Magnetos - Late Qualcast Mowers

[Cleggy]

Hope this is the right sub-forum.

I am working on the above; it's one of the later Suffolk Punch, with the aluminium engine. There's no spark so I removed the magneto, which turns out to be a 3-pole item and can be found on Google described as an electronic module, originally made by Wipac. I cannot find any Wipac info on it, but I suspect is may be an similar device to the Briggs and Stratton "Magnetron" magnetos (see attached). I think that Qualcast eventually used B&S engines.

I'm getting fairly good resistance values on the primary and secondary (0.5R and 2k8 resp), but that may not mean much.

Anyone experienced with these?

B

[Refugee]

0.5 ohms is a bit low for an ignition coil.
I came across one on a scooter that was as low as that. A new solid state driver was fitted against my advice and it blew up.
You can test the transistor with a good diode tester if it can do two times VBE and it is unlikely to be any good.
The trigger coil can be tested with a small 2W 12 volt bulb.
Both parts would need to be replaced as a pair.

[Kalee20]

Agree with Refugee, 0.5 ohms is rather low.

If it was a traditional magneto with contact-breaker points, then it could be anything lowish. But if it has electronic make and break, then it's going to be wound to operate at several volts at least, at maybe an amp or so, which means more turns and more resistance.

Having said that, I'd be surprised if the LT winding failed, as it is not highly stressed. The HT winding, more likely - or perhaps the electronickery has failed and is shunting the coil.

[Murphyv310]

Hi.
Before condemning the coil, try ringing it with a scope. You could also wire it up like a conventional coil, points (just some wire to make and break) and a 0.22uf cap across your home made Points. Try with 6volts whereupon you should get a healthy spark, mind and disconnect all electronics. Did it this way on many engines that we did in my previous job. Never had a coil duff, was always the electronic control that was at fault.

[Refugee]

That transistor looks like it is wired to act as a diode one way and then to allow a half cycle the other way to produce the spark.
It would be a special transistor with a BC junction that can take full current.

[ppppenguin]

I thought that was pretty common in TV line output transistors where the alter ones didn't need a separate efficiency diode.

[Murphyv310]

The line output transistors I think you mean have an extra internal diode eg BU508D and some have a resistor between base and emitter as well.

[ppppenguin]

Trevor, the BU208D does indeed have a reverse diode and resistor: http://www.alphacron.de/download/hardware/BU208D.pdf

It strikes me that this is an ideal sort of device to use in an ignition system too. In the absence of a diode the CB junction is pretty tough and if the base is driven from a low Z source it will give some energy recovery without blowing up. Exactly how much is another question.

[Murphyv310]

Hi Jeffrey.
I used to make up two transistor assisted ignition drivers. These were for reducing the current through the points. Adding a reverse diode or a transistor with inbuilt reverse diode didn't work, you always had to have a cap across the collector emitter junction of between .22 and .25uf effectively to tune the coil. I did use BU208D transistors but also standard BU208's never had a failure on either type.

[Refugee]

I agree about a C-E diode not working.
There may be a diode that was not spotted when the circuit was reverse engineered. It would go from B to C to help the B-C junction to charge the coil.
At about 1.5 volts the transistor would shunt the coil collapsing the field and producing the spark. Perhaps the coil resistance is all right.
If this is so the coil would be charged at about 2 amps before it gets shunted by the transistor.
With the electronic bit removed connect a brake light bulb and 12 volt battery in series with the coil and then short the coil and see if you get a spark.
If this works then the circuit can be reproduced to get it going again.