Motoplat Unwrapped

Many years ago, I raced a 125 Husqvarna, it was a great little bike, but as it aged, it became hard to start.  After more time,  it became push start only, then we discovered that it would only start if we made the spark plug gap very small.  Finally the bike sat for a number of years because it just wouldn't run below 4000 rpm. Now that I'm older and more nostalgic, I've started thinking about restoring this bike.  I bought a second coil dirt cheap (like me!) and it was worse than the first. A friend of mine let me have both a coil and stator/rotor combo from an old Maico. I set up the stator in my drill press, and wired it up and proved it was a working system. Next, I installed this known good on my poor Husky, it is started right up and all was right with the world...  Being the inquisitive type, I just had to figure out what was going on with the ignition,especially those coils. This is what I've found out so far. 
















                   I decided I had to see just what was inside the Motoplat coil, so I took the coil down to my local plastics shop to see if they had some solvent that might make it easy for me. That was just not the case. One night I went out into the garage, clamped one of the coils in the vise, took a wood chisel, and started  chipping away at the terminal end of the coil. I didn't take any pictures of this first coil, but I found beneath the first layer of epoxy a circuit board which lead me to understand that most of the electronics of the CDI system are stuck in the coil. I resoldered one of the joints on the circuit board and tested it in my drill press setup, and it fired right up. I gooped clear silicon seal all over the wound, and that coil is still running today, in my brothers 250 husky.





This is a picture of another one of my bad Motoplat coils. It's the first of the newer ones, you can tell by the curved exit of the high tension lead, not to mention the tach lead. The capacitor and SCR were on the bottom, and have been chipped away already, but this picture shows the soldered rivet where the primary coil is grounded as well as where the other side of the primary disappears into the epoxy.


I've removed the rest of the connectors, and the plan is to replace the original circuit using modern components.








My original Husqvarna manual has this nice diagram of the Motoplat circuit, which only leaves the actual components used up to the imagination.  It turns out that the blue wire has a connection to both the trigger and the charge coils in the stator. The black wire must be the connection to the SCR gate, which means the ground is the only other connection, through the diode in the stator.

I'm having a little trouble resolving the differences between the black wire coil and the red wire coil.  I believe the red wire coil has an extra diode which allows it to work in place of a black wire coil, but a black wire coil cannot be used on a later system. I don't know if this is true, or where an extra diode could be inserted. Maybe some has the older circuit diagram.


Above is the first circuit I made, and it works like a champ using the drill press. On the back side is a diode 1000PIV that can't be seen.  The SCR is an NTE5457 (200V 4A) only because it was on the rack.  I plan to use the MCR72-6 which has a little higher rating (600V 8A).  I'm starting with a 1uF 400V cap.  Note: in test both 1 and 2uf caps seem to work fine, but the high RPM results should show which is the better candidate.

Here is my test circuit hooked to the drill press stator etc,  the magic is back!

After this picture,  I fried the SCR with my super hot soldering iron, so any further pictures will show a new SCR. I just need to find the correct value for the resistor in the schematic, and I've got an ugly but operational replacement coil.