Adding Varispeed to Dynacord Tape Delay

[Polyphemus]

Hi there. I am trying to figure out how to drive the capstan motor of my Dynacord Eminent 100 mixer with a varispeed instead of the fixed 50 Hz. This is still just ideas and i hope some of you could help make this into a viable solution, without me burning out the motor.


The mixer has a built in amplifier which i don't use, so i think i can reuse it in this application.

First of i need to make an oscillator, any simple design will probably work. Maybe from 40-60 Hz? I'm not shure how much is neccesary.

Second i would amplify it with the onboard amp. From the schematic (For the updated version, the Eminent 100a, which seens to have the same output circuitry) ,it looks like if i apply ~720mv at the input, it would swing 18v on the output.
Here's the schematic: https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&cad=rja&uact=8&ved=0ahUKEwiUhpmggoLLAhVFuw8KHbCWB90QFgg5MAc&url=http%3A%2F%2Fwww.peel.dk%2FDynacord%2Fpdf%2FEminent%2520100A%2520SM.pdf&usg=AFQjCNFsfHQsuNxh3f25JEh8Y3hwE1ZA9w&bvm=bv.114733917,d.ZWU The schematics are on page 17.

Third i would need a step up transformer. A normal 230v-12v hooked up in reverse could work, and bump the voltage up in the ballpark. I don't know what the rating of the transformer should be though...

Now, in my mind this seems as if it might work. But theres probably pitfalls with no end, so if anyone has good ideas, i would be very thankful to learn.

- Jon Marius

 

[PRR]

Welcome!

> Here's the schematic : https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&cad=rja&uact=8&ved=0ahUKEwiUhpmggoLLAhVFuw8KHbCWB90QFgg5MAc&url=http%3A%2F%2Fwww.peel.dk%2FDynacord%2Fpdf%2FEminent%2520100A%2520SM.pdf&usg=AFQjCNFsfHQsuNxh3f25JEh8Y3hwE1ZA9w&bvm=bv.114733917,d.ZWU

That's not the schem, that's an obscenely long side-trip through a search engine with tracking numbers.

Page: http://www.peel.dk/Dynacord/Eminent_100A.html
User Manual: http://www.peel.dk/Dynacord/pdf/Eminent%20100A%20M.pdf
Service Manual: http://www.peel.dk/Dynacord/pdf/Eminent%20100A%20SM.pdf

The User Manual is vital: that's how we know this amplifier is 80 Watts RMS!!

So the question is: how big is the motor? I can't even find it on the schematic....

Ah, B14 B1, comes from 220V winding via 0.16A fuse. We would assume it is less than 35.2 Watts. An 80W output is suitable. Since you seem to want low-frequency, the transformer should be large for 50Hz. For 35W load I would round-up to a 50VA transformer, and keep a hand on it to check for heat.
__

Re-think: the power amp is DC-coupled. Speakers will have a few Ohms of DC resistance, but a 18V 3A transformer will have a very low DC resistance. The power amp's DC balancing is not exact. There could be some tenths of a Volt offset at the output. In speaker this causes a part-Amp of DC current, unimportant. In transformer this could be several Amps of DC current, which is a lot of waste heat.

Add a series element between amp and transformer. Simple would be 1 Ohm 10 Watts. This does cut power a bit, but also eases the amp's start-up into the non-spinning motor. Alternative would be a large DC block capacitor. This should be well over 1,000uFd 35V, and possibly 2,200uFd 50V. So an electrolytic. Start with a resistor (any resistor) and put a volt-meter across it to see which side is + and which side is -. Put the polarized cap in that way.

I suspect (never verified) that for casual motor driving (this isn't a steel-mill or even a wood saw), a triangle wave will work fine, and is a LOT easier to build and vary. To get good torque you may have to drive the peaks into clipping, but this just makes it more sine-like. However it is possible the sharp waveform will throw harmonics all around the tape heads and put a whine in the audio. I'd still start with that. If whine is an issue, a triangle can be soft-clipped and filtered very sine-like with a lot less trouble than making a true variable sine. (There is a chip, '8038?, which does sine and others and voltage-controlled; out of production but lurking in corners of the internet.)

 

[Polyphemus]

Thanks, and sorry for the odd link.

Thank you very much for your help!

Heres whats written on the motor. It doesn't seem to give away it's wattage rating (Picture attached, if it works). Its rated for 50 Hz operation, but +- 10 Hz wouldn't put to much stress on it? I am quite unfamiliar with motors in general; more so with AC motors.

A question regarding the transformer:
In the first part you wrote about that the rating of it should be 50 VA and in the second part 3VA.
Was the second  hypothetical, or would it be enough, with the series resistor in place?

And, yeah, i think i'm going to filter the oscillator quite heavy, since i only need very low frequencies.

 

[PRR]

> second part 3VA.

Read again. 18V 3A. 18X*3A= 50VA (rounded).

The schematic shows 220V with 0.16A fuse. 220V*0.16A= 35VA.

Because you also ask for lower Hz, you need a bigger transformer; 50VA is a reasonable step-up from 35VA.

Further: 3VA is a Clock Motor. I once knew a record player with two clock motors; it was very weak. I have never seen a tape deck with a motor that small. (Maybe cassette, not full size tape.)

I put a booster fan in a 6" (15cm) hot-air duct in my home; it is 50VA. It is similar to a tape-deck motor (except higher RPM, runs hotter, and less stable).

Running AC motors at lower Hz for low RPM is always a "maybe". Current will rise, so the motor will run hotter. If the motor fans itself, the fan turns slower, so it runs hotter.

This may not be an issue on a tape drive. It isn't heavy work. The motor is often "large" for stability, not to do the work. So it may not run hot at normal Hz/RPM, and may tolerate lower Hz for some time.

In another field, well-pumps, it seems to be common to run "60Hz" motors at 30Hz-60Hz, even 30Hz-80Hz. However these motors work hard, but have an exactly known load (lifting water), and are also water-cooled. And the nature of the job makes it unlikely that they will be run part-speed for many hours.