1978 Igniter Exposed! (1 Viewer)

[Coolerman]

OK, recently someone asked if there was a schematic available for the 1978 Igniter so he could repair his bad one. I happened to have one in a unknown state so I tore it apart last night. I was tired of working on the yard!

The first pic is the igniter I took apart...

In the second pic I have taken the metal case off. It just pries loose. This exposed a very old school circuit board with discrete components on one end of it, and a large power transistor mounted on a heat sink on the other end. The discrete components make up a pre-conditioning circuit for the pick-up coil from the dizzy. I'll post a hand drawn schematic of this section later tonight.

The third pic is the back side of the circuit board.

 

[Coolerman]

While tracing the board I noticed a line of pins in the center of the board. These appeared to go up into the heat sink. Hmmmm....

OK we are committed to showing MUD the whole story on the igniter so I continued to take it apart. I first removed the power transistor then tried to pry the 'heat sink' off. No dice, so I unsoldered the line of pins and was finally able to remove the assembly from the circuit board. When I flipped it over I was surprised to see that what ever the line of pins went to it was 'potted' with some silicone looking stuff. Sorry no pic of it. I proceeded to pic this stuff out to expose the real surprise!

A SURFACE MOUNT circuit board! I could not have been more surprised! Talk about a mix of old and new technology! I still have not managed to remove this board from the heat sink to draw it's schematic but truth be told it would be pointless. Unless the preconditioning circuit had a bad component or the power transistor is bad it would not be worth tearing apart and attempting to repair the surface mount board. But here it is for your curiosity!

 

[Coolerman]

OK so I was bored at work today and drew up a schematic of the 1978 Igniter. It's as accurate as I can make it and really helps to understand how this thing works. I took some liberty with some symbols so you real electronics experts out there feel free to send me the correct Visio shapes! The labels (R1-5 ect.)were assigned by me, the board is not marked in anyway. The 2 clear red diodes have no markings but appear to be germanium low power signal diodes.

I drew the surface mount circuit board as a block since I have not figured out how to get it out of the case without destroying it.

If you really know your electronics, after looking at this schematic, post up your description of the function of this circuit. My basic understanding is this: The pickup coil produces a pulse which is cleaned up and conditioned by R5,D3,R4,R3,T1 and C1. This is fed to the base of Q1 causing it to turn on. This turn on signal goes into the 'black box' where who knows what occurs, but it then turns on Q2 which grounds the coil causing it to fire the plug.

For those wanting to repair this igniter it looks like Q1, the amp for the pick-up coil, or Q2 the output power transistor would be the most likely things to go. These two parts are easily replaceable but you may not find a replacement easily.

I have been unable to track down either part Q1 = C1707HC or Q2 = D626.
Those in this business will have more luck I'm sure.

Cooments needed and welcomed....

 

[Vicm]

That is cool,,
I am sure I will save it incase I need to repair one in the future, Thanks for going through the troubles.

 

[webelk6]

Here is what a 75 looks like.

 

[yotanut]

And just think, all that bull**** replaces about a $.50 condensor.
After replacing two in high school and going broke, I finally figured out to just follow
the simple steps in a haynes manual for when these go out. 14 years later and same condensor.
Thats my $.50

 

[Coolerman]

Here is what a 75 looks like.

Thanks for the additional info. Looks like the 75 igniter is the 78 without the 'black box'.

I have a 76 I'll eventually get around to taking apart...