Old Mitsubishi circuit board mystery  - Page 1
Forum section: electronics / beginners

author: jlimburg2707
author_rank: Newbie
timestamp: May 24, 2025, 09:45:46 pm
content: Hi All, i am trying to reverse-engineer a motorcycle CDI (Capacitor discharge ignition) that was built by Mitsubishi in 1969-1970. Unfortunately the PCB is completely encased in some kind of epoxy so it's just a block when you take it out of the metal enclosure. I used a heathen to remove the epoxy as best i can but there is always damage to the components :-( In any case, after several tries i now have a very good ides what the circuit looks like, except for something i have never seen before. There are little rings, or cylinders, between solder points that are being held in place by tiny metal strips. They get lost easily when i apply too much force to free the board of the epoxy but i do have some images which i will attach here. Best i can come up with is that they maybe are ferrite and have some purpose blocking EMI but i am hoping someone can shed some light on this. Images attached are before and after sanding down the boards to clean it up. Many thanks in advance :-)

author: amyk
author_rank: Super Contributor
timestamp: May 25, 2025, 02:39:11 am
content: You used awhatto remove the epoxy!?Yes, they look like ferrite beads. Did you consider finding someone with an x-ray machine to image the module instead of doing it destructively?

author: jlimburg2707
author_rank: Newbie
timestamp: May 25, 2025, 02:46:06 am
content: Well, i used (tried) heat, sulfuric acid, muriatic acid, acetone, harsh language, staring at it a long time... Nothing worked except the heat gun :-) Contrary to what people say on the interweb the epoxy does not turn to rubber as it heats up but it does soften enough that you can pry it away. Does not always work accurately enough so it happens that a component gets damaged. Fortunately i have a bucket full of broken CDI so i'm still good.

author: jlimburg2707
author_rank: Newbie
timestamp: May 25, 2025, 02:46:50 am
content: Thank you Sir, i have done an image search and found the ferrite beads and i am reading up on them :-)

author: jlimburg2707
author_rank: Newbie
timestamp: May 30, 2025, 02:41:56 pm
content: Hi again folks, i have done a lot of reading and watched bunch of videos on ferrite beads and i get what they do, so thanks again for pointing me in that direction :-) There is one more thing i don't understand, and that is the physical arrangement on my boards here. Everywhere i look, the wire with the signal goes through the ferrite bead. On this board, they just soldered these little 'springs' for lack of a better word that push on the bead. These do not touch, and ferrite does not conduct, so what goes on? It it just the proximity of the metal pieces? Again, any help id much appreciated! In the meantime i will send a few signals through the board and will try to see what goes on. The issue with that is that due to the intense heat necessary to remove the epoxy some components may have expired :-)

author: Terry Bites
author_rank: Super Contributor
timestamp: May 30, 2025, 03:34:49 pm
content: By the time you've desolved the potting gunge the componets will be likely unidentifiable.Why waste your time, there are plenty of designs you could build.What bike is it for?

author: jlimburg2707
author_rank: Newbie
timestamp: May 30, 2025, 03:49:54 pm
content: It is about originality. This particular bike is a 1970 Kawasaki H1 500. I have all components identified, it took taking apart 5 broken units to piece it all together but i have a good level of confidence now. The only thing left is these little beads and their function in this particular circuit. It did build a couple CDIs from existing schematics and they work fine in a test setup, haven't tested on the bike in real life so i don't know how they will behave at higher RPMs etc.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 30, 2025, 04:29:59 pm
content: Whos to know it aint original?I'm guessing you haven't seen this page, it has a good simplified schematic of the Kwak CDI:https://www.kawatriple.com/manuals/maint/section7.htm

author: jlimburg2707
author_rank: Newbie
timestamp: May 30, 2025, 05:42:43 pm
content: i basically live on that site as i have a few triples :-) That schematic is overly simplified, and actually wrong. The unit "a" does not have a transistor driving the SCR, just passive components.

author: jlimburg2707
author_rank: Newbie
timestamp: June 03, 2025, 12:38:01 am
content: Hi again All, aside from the issue with the ferrite beads, i am still somewhat puzzled with this circuit!I am attaching below the board layout i have copied from now 6 units that i have broken open :-) I identified all components and drew them in place. I also took some voltage readings which i added where i took them. I created a diagram from this which i simulated in CircuitJs1, but so far the circuit does not make sense at all. I am sure it is just my ignorance so maybe someone can shed some light on this for me :-) The signal from the pickup coil comes in at 50Hz - 400Hz and 1.5V-12V depending on engine RPM. This is a triple two stroke so reach revolution there are three sparks which means at 1000 PRM idle we would have about 50Hz. I can follow the signal on an oscilloscope until just past the 20k resistor, after that it doesn't impact the 12V circuit at all anymore. The setup for this is as follows: This unit (unit A) is meant to receive the ignition pulses from the engine pickup coil and use it to trigger an SCR which will connect to 'unit B' which has a capacitor ready to discharge 400V which will trigger a spark from the ignition coil. I am assuming the whole 12V circuit is meant to offset/amplify the signal so that the power SCR has enough gate current to turn on. Unfortunately that is not happening. I also attached a screenshot of the scopes, one for the input signal, then after the rectifying diode and after the coupling capacitor. Thanks for reading :-)

author: jlimburg2707
author_rank: Newbie
timestamp: June 03, 2025, 03:06:01 am
content: Sorry seem the other two files weren't attached.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 03, 2025, 09:51:04 am
content: Sorry, but I can't make sense of the circuit, but I'm writing this so you don't feel ignored. An expert will come along soon.One thing that puzzles me is that only negative-going pulses get through the first diode, but positive-going pulses are needed for the SCR. I'm looking forward to having it explained. 😊

author: Andy Watson
author_rank: Super Contributor
timestamp: June 03, 2025, 10:28:28 am
content: Quote from: jlimburg2707 on June 03, 2025, 12:38:01 amI'm not seeing any ferrite beads. Are you talking about the round-planar objects that all appear to have three connections? My guess would be that they are some kind of active silicon device. Can you bleep them out with a meter and confirm if they resistive or some form of non-linear device?I would say that your circuit contains a disproportionately large amount of Rs and Cs in relation to the number of active silicon components!

author: jlimburg2707
author_rank: Newbie
timestamp: June 03, 2025, 01:11:23 pm
content: Thanks Gents for the responses! Andy Watson - brilliant idea, it never occurred to me that they could be active devices!! I will see if i can isolate and test them. If they are, the circuit would maybe make sense, because like Steve said it doesn't right bow :-)

author: amyk
author_rank: Super Contributor
timestamp: June 04, 2025, 06:04:48 am
content: If they have 3 terminals they can't be ferrite beads. Your second photo actually shows that a lot more clearly, it reminds me of a puck-style thyristor and that's what I think they could be.

author: Andy Watson
author_rank: Super Contributor
timestamp: June 05, 2025, 04:04:02 pm
content: This is how I think your sketch of the tracks would be translated using NPN transistors.I have corrected the orientation of D3.TR1 is my guess at how it is orientated. Its lead-outs are not consistent with TR2 and TR3. Perhaps it's a different type?

author: Terry Bites
author_rank: Super Contributor
timestamp: June 05, 2025, 05:19:32 pm
content: Here's a breakdown of what this circuit does.From left to right.Bubble 1 Provides AC gain (~100x) it "squares up" the pickup impulses.Bubble 2 Pulses from the amplifier are used to trigger a fixed pulse width circuitAka a monostable. It creates controlled short pulses (~10uS) to trigger the SCR.Bubble 3 These parts are there to prevent misfiring and to protect the SCR.The amplifier is fed with a cleaned 6.2VFerrite beads are there to reduce EMI to acceptable limits.Blaring EMI is very antisocial.