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GyroDec blog

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    Posted: 13 Sep 2015 at 9:43pm
I've picked up my GyroDec exploration from another thread. Mine was bought recently and is essentially a mark III with a Papst AC motor and GyroPower psu/filter from the mid-nineties. It had been retro-fitted with the later suspension turrets and springs but still had the armboard attached with long bolts and aluminium spacers.
Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo UL, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links. and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 13 Sep 2015 at 9:50pm
I found the AC motor cylinder wouldn't fit in the middle of the chassis hole despite my wiggling the unit about it's fixed point... The motor unit sits above a round hole in the plinth (it's a full GyroDec not the SE) fixed in place with one bolt and rests on a pole on the other side. I suspect the original owner had the springs extended so the chassis was riding very low on the cotton pads and so below the motor unit.

This was issue was fixed by drilling a new hole (M5) about 1.5mm nearer the centre of the circle. Result: I now have a level chassis with the washers floating 2-3mm above the cotton pads.
Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 13 Sep 2015 at 10:04pm
Motor ideas. The GyroDec is a very recognisable turntable and on my wish list for some years. Working on my P3-24 and SL1210/II made me aware of the Rega's AC synchronous motor with anti-vibration circuit and the servo driven DC motor on the SL1210/II. So I bought the GyroDec thinking I might fit a DC motor in due course. Then I read about the Papst motor and found a little cleaning and oil gave me a pretty quiet unit.

The GyroPower is the middle AC power supply, reducing the mains voltage whilst filtering it to give a 19 volt ac supply for the blue and green feeds to the motor. It uses a 22uF starter capacitor to generate the 90 degree shift from blue for the red feed. I found out that the Gyro QC supply went further by synthesising a 50 Hertz sine wave and then amplifying it. That made me think about the Rega TT PSU I have and whether it could power the GyroDec, certainly the switchable 33/45 would be nice.


Edited by Fatmangolf - 13 Sep 2015 at 10:06pm
Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 13 Sep 2015 at 10:22pm
So I started by measuring the existing setup i.e. the GyroDec fed by the GyroPower. Well the first surprise was finding the green return leg had 290 milliamps running through it. The voltages varied over the hours I experimented but I confirmed the rms voltage between Red and Blue was about 1.4 times that between blue and green, as I expected for 90 degrees phase shift.

I tried different starter capacitor values including 10uF and 15uF. It felt like there was more motor vibration (hand on the motor casing) with these than the stock 22uF, although the current drawn on green went down to 260mA and 210mA. That might be helpful.

The GyroDec runs on about 19 volts AC where the Rega TT produces 24 volts so I turned the gain present down and put a current limiting resistor in series before connecting up. The tiny transistors got warm quickly but they could supply the higher current. Putting out 260mA and delivering a steady 33 1/3 RPM according to my strobe and test disk. I don't have much call for TO126 heatsinks so popped out to Maplin and picked up some 25 degrees /Watt clip-on heatsinks for TO220, which helped.

Now I have proved the concept I'll try with a high wattage load resistor in place of the GyroDec. Much smaller for the table top testing and I can play records whilst I do it.

Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 13 Sep 2015 at 10:30pm
And I added some draft excluder to fill the gap between the back of the plinth and the lid of the GyroDec. Some dust had crept in through the 3/4 inch gap whilst I was away and that'snot good!
Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 2015 at 6:16pm
Looking at the PCB and thinking through the next steps. Because the TT PSU needs to output 24 volts ac from a 24 volt ac wall wart (it's a transformer with 24 Volt 350mA written on it) the designer has used a split rectifier or voltage doubler to get the +/- supply rails after programmable regulators. (24 volt ac is stated as RMS and is about 34 volts peak to peak so the output stage needs more than +/- 17 volts on the supply rails, with several volts more to cover the voltage drop across the regulator IC's.

The split rectifier gives twice the voltage but half the current of a bridge rectifier because one half cycle feeds the positive rail via a single diode and the other half feeds the negative rail via another diode in reverse orientation. That means a lot of smoothing is needed on the supply rails and I'm sceptical about the regulation provided by one 1,000uF capacitor on each rail. Moreover the Rega wall wart is delivering all of its "300mA" current. That may explain why the TT delivers 260mA where the GyroPower manages 290mA.

I shall be looking at the supply rails on an oscilloscope to see if they are flat, or rather how much ripple is visible. And the output waveform to see what the 50 Hertz and 67.5 Hertz sine waves look like.

Assuming everything else holds I would be trying a lower output of 19 or 18 volts ac, which could allow lower supply rails of say +/- 15V. That's a 500mA 15/0/15 mains transformer and a bridge rectifier with smoothing capacitors in front of the voltage regulators. There are a pair of 15 volt regulators already there that could be reused.

In theory any audio amplifier or 10 watts or more could drive the turntable motor but the Premotec motor in the P3-24 had an impedance of over 300 ohms so the output voltage of 24 volts is needed. 24 volts into 8 ohms is 72 watts, which is a quite meaty amplifier. I can see why the designer went for higher dc supply rails and the BD139/BD140 push-pull circuit. It was cheaper than using an output transformer to convert a typical low voltage/high current 10 watt power amp IC output to 24 volts AC to drive the Premotec motor. That said the little transistors are rated up to 1 amp each so dissipating the heat is the main issue, assuming the wall wart and internal psu circuits are up to it.



Edited by Fatmangolf - 15 Sep 2015 at 11:13pm
Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 2015 at 8:27pm
Jon,
Fascinating stuff. I'm keen to see if a well engineered AC supply and motor can match the supposed benefits of a DC supply and motor.
One can only surmise that such a design (for the OEM supply) is the result of applied value engineering, with a modicum of accounting topped off with a large amount of sales spin.
Such designs really do make you wonder, does all their equipment receive the same dedicated engineering?
Just listen, if it sounds good to you, enjoy it.
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