A Switched Loop Gain Option

[Wakefield Turntables]

Oh fuck another tweek I don't Think I'll be trying this in a hurry after my last experiences with pcb modifications

Good luck guys, can't wait to see what your experiments yield.

[Mike_New]

Hi Karl,
The block schematic you show does not agree with mine.
to the right of your diagram I have two more resistors the junction of which go to
pin 6 of the AN6675. not as you show it. Maybe there is more that one set of schematics. which would not be surprising given the time span over which theses circuits have been in production.
I have to agree with you, the circuit must be understood before we mess with the values, that is why I have solicited peoples help on this.

[StanleyB]

Adjusting the loop gain is one thing. But this can in turn cause feedback oscillation. One of the Technics linear tracking turntable had such a fault which resulted in a set of components eventually going up in flames literally.

Another example of tampering about with the speed locking circuit and buggering up the response is the Micro Seiki DQX-1000. It is no where as good as the unadulterated DDX-1000.

A better and saver idea would be to use a temperature controlled oscillator, but that sort of solution is mainly reserved for DX/TX equipment where even 1Hz at a 50MHz operating frequency can be critical.

[OneyedK]

The block schematic you show does not agree with mine. to the right of your diagram I have two more resistors the junction of which go to pin 6 of the AN6675. not as you show it. Maybe there is more that one set of schematics.

Yup, that's the faulty block diagram of the 1200mkII limited.
The schematic however is correct, a few values are strange and I very much doubt that they were ever used.
I've found the following service manual to correspond with all 1200mkII's I've seen until now.
http://www.vinylengine.com/ve_downlo...10mk2_supp.pdf
(sorry for the off-forum link)

Adjusting the loop gain is one thing. But this can in turn cause feedback oscillation. One of the Technics linear tracking turntable had such a fault which resulted in a set of components eventually going up in flames literally.

No problem in the case of the 1200mkII.
As long as the values stay reasonable, there will be no fire
Have a look at the schematic I've drawn and you'll see what I mean.

A better and saver idea would be to use a temperature controlled oscillator, but that sort of solution is mainly reserved for DX/TX equipment where even 1Hz at a 50MHz operating frequency can be critical.

That would be a great option.
Alas, the motor driver is DC controlled, not frequency
One could design an entire motor drive, but that's beyond my capabilities.

[OneyedK]

With component values as found in actual 1200mkII's.

[Mike_New]

Hi Karl,
I'm glad I asked you to join us,
Yea you have got about what I have except I could not
find a place for the sixth resistor shown on my block schematic. which appears to go from the junction of C210 and R208 to the output of the OP amp. at the point where C211 and R206 connect to the output. There are only 5 resistors shown in the circuit diagram, so I guess the block diagram is where things are wrong.

Your OP amp schematic clearly shows what is happening so now we only need to set up some RC conbinations.

The feedback resistor R207 could also have an affect on the response.
although all my information shows changes only to R209, R208 and C210.

In order to arrive at a set of possible and meaningful values over a range of responses, do you think it would be possible to take leads from the connect points on the resistors and OP amp to a tempory circuit board outside the SL1200. The possiblity of oscillation due to the wiring is what concerns me.

[OneyedK]

From what I've tried until now, my experience is that you better not change
R209, R207, R208, R210.
They control the startup voltage of the deck.
Mess with those values and your filter starts "looking" for the right voltage (and speed).

C211 is there with the same purpose, prevent the output from added (superposed) AC voltages.
You can compare this part with an LC loaded rectifier, where a "small" capacitor is placed on the input.

Now that leaves us with R209, lower more gain, higher, less gain.
Maybe a 250k pot in series with a 56k resistor (we don't want crazy high gain).
Wiring could be a problem, since the phase signal is AC, but still not HF.
So I'm guessing you'd be ok.
C210 should be adapted too, lower R209, larger C210.

My question is, wouldn't it be wiser to mess with C212?
A larger cap would not change the initial DC value (R206, R207, R208 keep it in check)
But the timing constant (R206, C212) would change, as would the rise and fall times of the output.
If I'm correct, T=60ms now, while I think the likes of 200ms would be ok (around 10µF).
Haven't tried it yet.

Another thing is how we think a heavier platter should be driven.
Do we want stronger "pushes" or gentler pushes.

[Marco]

Oh fuck another tweek I don't Think I'll be trying this in a hurry after my last experiences with pcb modifications

Good luck guys, can't wait to see what your experiments yield.

+1.

Much as I applaud Mike's endeavours to realise even more potential from the Techy, this is a step too far for me. Until I see/hear irrefutable evidence that such modifications have been totally perfected, are 100% safe, and above all, worth doing, sonically, I'll be steering well clear!

Marco.

[Mike_New]

Hi Marco,
I must concede that this mod does require some knowledge of electronics. However it is somewhat less "radical" than the voltage mods which does involve some heavy playing with the voltage distribution.
The proposed switching only involves a relatively passive allbeit important part of the circuitry and should not caused PCBs to be damaged.
In fact if the idea comes to anything the local electronics service people should be able to implement the mods at only a modest charge.