Yes, I don't know exactly where to cut and solder on your older Bosch circuit board, since the capacitor is located on the opposite side of the plate as compared to the newer ones that Foggysail modified.
It is noteworthy that in the later design implementation, Bosch moved the capacitor to the other side of the "can" where there is more volume.
Let's see if we can reason this out:
You mentioned being able to solder one side of the new capacitor to any ground available: Looking at Foggysail's solution, he chose to ground one side the new capacitor using a wire with a lug crimped to it, and attached that lug under one of the high voltage spark coil, hold-down screws. This would certainly provide a path to ground for one side of the capacitor.
The odd thing is, the Bosch circuit diagram does not show either side of the factory capacitor being directly grounded: One side goes to the anode side of the Thyristor and the other is attached to a ground path that goes through the low voltage windings of the ignition coil and to one side of the high voltage coil windings. At least that's what I read from the circuit diagram from the shop manual.
What I don't understand about Foggysail's solution, is why he grounded one side of the capacitor and why that allows the circuit to function properly to fire the spark. I would have thought the "fix" would entail circumventing the existing capacitor by disconnecting both capacitor leads and then connecting the new capacitor with wires which soldered to the appropriate circuit board connections, (after having cut the leads that went to the bad capacitor), effectively removing both sides of the bad capacitor from the circuit and providing the new capacitor the correct circuit connections to the Thyristor and to both sides to spark coil per the circuit diagram.
My understanding of electrical circuits is very limited compared to Foggysail's. Too bad we can't ask him what to do, but he hasn't been online since 2021. (or at least responded to a thread on the forum)
I think you perhaps you have mis-understood and mis-labled which is the primary and secondary spark coil connections in your labeled picture. I think you have reversed them; I have uploaded a picture which shows what I believe them to be. The Stihl shop manual circuit diagram shows the low voltage wire coming from the capacitor to the low voltage side of the coil to be the left wire on the right side of the spark coil and the right-most wire is connected to one side of the high voltage coil windings.
I have uploaded an edited picture which shows the wires labeled as I think they should be.
If I were you, I'd try to cut the lead on the circuit board that goes between the upper capacitor connection and the Anode of the Thyrister. (and the cathode of diode number 2) Doing so would isolate the bad capacitor from the circuit. Then if you can solder a new wire upstream of the cut lead (attaching it to the Thyristor and diode 2, and run that wire to one side of the new capacitor, it should be a simple matter to solder a second wire to the point on the circuit board which goes to the primary side of the spark coil (and secondary side).
I've uploaded a diagram showing the suggested locations. Severing the one side of the capacitor circuit should isolate it and enable the circuit to be completed by the new soldered wires and new capacitor located out from under the hot flywheel environment.
Examining the photograph you made of your stator with the epoxy removed, is very difficult for me to make out each of the electrical components and their soldered connections that you've exposed. So I can't tell you where exactly to cut, except to do so, by the upper factory capacitor attachment point to the circuit board. I can't tell where the Thyristor is located. If you could locate the anode side of the Thryistor, and solder one wire to it, after cutting the lead to the bad capacitor, you'd be halfway there. Then just solder the second wire to the wire that goes to the primary side of the spark coil. That should work.
Again, I don't know why Foggysail's solution involved grounding one side of the new capacitor, but my proposed solution, should work as well.
Hope this helps. Good Luck!
It is noteworthy that in the later design implementation, Bosch moved the capacitor to the other side of the "can" where there is more volume.
Let's see if we can reason this out:
You mentioned being able to solder one side of the new capacitor to any ground available: Looking at Foggysail's solution, he chose to ground one side the new capacitor using a wire with a lug crimped to it, and attached that lug under one of the high voltage spark coil, hold-down screws. This would certainly provide a path to ground for one side of the capacitor.
The odd thing is, the Bosch circuit diagram does not show either side of the factory capacitor being directly grounded: One side goes to the anode side of the Thyristor and the other is attached to a ground path that goes through the low voltage windings of the ignition coil and to one side of the high voltage coil windings. At least that's what I read from the circuit diagram from the shop manual.
What I don't understand about Foggysail's solution, is why he grounded one side of the capacitor and why that allows the circuit to function properly to fire the spark. I would have thought the "fix" would entail circumventing the existing capacitor by disconnecting both capacitor leads and then connecting the new capacitor with wires which soldered to the appropriate circuit board connections, (after having cut the leads that went to the bad capacitor), effectively removing both sides of the bad capacitor from the circuit and providing the new capacitor the correct circuit connections to the Thyristor and to both sides to spark coil per the circuit diagram.
My understanding of electrical circuits is very limited compared to Foggysail's. Too bad we can't ask him what to do, but he hasn't been online since 2021. (or at least responded to a thread on the forum)
I think you perhaps you have mis-understood and mis-labled which is the primary and secondary spark coil connections in your labeled picture. I think you have reversed them; I have uploaded a picture which shows what I believe them to be. The Stihl shop manual circuit diagram shows the low voltage wire coming from the capacitor to the low voltage side of the coil to be the left wire on the right side of the spark coil and the right-most wire is connected to one side of the high voltage coil windings.
I have uploaded an edited picture which shows the wires labeled as I think they should be.
If I were you, I'd try to cut the lead on the circuit board that goes between the upper capacitor connection and the Anode of the Thyrister. (and the cathode of diode number 2) Doing so would isolate the bad capacitor from the circuit. Then if you can solder a new wire upstream of the cut lead (attaching it to the Thyristor and diode 2, and run that wire to one side of the new capacitor, it should be a simple matter to solder a second wire to the point on the circuit board which goes to the primary side of the spark coil (and secondary side).
I've uploaded a diagram showing the suggested locations. Severing the one side of the capacitor circuit should isolate it and enable the circuit to be completed by the new soldered wires and new capacitor located out from under the hot flywheel environment.
Examining the photograph you made of your stator with the epoxy removed, is very difficult for me to make out each of the electrical components and their soldered connections that you've exposed. So I can't tell you where exactly to cut, except to do so, by the upper factory capacitor attachment point to the circuit board. I can't tell where the Thyristor is located. If you could locate the anode side of the Thryistor, and solder one wire to it, after cutting the lead to the bad capacitor, you'd be halfway there. Then just solder the second wire to the wire that goes to the primary side of the spark coil. That should work.
Again, I don't know why Foggysail's solution involved grounding one side of the new capacitor, but my proposed solution, should work as well.
Hope this helps. Good Luck!
