I have a 1971 Evinrude 60hp outboard motor that has a stock dual point trigger, a CDI(capacitive discharge ignition)amplifier, ignition coil, distributor cap and rotor. This is a 3 cylinder engine. The CDI amplifiers on these motors are prone to failure from low and high voltage surges(weak battery during cranking, over-voltage during battery charging, jump starting...) and are expensive($300) to replace. I would like to replace the amplifier with a more modern multi-spark unit that is less prone to failure and cheaper to replace.
Here is my problem:
The original units, when working properly, generate approx. 30kv primary voltage to the distributor cap. I've been told that the standard ignition coils increase the amplifier output voltage by 10, so i am assuming the the original amplifiers output potential to be approx. 300V. I don't have an operational amp. to measure.
The new CDI amplifier output is 520V. One fellow i talked to that converted his motor this way, burnt a hole in his distributor cap(that is no longer available), so he replaced the cap and wired 2 ignition coils in parallel, grounding the secondary output of the second coil to reduce the first coils output by 1/2 or 26kv. He said this is working fine for him.
I'd like to install only 1 coil but need to reduce the CDI output from 520V to approx. 260V to achieve a 26kv available firing voltage.
What would be the best device i could use to accomplish this? This would be a very low current application i would think.
Thanks, Tony

 

How fun

I miss the 1955 7-1/2HP Evinrude twin we had when I was a kid.

A few observations here for the interim:
1) Make sure your boat's battery is properly maintained, kept charged and is in good shape. If it's more than a couple of years old, replace it.
2) If battery voltage is below 11.7v, use a rope pull to start the motor.
3) Don't try to jump-start it. Charge the battery fully. Then remove charger, and start normally.
4) To avoid the problems in 2) and 3), refer back to 1). A $70 battery is much cheaper than a $300 ignition part.
5) If you use battery power for cabin lights, running lights, radio, etc - use an aux battery. The battery you pulled from engine starting duty after a couple of years would make a fine aux battery.
6) Charge the aux battery via conduction through a light bulb and a high-current Schottkey diode. The light bulb will limit the current, preventing a big voltage surge at your outboard's alternator. Keep the aux battery charge circuit turned off until the motor start battery has fully charged.

Helpful Hints: you can repair distributor caps using the original J-B Weld. Do your absolute best to remove any signs of carbon tracking. A pencil eraser can be quite helpful in this endeavor. Sacrifice a brand-new pencil that's never been sharpened or used. Graphite contamination on a distributor cap will be quite counter-productive.

The original units, when working properly, generate approx. 30kv primary voltage to the distributor cap.

Actually, it's the secondary coil winding that has 30kV. Minor technicality.

I've been told that the standard ignition coils increase the amplifier output voltage by 10, so i am assuming the the original amplifiers output potential to be approx. 300V. I don't have an operational amp. to measure.

Sounds like a reasonable assumption, with the information you've been given. I do have my doubts.
Oh, you wouldn't use an operational amplifier to measure such high voltage. You would need an oscilloscope with a voltage divider probe to get a reasonably accurate measurement. However, you could also use a high-voltage diode and a capacitor with a digital voltmeter to get a pretty accurate reading.

I'm going to have to get back to you later - it's after 3am here, and I'm exhausted.

 

At least 2 errors in my original post.
Quote:
The original units, when working properly, generate approx. 30kv primary voltage to the distributor cap.
Actually, it's the secondary coil winding that has 30kV. Minor technicality.
______________________________________________________________________
And "I've been told that the standard ignition coils increase the amplifier output voltage by 10,".........actually it should read a 1:100 increase(300V in 30kv out)

 

The new CDI amplifier output is 520V. One fellow i talked to that converted his motor this way, burnt a hole in his distributor cap(that is no longer available), so he replaced the cap and wired 2 ignition coils in parallel, grounding the secondary output of the second coil to reduce the first coils output by 1/2 or 26kv. He said this is working fine for him.
I'd like to install only 1 coil but need to reduce the CDI output from 520V to approx. 260V to achieve a 26kv available firing voltage.
What would be the best device i could use to accomplish this? This would be a very low current application i would think.
Thanks, Tony

I don't have any experience with the Evinrude CDI; however, I do have experience with CDI for model aircraft. Based on your description, they are probably similar. Their amplifier boards are of at least 2 types: 1) simple rectifier from transformer to output 300V (American type shown below); and 2) rectifier and voltage doubler to 300-400V (Chinese type shown below). Based on that, if you can access the board of the 520V unit, you may find a voltage doubler, which you could remove. The doubler is probably just one stage. A key to it will be and extra diode and capacitor. (There may be a small capacitor across the transformer in both types, I am referring to a cap with a capacitance something like the one connected to the coil.) I have attached 2 schematics from model ignitions, the Chinese one on the right has the "extra" capacitor, which is labeled C4. The American schematic is on the left. "American" and "Chinese" just happen to be labels I had on the devices before reverse engineering them. Regular and voltage doubler versions are probably made in both countries.

I fear everything may be potted, which means you will not be able to see the components or modify them. Have you considered trying to adapt an automotive unit that outputted lower voltage?

Finally, ST has an application note on CDI (AN819) that you may find useful.

John

Edit: Here's the ST link: http://www.st.com/stonline/products/literature/an/4347.pdf

 

This amplifier is potted, go if i can't reduce the output voltage from 520V to 260V with a simple device, then i'll have to go the dual coil route.
Thanks to all who've helped.
Tony Garbarino