CDI GURU's need HELP PLEASE!!
 

I'm stumped. 3pin CDI
First of all I fixed 10 of these and they were pretty easy. I always change a slew of components even before I plug it in, including all caps, power diodes, all semiconductors except the input stuff. which checks out all good.
Second I've had to change the 22v zener diode(ZD3) that's weird. It was SHORTED, which leads me to believe the battery got hooked up backwards?

The problem is when powered it it goes into oscillation 9kHz instead of the normal 3kHz and the box is constantly discharging the output cap with no input. If I ground the intersection of T9,R9, R7, the power supply works, but only charges the cap(C16) to 150V. Perhaps because of the high frequency?


I measured the resistance of the transformer and the readings seem correct.
W2 measured good, but since W3 & W1 is a shorted wire, but compared to another good box they were the same.
Had to borrow a meter for that.

PLEASE HELP it would be greatly appreciated
THANKS!!

Steve Wong or Ingo, send them a PM.....

Send to Pelican Parts, they provide a Bosch CDI rebuilding service.

Isn't it interesting that few Pelican Parts forum members rarely if ever promote Pelican Parts for the
rebuilding services they provide?

Dave,
Doing this for the fun, like I mentioned above I've fixed 10 of these already. It's me hobby.
I do electronics. I fix them for nothing, just parts. These are from a guy in Illonis. ALFA Montreals use the same CDI Box.

Joe I'll write Steve and Ingo.

Thanks

I've never worked on a Bosch 3 pin, so take what I am suggesting with a big grain of salt. It seems to me like the secondary of the power supply transformer is shorting internally. This would affect the frequency and explain the low charging voltage on the discharge cap. If the shorting occurs at a certain threshold voltage, it would also account for the spurious firings of the SCR. Fred

My conclusion as well. Just want to confirm this before I rip out the transformer. I got the materials in house to fix it. I've done it before.

Thanks Fred!!

We are either both right or both wrong. May I suggest that when you wind the new transformer that you do not impregnate the windings with any compound as used to be done in the past before magnet wire insulation improved. It's my pet theory that in climates (read northern USA or Canada) where the copper windings are exposed to extremes in temperature, that work hardening of the copper can lead to breaks after 15 to 20 years if they are restrained by impregnating with shellac or epoxy. As long as there is no fretting from magnetically induced vibration, modern magnet wire insulation should last nearly forever without such a treatment. I even lay down a single layer of kapton tape before I start winding the bobbin just to ensure there is some 'give' for temperature expansion and contraction. Fred

Fred,
I fix everything electronic you name it, even tube stuff. Just got interested in fixing CDI's cause a few ALFA buddies didn't know how or where to get these boxes fixed for their Montreal's.
So I decided to take a wack at it, and I like them so much I have them in both my 75's now, easy upgrade, and better performance, especially in my 75 Sedan, Race car just starts better.

One of the boxes I fixed 5 years ago is still running great and the guy's in Maine.
Here's a LINK to see.

Thanks again and I think we are right, I changed everything else, what's left but the transformer!
The one I fixed before was obvious. This one through me for a loop.

Porsche People are COOL!!

John

http://forums.pelicanparts.com/porsche-911-technical-forum/376126-transformer-bosch-3-pin-cdi-defect.html

Good for you on redoing the transformers. That's when most people would give up. By the way, I just bought two compu-spark CD ignitions from the early seventies. One actually works and is a proper multi-spark that gives two bi-polarity sparks back to back for a total duration of 400µS. (a bit like Jonny's in that the multi-sparks are close enough together to be useful) The other unit has a failed transformer so I might take the plunge and re-wire it. There is a design flaw in the working compu-spark in that it will completely misfire about 10% of the time above 1000rpm. I'm sure that can be fixed too. Fred

http://forums.pelicanparts.com/uploa...1478481451.jpg