Revox G36 and 50Hz Issues!

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I've gained a TON of insights via the long G36 thread elsewhere on the site.

However, this is a more general question about frequency conversion and the best way to accomplish it. Revox does provide instructions on converting the motors from 50Hz to 60Hz, but this also requires a high-speed 60Hz capstan, which I'm fairly certain I'll never find.

So it appears that the easier route is to provide frequency-shifted AC. Seems like there are two "simple" ways to do this.

1. I have seen some tabletop units marketed as "overseas electronic device testers" that provide variable AC voltage a number of frequencies. These units seem relatively rare, and expensive.

2. Some folks have suggested rectifying AC, then using an inverter that puts out 220VAC (typically) at 50Hz. A common way of doing this is using a battery charger to power a deep-cycle marine battery (or possibly a capacitor and choke?), then the inverter.

Anyone have any insights? I would truly appreciate them!
 
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You will need a very stable 50Hz power source if you expect any sort of speed stability at all. The typical battery inverter produces either a square wave or stepped sine (a misnomer since it is really a ramped square wave / trapezoid stacked on top of another - neither are suitable for powering motors. Frequency stability may not be good as well.)

Some of the older PS Audio Power Stations might be suitable as I believe they can be programmed for 50Hz output. (Investigate before buying)

You might be better off finding a 60Hz parts unit and pulling the capstan motor.
 
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You probably don't want an inverter, you want a synthesized 50Hz source like you talked about or something along the lines of the PS Audio power station.

At minimum something with a crystal controlled source and a perfect sine output.

Don't fall into the trap of getting something too small, remember the motor starting currents will for the reel motors will be high. A 1kVa source may not be too large.

I think you would like to minimize speed variations and be able to play things back at the exact same speed as you recorded at.

The other option might be to machine the capstan to the correct diameter for 60Hz operation.

Having the high speed capstan is desirable, but complicates things.
 
Yes, but the deck will accept anything from 110VAC to 240VAC, so I'm not too concerned about that.

Kevin: Many thanks, sounds like the PS Audio P300 will work well for this application. Of course, I'd like to do it cheaper, but that may not be a real option here....
 
Hold on a minute guys;

Slor, I gather from your posts that your machine is SLOW speed (3-3/4 - 7-1/2) and you'd like to make it HIGH Speed (7-1/2 - 15ips). I believe that the motor is 220V hysteresis synchronous, which means that it locks on to the frequency of the source. If you can't get (or have made) a larger capstan of twice the circumference, you would have to run the motor at twice it's "normal" speed (which it may not be designed for) in addition to the possible frequency conversion - so you are looking at a sine wave inverter that puts out a very stable 100 or 120hz. And the C70 phase shift capacitor (being frequency dependent) would have to be recalculated.

The "stock" motor is powered from the 220V section of a 120/220 winding on the main power transformer. You indicate that you can apparently switch taps on the input of the transformer, so you could match the output of the inverter to one of those taps, and the transformer should work correctly even at twice the frequency.

Wait a minute, maybe not - the supply and takeup motors are powered from the 120V portion of the winding - not sure if they are induction or synchronous (if you double the frequency maybe they would try to spin twice as fast - !!!???) OK, you could fix that by unwiring the capstan motor from the power transformer and running it on a separate 220V feed from the inverter. Run the power transformer from the closest voltage tap to your mains voltage. To maximize torque, the C71,72,74,75 caps on the supply/takeup motors have to be sized for the mains frequency.

My head is spinning. Sorry, but if you can't get the "correct" capstan, I'd consider giving up the project.

Charles
 
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