I'm trying to fix an old direct drive turntable and I'm stumped.

I know there are some electrical engineers on this site. Would anyone be interested in helping me diagnose the problem? I have limited experience but I want to learn.

Thanks in advance.

NOTE: I posted this in the lobby and someone suggested I post it here. Sorry about non-LEX related.

 

What is the problem? What is your turntable model?
I'm not only an audio engineer by day, but also a DJ, so turntables are my thing. I'll try to help here if I can.

 

Well, it's a Pioneer PL-400 direct drive turntable from the 80's. It sounds great, but about once a revolution it slows down briefly and then gets back up to speed. I've taken it apart and don't see anything visibly wrong.

I guess the first step is to get schematics from Pioneer. Even if I can't fix it myself I would at least like to figure out what's wrong before I have the pros take over.

Would you like me to snap some pics of the circuitboards? It doesn't look like it would be too complicated for a pro but I'm not experienced.

Thanks for your interest, it really feels good to get some help.

 

This is a very tough problem to have to fix without having the unit to test in front of you, with test equipment handy. The first thing I will ask is: Is the slow-down exactly once per revolution, in the same place every time?

The signal to the motor is normally very repetitive and continuous, as with any motor. There may be a variety of things that can be wrong, but I would initially suspect something wrong with the motor itself, i.e. winding problem or other asymmetry. The drive circuit to the motor does not vary, it is doing the same thing continuously. If you can verify that the problem occurs exactly at the same point every rev, then the problem is mechanically linked, not strictly electrical. If that point varies, then it probably IS electrical, something has fried or gone wacko (technical term).

Is the motor driven by a quartz phase-locked-loop circuit? (If you do not know offhand, you can tell by creating a bit of drag on the table - if the table counters by pulling back to correct its speed, it is quartz-locked. You can see this by watching the strobe/dots.) If so, there could be a problem with one of the sensors.

Unfortunately, taking a pic of the board and having a schematic will be of limited help with this kind of problem. Time-sensitive voltage waveforms need to be observed, and also some physical aspects of the guts. Can't tell just by looking at the parts.

Assuming your best approach is to have it serviced locally, you may want to first get an estimate, and just so you know, if you'd like to replace the turntable at that point, there are - believe it or not - more good turntable choices now than ever before, with the advent of turntablism. Technics, Vestax, Stanton, Numark, Gemini, and others are making great products at reasonable prices, with features and performance levels unseen before. And no, I'm not a salesman. Just another option.

 

Thanks. That is good advice. I found a shop in Houston with really nice people and when I am there for Thanksgiving I'll let them take a look. If it's too complicated to fix they can just sell me another used one and I'll give them mine for parts.

Since it is quartz locked like you said it might be one of the sensors. I don't see anything that looks like a sensor, though.

 

Don't know if I have something much to add to this but if the problem is exactly once every revolution, it sounds like it might be more mechanical than electrical. I think engin-ear was getting at this but as far as the electronics are concerned, the difference between one control signal and another is really nothing. Maybe there is a sensor that is not getting the correct input or just a bent shaft somewhere. Don't know if we can really do much for you but if you rotate the table by hand do you feel any drag or hiccup anywhere? Might be a very small problem, the direct drives are not known for their torque, don't need it. But if you can actually feel something, you might be able to explain better to the guys in Houston what is going on. Good luck.

 

"Guys in Houston" can do anything, can't they?? They sent a guy or two to the moon... ;-)

Ron, just FYI, the new wave of direct-drive tables (Vestax, Numark) have HUGE torque. They are meant to get up to speed extremely quickly (like in 1/8 of a rev or so) - for DJ applications.