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Sony PS-X70: Deciphering motor issue
Sony PS-X70: Deciphering motor issue
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Old 15th May 2019, 02:30 AM   #1
dbxdx5 is offline dbxdx5
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Default Sony PS-X70: Deciphering motor issue

I have a Sony PS-X70 on my bench that isn't doing the one thing every turntable needs to do -- turn! When either Start or Auto Start are pressed, there is no movement of the platter. Nada.



After some work on the power supply board, all of the PS voltages to the main board are correct. Working backwards from the motor, it appears that the issue starts at IC3, prior to the gain adjustment for the Hall Sensors and the motor drive circuit. I should be getting around 1.8V at pin 1, but instead it shows zero. When I bypass IC3 and apply 1.5V DC from an external source, the platter turns. I'm also seeing zero at Pin 3 and at the emitter of Q5. Given these measurements, I'm trying to understand how the circuit is supposed to function to send the correct voltages to pins 2 and 3 of IC3. Is it via Q2, Q3, Q4, and Q5? Or through IC1?

Schematic — imgbb.com



Click the image to open in full size.

Last edited by dbxdx5; 15th May 2019 at 02:33 AM.
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Old 20th May 2019, 07:34 PM   #2
Mooly is offline Mooly  United Kingdom
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Sony PS-X70: Deciphering motor issue
Do you have the sawtooth waveform on pin 1 of the CX193?

Do you see a change in logic level on pin 7 as you operate the start/stop button?

Do you have a very high frequency clock signal on pin 16. This should be running at the frequency of the crystal nearby. It doesn't say what that is in the parts list. Usually if its running its going to be OK though.
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Old 21st May 2019, 12:01 AM   #3
dbxdx5 is offline dbxdx5
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Actually, I don't have a sawtooth waveform on pin 1 of the CX193. This is what I'm seeing:


Click the image to open in full size.


The signal at pin 16 looks more like what's shown in the schematic:


Click the image to open in full size.


For pin 7, the DC voltage changes from 2.0V to a lower value every time I press Start/Stop.



And there's a square wave at pin 8 of the correct peak-to-peak voltage when I turn the platter by hand.
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Old 21st May 2019, 07:01 AM   #4
Mooly is offline Mooly  United Kingdom
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Sony PS-X70: Deciphering motor issue
Pin 16 looks OK, not so pin 1. The big question is whether the sawtooth should be present all the time or whether it is present only in play mode. In other words is something inhibiting the IC or is it a real issue around there.

Might be worth checking C6 by substitution and also making sure that the resistor chain and presets are OK. No need to measure anything for those, just dab a 470k from C6 to the 5 volt rail and see if anything changes.
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Old 21st May 2019, 07:11 PM   #5
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The service manual lists C6 as "polyethylene" which I found odd. I had a .01 uf polyester film cap in hand, so I used that. No change in the signal. Nor did it change when I touched a 470k resistor between C6t and the 5V rail. Now for what it's worth, the image I got for pin 1 yesterday was not when using my scope probe, but rather a BNC terminated length of R58 A/U that has mini grabber clips on the test end. When I used my probe today, this is what I see (best picture I can get of it, since it's always moving).



Click the image to open in full size.
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Old 21st May 2019, 07:31 PM   #6
Mooly is offline Mooly  United Kingdom
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Sony PS-X70: Deciphering motor issue
What you are seeing on pin 1 looks more like internal leakage of some high frequency signal... assuming it is high frequency, can't tell without the time-base setting on the scope... but its obviously not what it should be.

I'm so reluctant to condemn the chip because usually faults often tend to be anything but 'the big IC' and yet at this stage I suppose you have to try it if only to rule it out.

When you have been around the chip looking at all the inputs to it... well then its time to consider.

What would I do... probably pull Q3 out and see if that changes anything. Is it doing something weird with the on/off flip flop input? Pull it and see.

There isn't much else I can suggest at this point tbh.
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Old 21st May 2019, 08:43 PM   #7
dbxdx5 is offline dbxdx5
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I hear you about the CX193. A few days ago I was ready to swap it out, but something keeps nagging at me that there's a different culprit at play.

Regarding Q3, I replaced it with a new one the other day, so probably not worth the test of pulling it I'm guessing.


Thanks for the help. I'll order a new CX193 and report back.

Last edited by dbxdx5; 21st May 2019 at 08:47 PM.
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Old 21st May 2019, 09:41 PM   #8
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One addendum. I just checked the voltages on IC2. The first numbers are what the schematic specifies, followed by what I measured:


1: 0V (+14V)
2: 0V (+14V)
3: 0V (+0.80V)
4: -14.5V (-14.64V)
5: 0V Play (0V)
6: 0V Play (0V)
7: 0V DC / 26V AC p-p (-0.30)
8: +14.5V (+14.55V)
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Old 22nd May 2019, 06:51 AM   #9
Mooly is offline Mooly  United Kingdom
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Sony PS-X70: Deciphering motor issue
IC2 is a dual opamp and is shown with only one half in use.

Its also shown with pin 3 floating which would be really bad design if true. In any case, the readings on those three pins could be anything depending on what pin 3 is connected to.

Its worth checking for curiosity. With pin 1 and 2 connected these two pins should follow the voltage on the input (pin 3). The board layout seems to show pin 3 connected to the top pin of a group of three.

If that connection is floating then that is why the readings are as they are (and that is bad design), your meter is reading a floating pin 3 and when the meter isn't connected pins 1 and 2 just reflect whatever pin 3 has floated to.

Its probably all normal and in any case you have checked the important FG signal from the other half of the opamp.
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Old 22nd May 2019, 12:13 PM   #10
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Interesting. Pin 3 of IC2 goes to an unused board pin that's next to the two for the FG signal. So I assume this and my voltage readings on pins 1 and 2 means it is in fact floating?
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