Hello everyone,

I working on an old motorola amplifier CDI system for an 1969 johnson 55 horse outboard motor. The PCB was potted deep in an aluminum case. Ive managed to remove the PCB with minimal damage, but after boiling it several times to get to the traces and components im skeptical ill be able to identify all the components. Ill be posting photos here and asking for advice along the way about testing and whether parts or equivalents can be acquired. I apologize in advance for any basic questions, as i have not done my due diligence and searched for days for specific answers to questions.

 

I posted an Electronic Ignition from ETI 1977 (it's the 3rd pdf) magazine here, using a Royer Oscillator, yours is probably the same design with a thyristor trigger circuit.

https://forum.allaboutcircuits.com/threads/eti-1977-projects-book-1-2.142188/

 

Thanks Dodgydave, I guess I had a basic understanding of the concepts of CDI, but was having some trouble with the details and why my amplifier CDI was different from any other CDI circuit out there. Ill continue on and see if i can find the parts to make a similar circuit. Makes me wonder if there isn't a better off the shelf solution, besides the $350 direct replacement.

 

@Hab_79
Welcome ro AAC.

I am assuming that is a 3-cylinder engine. What's it maximum rpm? The biggest problem on model aircraft CDI's is to get enough current from the inverter. The higher the RPM or number of cylinders, the more current needed.
Have you considered a ready made replacement like this: https://www.westmarine.com/buy/sier...evinrude-outboard-motors--3735685?recordNum=4

Sorry to be so late replying. AAC went down just as I hit send.

 

Hi jpanhalt, it is a 3-cylinder, I don't know max rpm off hand, but some sites say 5000. So, yes I have considered the ready made solutions, but I don't know what the limiting factors are for this system. Can I condense any CDI ignition module to work on my boat (i.e. supply, trigger, and discharge)? I started looking at automotive CDI modules, but they claim to be for 4,6, or 8 cylinder only. My limited knowledge of the CDI tells me the # of cylinders doesn't matter, but I don't want to chuck $100+ at a theory.

 

My experience with CDI is limited to one and two-cylinder engines. In those, you typically do not use a distributor, but simply spark both cylinders at the same time, even if there is alternate firing (compression strokes on opposed cylinders). With a 3-cylinder engine, that might not work so smoothly.

The other reason for knowing the number of cylinders is to calculate the rate of firing. That requires charging the capacitor faster. Simple model-type ignitions tend to have more problems with robustness of the inverter at more than 12,000 to 15,000 sparks per minute. One of the members of a DIY CDI blog races model boats. Those need to go to 20,000 rpm, and that was always an issue he raised.

 

Hi jpanhalt, it is a 3-cylinder, I don't know max rpm off hand, but some sites say 5000. So, yes I have considered the ready made solutions, but I don't know what the limiting factors are for this system. Can I condense any CDI ignition module to work on my boat (i.e. supply, trigger, and discharge)? I started looking at automotive CDI modules, but they claim to be for 4,6, or 8 cylinder only. My limited knowledge of the CDI tells me the # of cylinders doesn't matter, but I don't want to chuck $100+ at a theory.

Hi the CDI units usually for cars or bikes have an oscillator and a Capacitor that charges up, then a Thyristor for firing the Capacitor across the coil for spark generation, this is then distributed via each cylinder. The limit of the Cdi is the speed of triggering at high revs as this effects the charging time of the Capacitor, and so not enough voltage is produced.

If your engine has a distributor then any Cdi for four cylinders will do.