Speed control for TT motor (AC) schematic wanted

[bigpandahk]

Hi

I am using VPI MW19 Mk4 and want to DIY a speed control similar to the VPI SDS for it, can anyone recommend a schematic or DIY kit?

[Chris Daly]

Originally Posted by bigpandahk

Hi

I am using VPI MW19 Mk4 and want to DIY a speed control similar to the VPI SDS for it, can anyone recommend a schematic or DIY kit?

A 555 timer will output a square wave, and triangle wave if confronted with capacitive coupling. see diagram: Its a relatively simple circuit to construct that as provided works as an inverter witha transformer as its load. It varies its AC output that is really a Triangle wave by adjusting the wiper of the 50K pot. The AC output at the 4700uf cap is typically 4-5vAC that can then be coupled to a suitable transformer to provide a higher voltage for driving a TT motor. I bench tested this same circuit on the weekend . suitable transistors TIP41 or BD139 NPN and TIP42 or BD140 PNP. You can also vary the Triangle wave output by lowering the output capacitor value, which also alters frequency. I tried as low as 10uf. to get closer to a Sine wave you need another cap to Gnd from the Negative of the output cap, but the AC output drops off appreciably. Possibly for a TT you need better precision - but the 555 is instructive to gather ideas how this is achieved. For better precision of control you may for instance investigate 74HC74 Flip Flops to store and release timing data

It might work in your application to vary speed. .

Cheers / Chris

[bigpandahk]

Thanks Chris for your prompt response. It seems tht I can use the principle of an DC/AC invertor for speed control.

[bigpandahk]

I search the web using all the possible wording and can't find any similar schematic, isn't there any DIYer think about this?

[kevinkr]

From time to time there are kits offered on eBay that might be suitable.

For good speed stability a crystal clock is usually used and divided down using programmable binary counters, subsequently this signal is run through a flip flop to assure the duty cycle is correct, (50%) and then LPF filtered to make a reasonably clean sine wave which is used to drive a power amplifier and step up transformer.

You can also do it with a weinbridge oscillator and power amplifier. Have a look at the old TD-125 design for possible inspiration.

[Chris Daly]

Hi bigpandahk
Linn Products provided some very good TT speed control circuit boards. There is a schematic here ( after registration ) : Linn Valhalla Owners Manual, Service Manual, Schematics, Free Download | Vinyl Engine

Comments suggest its an older schematic than the image shows, but might be a good start. Not much variation on speed either but Linn instead provided a pulley system for 45rpm. Linn can be contacted http://www.linn.co.uk/support.

Cheers / Chris

[bigpandahk]

Thanks Kevinkr and Chris, Happy new year!!!