Diode board replacement

[dogshome]

Love it. 

Are you have two pick up coils and twin ignition amps and coils then? Oh, no, one coil one pickup and dual reluctors.

The brief bit I read on the HEI is that it retards coil charging at low revs to conserve energy, then advances it as RPMs go up and uses current limit to saturate the coil without burning it. There is a feedback loop in there for that but I didn't think it altered the actual ignition point i.e. so it is just getting the two pickups to trip at exactly 180 apart physically that is the problem? The bean can isn't a great diameter for fine adjustment.

[mrclubike]

Just one trigger but 2 lobes on the reluctor

In the screen shot below
The lower trace is the ignition coil primary voltage on the NEG side of the coil 
Upper trace is the trigger coil out put
 
point A is were the dwell starts 
point B is where the current limiting of the coil starts (drops voltage to around 4 to 6 volts I think
point C is where the spark occurs

It adjust the dwell according to the amplitude of the trigger coil output
The dwell time is not that  critical on a 2 cylinder compared to a 8 cylinder
So as long as I get it to spark at cranking speed I should be okay
I may have to bump up the base time a couple deg because of the slight retarding caused by the dwell lengthening
 
When I was testing it on the bench with the drill press as a distributor tester  the coil stayed cool and the Ign module didn't get hot at all as long as it was clamped to a heat sink 

Drill press is the same as a Pillar drill

[dogshome]

Some pics of regulator. I can measure 0V drop across the IRF4905 MOSFET (low RDS of 0.02R) and as expected the switching is very positive with the hysteresis. Voltage looks good, so will fit in the bike. Note higher current on the same bulb test load than the cheap ebay one as the regulator is not dropping any voltage. Doesn't need a heatsink really 3A X 3A X 0.02 = 0.18W.

[mrclubike]

It will be interesting see what the charging voltage is at 1100 rpm 
It has to be better than the stock regulator

[dogshome]

More fun and games. The MOSFET regulator works, but the capacitor values are far too slow for our little alternators. The field collapses when it switches off and then takes a second to get going again. Car alternators (for which this is designed) probably have a much bigger field with more inductance (more iron basically) which will tend to hold up between switch points. I will pull the 100uF and 10uF and put (say) 1uF and 100nF.

Anyhow, now the original part is off the bike, I've tested it. It goes off around 14.25V and back on at 14.05V. The transition is linear-ish due to the BJT output. That is similar (but not as slow) as the cheap Ebay type. However it does only drop 0.7V when fully on which is expected and good.

It's pretty easy to put a MOSFET output stage in though like this (see pics). The Darlington pair of BC327 and BD244 is removed, the base (now gate) resistor of the MOSFET increased to 2K7 and where the base and collector of the BC327 linked out. This works in the same way as the original (+-) but has no drop when the FET is switched fully on.

Pics.

When the BJT is off, the FET sees -9V which is plenty to turn it on.When the BJT turns on, the FET sees near full voltage which turns it off.

[dogshome]

The 100uF cap replaced with 100nF and the 10uF with 10nF. With the hysteresis and looking at the (basically speed-up and RF filter) caps in the Darlington BMW design I hope this is fast enough! I love the smell of burning fresh epoxy-on-soldering-iron in the morning  I like that one, so here's another 

[This all counts a Continuous Personal Development I suppose. Plus what else would I do on a Monday night during an increasingly annoying pandemic!]
https://youtu.be/Qu5bRDmlrbI

[dogshome]

The smaller caps worked and she keeps the fire lit on the stator without dropping out now. Idle voltage and 2,000RPM volts about the same as they were (12.5 and 13.8 on my USB socket voltmeter), but the engine is stone cold and I'll see once it warms up. After measuring the standard regulator, I'm not expecting huge changes. Like the loss of 1 gramme on the hummingbird (size) of the alternator, it might make a difference. I'll also put the DVM on the battery itself to see what is going on there.

It went from -3 to +10C today and the difference in starting performance is immense. Cold, thick oil, high compression and cold battery.

The standard electronic regulator design is obviously spot on and with a tweak of the pot can be upped a little for AGM batteries. Changing the BJT output devices to a modern enhancement mode P channel MOSFET is also extremely simple and functional. The IRF4905 is rated 74A /260A pulse vs the BD244 6A /10A in the same package. What a difference 40 years makes.

[mrclubike]

I stared my R65 at 19F deg
When I first hit the button the motor would not turn over
I thought maybe I hyd locked it because I left the fuel turned on
So I  turned  the motor over  a couple times by hand and I hit the start button again and it turned over and started
It was probably not a good idea to start it at that temperature
Some times the oil filter can collapse because of the thick oil
It is synthetic so it's  probably OK   

[dogshome]

A run around the town tonight and the alternator is behaving exactly as per the original BMW part. The local motorists are also behaving as they usually do, good, bad, hopeless and arrogant  Yes I know your 5L Mercedes does 200 MPH, but I can nip off from the lights faster than you. It's a bike, live with that! Going past me at 70 in a 30 a few seconds later is not going to change that or make me ride unsafely. On you go, have a good evening.... Destination F.

Conclusion: The original part is obviously tuned to the somewhat puny alternator. i.e fast response to keep the field on the boil. If the device has failed, than an easy swap is to a modern MOSFET which should last forever and then some. Otherwise (apart from maybe tweaking the voltage up a tad on the pot to match modern AGM batteries) leave alone