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Sony PS-X70: Deciphering motor issue
Sony PS-X70: Deciphering motor issue
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Old 22nd May 2019, 01:32 PM   #11
Mooly is offline Mooly  United Kingdom
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Sony PS-X70: Deciphering motor issue
Sounds like it.

With pins 1 and 2 connected the opamp behaves as a voltage follower (buffer) and the output will be whatever is applied to pin 3. Its very bad design to leave input pins floating and it would have been far better if they just connected pin 3 to pin 7 so that the unwanted half just followed what the other side was doing.
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Old 28th May 2019, 06:13 PM   #12
dbxdx5 is offline dbxdx5
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Quote:
Originally Posted by Mooly View Post
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.

So we've ruled out IC1. I replaced it with a new CX193, and there's no change. Can't say I was surprised.
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Old 28th May 2019, 06:23 PM   #13
Mooly is offline Mooly  United Kingdom
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Sony PS-X70: Deciphering motor issue
It had to be tried I suppose

One area worth exploring... trace pin 2 back to the user controls for pitch adjustment. What going on around that area?

It looks as though with the pitch option set to OFF that there should be a half supply reference voltage (2.5 volts) at the junction of R5 and R6.
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Old 28th May 2019, 08:03 PM   #14
dbxdx5 is offline dbxdx5
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I have 2.8V at the junction of R5 and R6. I'm also seeing the full reference supply voltage on the one side of R4.
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Old 28th May 2019, 08:11 PM   #15
dbxdx5 is offline dbxdx5
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One related observation: I see 2.8V at pin 7 of IC3, but I should be getting 0V. Same goes for the inputs of that opamp: Both pins 5 and 6 of IC3 are showing 2.8V, which originates at pin 21 of IC1.



Also, the 2.8V between R5 and R6 is there regardless of the Pitch control being on or off.

Last edited by dbxdx5; 28th May 2019 at 08:29 PM.
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Old 29th May 2019, 07:14 AM   #16
Mooly is offline Mooly  United Kingdom
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Sony PS-X70: Deciphering motor issue
Quote:
Originally Posted by dbxdx5 View Post
I have 2.8V at the junction of R5 and R6. I'm also seeing the full reference supply voltage on the one side of R4.
That sounds OK.

The static DC voltages at the output of IC3 should follow what you see on pin 21, however IC3 is configured as a filter/integrator meaning it provides an average DC output based on the duty cycle of the squarewave on pin 21, which I'm assuming isn't present and its just a static DC voltage you are seeing there.

Also there are three voltages marked which don't seem to agree with each other.

I think you have to concentrate on why the sawtooth oscillator (pin 1) isn't running and whether that oscillator should run all the time or whether its inhibited by the logic in some way.
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Old 29th May 2019, 11:35 AM   #17
dbxdx5 is offline dbxdx5
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Originally Posted by Mooly View Post
That sounds OK.

The static DC voltages at the output of IC3 should follow what you see on pin 21, however IC3 is configured as a filter/integrator meaning it provides an average DC output based on the duty cycle of the squarewave on pin 21, which I'm assuming isn't present and its just a static DC voltage you are seeing there.

Also there are three voltages marked which don't seem to agree with each other.

I think you have to concentrate on why the sawtooth oscillator (pin 1) isn't running and whether that oscillator should run all the time or whether its inhibited by the logic in some way.
If I recall correctly, I saw a sine wave on 21 instead of the expected square wave.

Also, fwiw I rechecked the waveform on pin 8 while spinning the platter, and it's a less than square square wave.

Click the image to open in full size.

I'll focus on pin 1 as you suggest. Thanks.
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Old 29th May 2019, 11:48 AM   #18
Mooly is offline Mooly  United Kingdom
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Sony PS-X70: Deciphering motor issue
Pin 8 is fine. Its just a feedback signal of frequency related to the platter speed.

I've no bright ideas on the oscillator tbh other than to cling to the theory it should either be running all the time or at least get enabled as the start/stop flip flop internal to the chip does its thing.
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Old 29th May 2019, 02:02 PM   #19
Mooly is offline Mooly  United Kingdom
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Sony PS-X70: Deciphering motor issue
Another thought on all this with it being such an involved and complex design would be to work from the block diagram at the beginning of the manual.

We've check pretty much everything around that chip and not really come up with anything.

Is it possible that some mechanical issue (all the switches) is somehow inhibiting operation. Look for clues. Do the 33/45 rpm lights change correctly, does the repeat LED respond to button presses and so on. There seems to be an optical auto return detector using a bulb and photocell. What about all the cam switches (lower right on the block diagram). There is a start/stop LED. Does that respond. Does the pitch LED light in response to the switch. Up/down button ? Is that arm lift.

I'm just wondering if some input is wrong somewhere and somehow inhibiting operation. Look for clues anywhere you can.
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Old 29th May 2019, 04:04 PM   #20
dbxdx5 is offline dbxdx5
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Good thought. All of the control LEDs light up as they should, the arm raises and lowers when Up and Down are pressed, and pressing Auto Start will cause the arm to swing out over the platter and lower (Stop will cause it to go back to the rest).



The only anomaly (if it is one) is that the neon light for the XTAL/speed window does not light up, even if I rotate the platter by hand. According to the SM, I should have 187V and 85V at the Red and White wire pins respectively that go to the neon bulb board. I measure 201V and 88V. There's also an 8.2K resistor at R411, rather than the 12K specified, though it appears to be original.
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