode, but since the diode flags as failed, it sounds like a bullseye.
 
Thanks, Phil! That was what I was hoping for as I know you have been around the block on these things. I figured it was just a matter of time before you chimed in.....let's vote for the diode rocket!!!
 
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