Technics Sp10 Woes

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Wolfgang Felber
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Re: Technics Sp10 Woes

Post by Wolfgang Felber » Sun Aug 04, 2019 2:47 am

Hi Nick, finally worked out how to measure the timebase, I have attached some photos, hopefully they are ok.
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DSCN5027.JPG
DSCN5026.JPG
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Nick
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Re: Technics Sp10 Woes

Post by Nick » Sun Aug 04, 2019 10:53 am

I don't have the service manual numbers to hand, but they look fine.
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Re: Technics Sp10 Woes

Post by Wolfgang Felber » Sun Aug 04, 2019 11:32 am

Hi Nick, now that I have got to this point, can you please explain the significance of the time base and the difference between the Sawtooth Wave form and the impulse wave form.

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Re: Technics Sp10 Woes

Post by Nick » Sun Aug 04, 2019 7:07 pm

Well (and I am not sure if this is your question), but its using the sample and hold to provide the phase lock signal. The saw tooth is locked to the frequency the platter should be rotating at, and the pulse is driven from the signal from the actual platter rotation. The phase error signal is generated by the point the sawtooth is at at the point the impulse happens, that level is held until the next pulse. So if the platter is phase locked with the ideal speed, the pulse will happen at the same point in the ramp each time and the output from the sample and hold will be a constant voltage as the ramp is at the same point each time. If the platter is turning faster than it should, the pulse will be at a lower point of the ramp as it will have happened sooner, so the output will be a smaller signal, growing ever smaller as the pulse creeps down the ramp. The phase signal is added the output voltage, so will reduce the speed of the platter until the pulse happens at the same point in each ramp. Conversely If the platen is going slower than required the pulse will happen later, so will cross the ramp at a higher point, so the error voltage will be higher, that will be fed to the drive circuit increasing the speed of the platter until its back locked.

Its a cunning control system that many of the direct drive systems uses. The rotation is speed controlled to get close to the right speed, then a phase lock is added to provide the final control. I do exactly the same thing in the turntable that I brought to Owston, but in that case its all done digitally, but the action is the same.
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Re: Technics Sp10 Woes

Post by Wolfgang Felber » Mon Aug 05, 2019 1:18 am

Hi Nick, absolutely my question, if I have learnt anything from this two year exercise, is you are never too old to learn. believe it not, your explanation does make sense, I have done a bit of research on Sample & Hold circuits to find out what it is all about. My next project is to make a plinth for the two SP10's I have. I have glued a number of sheets of birch ply together for the plinth. My other SP10 is running a Jelco 750E 10 inch arm with a Stanton 881s moving magnet and a kit Tim Paranvinci valve phono stage going to a full Accuphase system and Revel Salon speakers. As you can see, there is a real passion.

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Re: Technics Sp10 Woes

Post by Wolfgang Felber » Mon Aug 05, 2019 11:19 pm

Hi Nick, I tested your explanation last night, if you put a little pressure on the platter to slow it down, you notice the impulse moves to the right and increases in intensity. Once the platter is back at the required speed the impulse moves back to its original position and the intensity drops back. Very ingenious. If you consider the age of the turntable, this is pretty awesome technology.

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Re: Technics Sp10 Woes

Post by IslandPink » Mon Aug 05, 2019 11:20 pm

'The joy of finding things out' as Feynman was so enthusiastic about :)
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Re: Technics Sp10 Woes

Post by Nick » Mon Aug 05, 2019 11:35 pm

Yep, as I found out with that turntable I worked on, the phase lock is the key to the performance. Simple rotation speed control will get you close, but (for example) cant deal with a bearing that’s tight for half a revolution but loose for the other. The phase lock solves that problem. The only thing that doing it digitally I could do that the sp10 didn’t was instead of just having proportional control on the feedback loop (both the velocity and phase parts) was to add an integral term, so the thing adjusts itself in use to do what you have to do manually setting up the turntable with the tuning pots. I did play with making it "learn" the bearing friction and adjusting the control signal before it was needed in a sort of feed forward system. But I realised that without any absolute measurement of the turntable speed I didn't know if I was learning the bearing drag or just the position sensor error.
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Re: Technics Sp10 Woes

Post by Nick » Mon Aug 05, 2019 11:40 pm

If you consider the age of the turntable, this is pretty awesome technology
Though what its built on goes a few more years back

https://en.wikipedia.org/wiki/Control_theory
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Re: Technics Sp10 Woes

Post by Mike H » Sat Aug 10, 2019 10:16 pm

IslandPink wrote:
Mon Aug 05, 2019 11:20 pm
'The joy of finding things out' as Feynman was so enthusiastic about :)
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Re: Technics Sp10 Woes

Post by Wolfgang Felber » Thu Aug 29, 2019 12:36 pm

Hi Nick , been off line for a while, making up two plinths for the SP10's. I have a question, I noticed that the neon flickers. Any ideas what is casing that. When I first started with the, getting it up and running, I noticed the two transistors that drive the neon used to get really hot. Do you think they may have reached the end of the effective life?

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Re: Technics Sp10 Woes

Post by Nick » Thu Aug 29, 2019 4:08 pm

Well, with mine, the neon would go out all together some times. I think partly due to age of the lamp, and maybe the 140v (or whatever it was) voltage dropping as caps age. I eventually removed the neon altogether and replaced it with a pair of blue LED's driven by the logic signal and not using the 140v line at all.

Just gone and checked and corrected the HV line voltage.
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Re: Technics Sp10 Woes

Post by Wolfgang Felber » Fri Aug 30, 2019 12:23 pm

Hi Nick, any chance that you have a photo of how to connect the LED to the logic. Will the LED flash at the correct frequency, so that the squares on the platter don't move.

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Wolfgang

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Re: Technics Sp10 Woes

Post by simon » Fri Aug 30, 2019 1:31 pm

Not wishing to derail Wolfgang's thread, with the scarcity of AC filament bulbs in favour of low energy LEDs etc. I have wondered about using an LED possibly with a small Arduino to control it. I haven't thought about it in any detail but I reckon it should be doable, but I'd be interested in how you did it too Nick.

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Re: Technics Sp10 Woes

Post by Nick » Fri Aug 30, 2019 2:22 pm

Something like

1. Take F3 out in the power supply to disconnect the 140v supply
2. Remove neon from strobe housing
3. Connect led in series with 330R (or so) resistor between 0v and IC15 Pin 3 on the logic board.
4. Position the LED where the neon bulb was

Or something like that, it was some time ago, You have 16ma to play with for the gate, so if you want to use more than one LED, or different colours, pick the resistor to suit. You can experiment with the led between 5v and the output and between 0v and the output and see which gives the better illumination and sharper edges, as the waveform at that point is not 50% M/S ratio.
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