"Kent Synchronous Converter": what is it?

[Uncle Bulgaria]

I'm sorting a box of lots of peculiar parts from my grandfather, and there are a couple of these in there. What on earth are they for?

[ms660]

My first guess would be for converting low level DC to AC (chopped) so that it can then be amplified without the drift that would have been associated with DC amplification back then, I think there should be a drive coil in it, that should operate a set of vibratory contacts that are used to chop the low level DC so far as I can make out.

From a Google books snippet:

The Kent synchronous converter is a single - pole - double - throw switch which operates continuously at mains frequency , and is designed for converting small direct currents into proportional alternating signals for drift - free ...

I think the manufacture could be George Kent.

Lawrence.

[Hartley118]

I agree that it's probably from a chopper amplifier. In these days of stable solid-state DC amplifiers, we tend to forget how difficult it was in the valve era to achieve drift-free amplification of low-level DC signals. A chopper works very much like the vibrator in a valve car radio to convert the DC input into AC for amplification. The attached diagram shows the principle.

A synchronous chopper system operates the demodulator from the same chopping signal as the input chopper. Chopper amplifers, using electronic rather than mechanical switches, are still favoured in high sensitivity detection, such as medical applications.

Martin

[TonyDuell]

I'll go along with the above, I am pretty sure I have the same type of device in the servo amplifier from a chart recorder.

The AC signal that it produced at the input to the amplifier was of course in phase, or 180 degrees out of phase with the mains (the chopper coil was normally run from the heater winding on the mains transformer). The output of the amplifier was applied to one winding of a small 2-phase induction motor, the other winding being fed from the mains (or the output of a mains transfomer) via a phase shift capacitor. Therefore the direction of rotation of the motor depended on whether the AC was in phase or 180 degrees out from the mains, and thus ultimately on the polarity of the DC input to the system. Just what is needed in such a servo amplifier.

[lesmw0sec]

Kent made strip-chart voltage and current potentiometric recorders. I suspect that the item in question was designed for the valve amplifier which drove the motor to move the pen in proportion to the signal applied.

[barrymagrec]

We had dealings with a sensitive Philips DC Millivoltmeter many years ago that used a mechanical chopper - may even still have one lying around somewhere. Bigger and squarer than that Kent unit though.

[MotorBikeLes]

That Kent synchronous converter is almost certainly the same as a Philips synchronous converter for 12v power supplies on their car radios.
But the Kent will be quite different and probably be out of one of their Mk 111 potentiometers, strip chart, circular scale or why? All part of a DC amplifier.
During the war, there was a push in the US to develop a reliable DC amplifier, Honeywell, Brown, Evershed and Vignoles and possibly others were chosen for the task, which they managed.
Evershed and Vignoles quietly licenced their current design to Kent, which resulted in the Kent Multilec. This used a sensitive, taut wire galvo as the central element of a potentiometer. A camshaft continually rotated at probably 10Hz with a complex mechanism that
(a) trapped the galvo pointer.
(b) checked if it was above or below the centre zero position.
(c) dis-engaged a clutch whilst moving another part into contact with a disc.
(d)Cause the disc to rotate a little in a direction that will move to offset the potentiometer to help null the offset.
(e) start the cycle again.
Actually, quite a bit more complicated than that, an amazing mechanical device. We had eight of these as chart recorders plus another two manually operated conventional potentiometers on two of our kilns.
After a few years Kents moved on to there Mk 111 which would be a conventional automatic potentiometer along the lines of the wartime designed US ones. Maybe they again licenced fom E&V, I don't know.This device would simply be the chopper to allow AC amplification with much higher stability than a DC amplifier.
I have a manual for the Multilec here, quite mind boggling to study.
Les.

[G6Tanuki]

Yes, a chopper used at the inpit to a DC-amplifier to convert the DC to AC for am0plification [followed by a reverse-process at the output].

Such things persisted well into the transistor-era, early transistors having thermal-drift characteristics that led them to look at valves with envy!

By the early-60s there were semiconductor equivalents: I remember a range of such made by Venner, gard-wired circuitry fitted into flattish grey cases and stuffed-full of OC71 multivibrators etc.

[Wendymott]

The Marconi TF2603 (DC Millivolt meter) used one as the DC to AC chopper in the front end. The two contacts on the top are the coil drive connections. The one used in the Marconi were made by Salford Instruments. I think I still have mine, from when I converted the Marconi to a Dual fet chopper.

[Radio Wrangler]

Moving parts concerned people, so some manufacturers moved to multivibrator-flashed neon bulbs illuminating CdS cells, some had plain bulbs and a motor-driven chopper disc. Eventually FETs took over.

There are usually DC choppers in RF/microwave power meter heads.

David

[Uncle Bulgaria]

Fascinating! Thank you for that. One has a piece of masking tape on saying it 'tested OK' in 1979. I can't imagine I'll have a use for them in the next 40 years. Perhaps they'll go back in the box for the next generation!

The 6.3V marking suggests it is indeed run off a heater winding. What's the mechanism for integrating the amplified signal back to DC at the other end? I understand that this object is just the switch to do the first stage chopping.