Foggy, I have a much better understanding , thanks to your patience, of how this thing works. I will probably never see another one. Can you give a simple explanation of the timing of the gate that lets the scr conduct? Bill PS. Have you ever dealt with a ignition system in a Lawnboy push mower? It might work similarly but is enclosed in one unit. Thanks
SUCCESS!!! Just repaired Stihl 045/056 Bosch electronic ignition!!!!!
- Thread starter foggysail
- Start date
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I'll try...
If I remember right, there are 3 magnets in the flywheel. Two magnets charge the infamous capacitor. The other magnet charges a capacitor essentially located between the gate and teh cathode of the SCR. Also note there is a resistor across that capacitor to discharge it after the SCR is fired. The slow voltage build up on this cap reaches the voltage level that causes the SCR to switch to the conduction mode.
That voltage is in the range between 0.7 - 3 volts. When SCR conduction begins is very close to the times that both the infamous cap is fully charged along with the time the gate voltage in at the needed level to cause the SCR to switch to the on (conduction) mode.
Now there is something clever happening here! The resistor in parallel with the gate cap have what is called a time constant which is the time it takes for about 2/3 of that voltage to discharge. Time constant is the resistor value in ohms multiplied by the capacitor value in farads. Getting to the clever........ When the saw operates at higher RPMs--- the discharge on the gate capacitor is lessened, some charge remains. The remaining charge means that it will not take as long for this new charge to reach the SCR's trip level.
Amazing!!!! This is a form of spark advancing as a function of RPM!
Another Satisfied Customer!
I just finished Foggysail's fix for the, (as Sthil refers to), "the high tension condenser discharge ignition" or MHKZ on my 056. I used the Phillips capacitor and mounted it per instructions. Idid find out that there are timing marks on the armature plate and one on the crankcase, all I did was line them up, hardest part was routing the wires to prevent chafing. and screwing the plug wire into the ignition armature.
I cleaned the carb, fuel and oil tank, assembled it filled tanks with fresh oil and premix put the choke on and the throttle lock on second pull it started to fire so I opened the choke half way pulled the cord and my saw runs again. Thank you so much!
I have attached a factory schematic view of the ignition system
I just finished Foggysail's fix for the, (as Sthil refers to), "the high tension condenser discharge ignition" or MHKZ on my 056. I used the Phillips capacitor and mounted it per instructions. Idid find out that there are timing marks on the armature plate and one on the crankcase, all I did was line them up, hardest part was routing the wires to prevent chafing. and screwing the plug wire into the ignition armature.
I cleaned the carb, fuel and oil tank, assembled it filled tanks with fresh oil and premix put the choke on and the throttle lock on second pull it started to fire so I opened the choke half way pulled the cord and my saw runs again. Thank you so much!
I have attached a factory schematic view of the ignition system
My first post and although I'm necroposting, thanks and best regards for the fix. I'm searching for the cap now.
These ignitions were also used on a lot of Dolmars and my 153 (Poulan S6000) runs well until it gets hot then dies. Let it sit a half hour and away it goes for a while.
These ignitions were also used on a lot of Dolmars and my 153 (Poulan S6000) runs well until it gets hot then dies. Let it sit a half hour and away it goes for a while.
powerking
ArboristSite Guru
...I have my "new to me" 056 on the bench now.....I should be checking spark soon......Anyone else find those caps? I don't see them anymore on Ebay......
Guys consider---
These caps are found over in Ebay and should work for you. The important thing is that the capacitor be metalized (MKP) polypropylene film AND MAKE SURE IT IS NOT TUBULAR!!! The rectangular caps are made with a stacking construction whereas the round ones are wound on a machine. The round ones have what is known as inductance built into the device via the windings. A pulse capacitor should have minimal inductance therefore I recommend the stacked which will be rectangular.
Now of course other capacitors will work..........for awhile........maybe. Why take a chance after going through all the work of a modification? Other dielectrics have temperature issues so what I am/have recommended is the best for you to use.
Now----GO FOR IT and good luck. Just take your time, this is not brain surgery ......follow my instructions and you will have working saws.
Foggy
- 2pcs-PHILIPS (BC) MKP378 1uF 400V 5% P:27.5mm Film Capacitor - 222237852105
- $4.00
- Buy It Now
- 59 sold
- From Taiwan
-
2pcs - WIMA MKP10 1uF (1µF) 400V 10% pitch:27.5mm Capacitor MKP1G041006D00KSSD- $4.30
- Buy It Now
- From Taiwan
-
2pcs - PHILIPS MKP379 1uF (1µF) 400V 5% P:27.5mm Polypropylene Film Capacitor- $3.80
- Buy It Now
- From Taiwan
-
16PCS 1uF 400V
These caps are found over in Ebay and should work for you. The important thing is that the capacitor be metalized (MKP) polypropylene film AND MAKE SURE IT IS NOT TUBULAR!!! The rectangular caps are made with a stacking construction whereas the round ones are wound on a machine. The round ones have what is known as inductance built into the device via the windings. A pulse capacitor should have minimal inductance therefore I recommend the stacked which will be rectangular.
Now of course other capacitors will work..........for awhile........maybe. Why take a chance after going through all the work of a modification? Other dielectrics have temperature issues so what I am/have recommended is the best for you to use.
Now----GO FOR IT and good luck. Just take your time, this is not brain surgery ......follow my instructions and you will have working saws.
Foggy
I did a preliminary check today to confirm the timing of the Bosch unit.
Spinning it with the variable speed drill it starts advancing at about 800 rpm. The drill maxes at 1650 rpm. I didn't put the degree wheel on it as I just wanted to confirm that it has an advance curve. T= TDC F= fire A= advanced.
I have an assortment of caps. Each has slightly different characteristics so I'll be busy testing for a while.
On this Dolmar saw there isn't room to fit the cap mod in the flywheel area. I remote mounted it in the airbox where it's easy to access should it fail etc.
Spinning it with the variable speed drill it starts advancing at about 800 rpm. The drill maxes at 1650 rpm. I didn't put the degree wheel on it as I just wanted to confirm that it has an advance curve. T= TDC F= fire A= advanced.
I have an assortment of caps. Each has slightly different characteristics so I'll be busy testing for a while.
On this Dolmar saw there isn't room to fit the cap mod in the flywheel area. I remote mounted it in the airbox where it's easy to access should it fail etc.
Attachments
See my post 423 above. I explain how they provided advance capability.............BUT THANKS for actually TESTING IT!!!
powerking
ArboristSite Guru
have gotten more than a few things,,from both places,,no problem.. and had faster shipping to me, than from a western state!!!!!!!!
Sakman --- Picture shows a nice installation. Now I am asking YOU for a favor to help others. Please start a new thread explaining there is a fix for Dolmars that use this ignition AND REFERENCE THIS THREAD FOR INFO AND INSTRUCTIONS. There could be many others out in the great beyond who have this problem and be unaware of a solution. My intention is to help others and only ask for thanks in return.
Thanks
Foggy
Perhaps a more useful approach would be to start a new thread detailing the fix for the particular ignition module itself regardless of application.
.
.
Maybe. Many people who have a failed saw and even relate the failure to an ignition problem might not have a clue what ignition is in the saw. And take another look at this thread's title where I specifically stated the repair was for a Bosch ignition.
And the 1uf will work?Guys consider---
Do a search on Ebay for 1uf 400v MKP capacitors. There is a boat load of them offered for sale.
- 2pcs-PHILIPS (BC) MKP378 1uF 400V 5% P:27.5mm Film Capacitor - 222237852105
- $4.00
- Buy It Now
- 59 sold
- From Taiwan
2pcs - WIMA MKP10 1uF (1µF) 400V 10% pitch:27.5mm Capacitor MKP1G041006D00KSSD
- $4.30
- Buy It Now
- From Taiwan
2pcs - PHILIPS MKP379 1uF (1µF) 400V 5% P:27.5mm Polypropylene Film Capacitor
- $3.80
- Buy It Now
- From Taiwan
16PCS 1uF 400V
These caps are found over in Ebay and should work for you. The important thing is that the capacitor be metalized (MKP) polypropylene film AND MAKE SURE IT IS NOT TUBULAR!!! The rectangular caps are made with a stacking construction whereas the round ones are wound on a machine. The round ones have what is known as inductance built into the device via the windings. A pulse capacitor should have minimal inductance therefore I recommend the stacked which will be rectangular.
Now of course other capacitors will work..........for awhile........maybe. Why take a chance after going through all the work of a modification? Other dielectrics have temperature issues so what I am/have recommended is the best for you to use.
Now----GO FOR IT and good luck. Just take your time, this is not brain surgery ......follow my instructions and you will have working saws.
Foggy
I have my great-grandfather's 056, really hoping this will work.
Could you just dig the old one out and replace it? Also, why the potting?
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I'm not suggesting that I will do it any different, just curious.
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powerking
ArboristSite Guru
POC.....I think that is the reason they failed in the first place. the heat, kills the cap. Mounting it externally is genius!
.....as opposed to the SEM ignition also used on the 1115 series Stihl saws for those who know the difference.
Not trying to be contentious, here. Just trying to assist with some clarification in applying your request for a new thread detailing the same fix for other saws -- not just Dolmars -- that use the same Bosch ignition.,..., including some early Husky 2100's, etc. Most folks attempting to revive/restore any such old-school specimens most likely know, (or are capable of identifying), the type of ignition setup on their particular project. especially if it's already been determined to be the primary reason the saw has no spark.
This is one of the best threads to come along in a while and can indeed help folks other than 045/056 owners.
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