AC PSU for synchronous motor

[jo-briggs]

So I've got an RF Generator from ebay for £25, and a Cambridge A&R A60 gathering dust; can I generate my 50/67.5 sinewave, feed it through the standard resistor/capacitor network to put the phases in quadrature, pass the resulting output through the amplifier and drive the motor? Or have I missed something?

The output to the R/C circuit should be as pure as it gets; the signal from the amp equally good; is the problem going to be that the amp expects an 8ohm impedence rather than the inductive load of the motor?

The reason I ask is because I have a TD125 with a PSU board cracked and broken in about 9 places - I could solder bridges across the defects, but I would never be able to either flatten it or re-attach it.

[AndrewT]

Hi,
My thorens td150 used a 115vac motor with that little circuit to convert the normal 240vac down to 115.
I found my motor needed a minimum of 90vac to start reliably and would run on 80v. I settled on 100v to play music. I used a 100w into 4r poweramp running into a back to front mains transformer to generate the hi voltage. Crystals can be used to generate the two frequencies exactly with very low temp co. The ones you need are very cheap because they are used in the tv industry. Sorry cannot remember the values, it was 30 years ago.
Thorens are a bit different from the low voltage motors fitted to other manufacturers' decks.
Check your motor needs.
regards Andrew T.

[analog_sa]

I see no problem with your proposal. The 125 uses a low voltage motor so any amp will suffice. In the original PS the phase shift is performed before the driving circuit and not at the motor as you suggest. You may consider using both channels of your amp for this purpose.

While the 125 will certainly spin under this ridiculous arrangement i see no reason to put up with two extra boxes as an excuse for your laziness

If i were you i'd simply build a new PS on perf board using chip amps for drive.

[jo-briggs]

I have the signal generator, and would expect to to get £40 for the amp on ebay.So I thought of another use for the components, the only thing I would have to make is the phase thingy to put between the generator and amp (TTs without PSUs have the phase shifter before the motor) - personally I would regard that as the application of intelligence. Both units would still be usable after I had established the principle, checked the relevant parameters, and then made a dedicated unit; which I might not, as my ability to calculate eletronics is limited to V=RI. The mechanics are a doddle, relatively speaking, but electronic calculation is outside my expertise and not down to laziness.

Were I capable of designing a scratch built PSU, I would not have posed the question - I have trawled sites like this and have not seen an actual schematic to either rebuild the TD125 PSU, or to build a replacement.

[Raka]

Let me jump into this...
I'm learning circuitmaker, so will stop to do point to point. And in the project pile is a very simple phasing circuit for my thorens I desinged. And yes, the GC will be very useful here. I'll post the circuit if I success.

[jo-briggs]

Evening Raka,

I'll be interested to see your proposal - In the meantime, having to hand a 13.8 Volt PSU from a CB set, and a handy Matsushita tape deck motor, I quickly jury rigged a setup using a 10k linear pot for speed adjustment, which was happy trundling away at spot on 33 1/3 when I last looked. I plan to let it run for about 10 hours and see if it is stable - more laziness I'm afraid!

[jo-briggs]

While trawling through the web and found this site

http://www.interq.or.jp/japan/se-inoue/e_ckt20.htm

A sine/cosine wave generator which gives 2 outputs at 90deg to each other, ideal for the purpose I'd say.

[scott wurcer]

Why not take a surplus portable CD or MP3 player and record the appropriate tracks on them. You could make them 30min or so for an automatic shutoff. A little trial and error with Cooledit or equivalent and you could get the speed right on then burn the tracks.

[Onvinyl]

It is not really worth it to repair the original circuit: it is not designed properly. Literally everybody who ownes a 125 MKII experienced that the circuit blows away some day -- So a decent and stable oscillator with enough power would surely better things!

@jo-briggs:
the circuit you linked uses the 2sk30 -- impossible to get these days...

greets

[rkay5]

Hello, On my Thorens TD150mkII the PSU I'm using is a Project Speed Box it will work only with a low power motor witch I have, 16V from a TD146 and speed box also drops 60Hz to 50Hz I had to get a new pulley.So it was a easy job to do and now I have a turntable that run's at a + or - 0.02% Good luck Robert