Need help reverse engineering transistor specs from my mercury outboard (no part #s!) - project :)

Thread Starter


Joined Nov 21, 2016
Hi all - I have a 1990 mercury/mariner 60hp and have had a cdi failure on the system. I have removed all the epoxy and exposed the entire board.

There were 6 transistors soldered in that i removed. Only one came out in good enough shape to get a reading. My friend plugged it into a transistor reader on his multi-meter and it registered on the npn type and showed 1500 on the reading scale.

Attached is a pic of the board, I also noted down which leg went where on the circuit board (to assist with gate / etc pinouts).

Since mercury have fun not printing any part numbers on their equipment I thought id give it a go and try get similar spec transistors in there..

I know the DC volt readings into the cdi (from the trigger) is in the 3.8-5.2v (as read on my dva adapter), so i think it may be quite easy to try replacing these and having a try - would any 5v transistors work?

I dont care if the thing goes poof, its lying in my garage waiting for some input.. if i can get this cdi working again ill be incredibly happy as I blew a 2nd one by mistake and they are burning a hole in my pocket!

Basically what happens is the stator charges the capacitor in the cdi, and then the trigger signals these transistors to "release" the volts from the capacitor to the coils primary side. The voltage on these (idling) is around 120v-140v.

Ill try make a better pic of the bottom / top showing where the ground is - ill have to go back to my notes.. are all npn transistors the same pinouts? Could I test them with a diode test to find the pins?

many thanks!

Also - on the one pic ull see a big grey thing connecting the main ground.. i broke one while removing epoxy - what could this be? there is one for each cylinder/track (3 total). Or should I de solder a good one and get specs from that? Would it be a resistor? The cap is a 250v one -



Joined Jun 4, 2014
The pinout of transistors does vary - I think all possible orders of pins are used. You can identify the base of a bipolar junction transistor with a multimeter but you can't detect which is emitter and collector.

I can't see enough of the circuitry to be able to suggest a replacement transistor type.

I can't see the 'big grey thing' to be able to identify it.

Thread Starter


Joined Nov 21, 2016
Hi thanks for the reply - ill get a better picture and trace some tracks - my elec knowledge is quite basic to say the least so sorry if ive asked anything unclear. ill take some better pics and post later..


Joined Feb 5, 2010
The gray roughly spherical components are diodes. The connection to ground on one end says they "may" be back emf.
Just a few closeups means lots of guessing from us.

I see bare copper wire in places... Are those coils? And if you scratched away the insulation on the wires it may have to be repaired before you try it.
Look up how to use a meter to check high voltage insulation resistance. You might have jumped in over your head with this project and perhaps should spend money on a finding a good used replacement cdi


Joined Sep 17, 2013
Judging by the various CDI schematics that Google throws up I think your 'transistor' may well be an SCR. Depending on your friend's meter construction it might interpret an SCR as an NPN transistor. My guess is that the 'grey thing' is a power diode.