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I have been following this thread with interest, lots of good info and thanks to all that have contributed. Relative to the issue of the vibrator and possible replacements, I just stumbled across a neat motor gear box in the latest catalogue from “American Science & Surplus” ( item 49163 , $5.00) that just might work if the proper lobed cam were installed on the output shaft. The motor is 1.5 to 12 VDC draws 250 to 750mA, at 9 volts the output shaft is 75RPM, Nice compact unit 3 X 3-1/2 X 2 1/2. … output shaft is 5/16 dia.
Might be worth while checking it out if one is in need of making a replacement vibrator. Someone smarter that me will have to figure out the cam lobe design at 75 RPM to give the desired pulse frequency.
Joe B
At 75 rpm the motor would need to have multiple cam lobes so that the points opened and closed at a respectable speed in my opinion. A point set on an engine running at 3000 rpm would be opening and closing 50 times a second if an engine was a single cylinder and the points were driven off of the crankshaft. If the points were driven off of a camshaft they would be opening and closing at 25 times a second since most 4 cycle camshafts rotate at half the engine speed. If the cam on a motor operating at 75 rpm had 8 lobes it would be opening the point set at 75 x8 or 600 times per minute which would be 10 times per second. This would equate to a very low rpm test of an ignition coil in my opinion and might not show up a problem that could be present at higher engine speeds. To increase the number of lobes on a cam to a higher number than 8 would probably result in a very low dwell number (the amount of rotation duration in degrees that a point set remains closed). A low dwell number could result in the point set not being left closed long enough to properly saturate the primary winding of an ignition coil before the point set opened and the coil fired. If you could find an electric motor that operated at a much higher rpm speed it might be better suited as a possible replacement for a bad vibrator. The other consideration is that there is not an abundance of room inside a Model 98 or Model 9800 unit for an electric motor.
Just as a matter of interest I will mention that in testing the original mechanical vibrator we found that the vibrator operated at a frequency of approximately 100 hz (100 cycles per second) and that during each cycle the two sets of contacts each opened and closed once. The duration that the contacts were closed created a duty cycle (closed time) of approximately 3 ms and an open time of approximately 2 ms during each cycle for each contact. This may sound confusing but the end result is that the vibrator emulates a set of points opening and closing approximately 200 times each second which would equate to a set of points opening and closing in an engine running at 12000 rpm if the points opened once every revolution driven off the crankshaft or 6000 rpm if the points were driven off of a camshaft on a single cylinder 4 stroke engine.
The electronic timer circuit I have been experimenting with and the circuit the other chap on the Smokestack website has actually working in his Model 9800 Mercotronic emulate the original mechanical vibrator very closely as far as frequency and duty cycle. The circuit that I have been working with has a frequency of between approximately 195 and 215 hz (depending on the voltage input 7.0 volts – 12.5 volts) and the on (points closed equivalent) duty cycle is approximately 3.5 ms and off for approximately 1.9 ms – this too varies slightly with the voltage level being used to test the coil. The other chaps circuit uses a small 555 timer module that can be purchased for less money than the individual components which is why he chose to go that route I believe.
One other thing we did learn from documenting the Model 98 and Model 9800 is that the original Mercotronic vibrators purpose was to emulate a set of contacts opening and closing such as what would be encountered in a points and condenser style ignition system. The two separate contact pins of the vibrator that feed interrupted current to the coil under test are actually connected together physically with a jumper wire in the vibrator socket. This is one thing that differs from how a radio vibrator is used as in a radio application one set of contacts powers half a transformer primary winding and the second set of contacts powers the other half of a transformer primary winding to create high voltage AC current at the secondary winding. We believe that the original vibrator that is used in the Mercotronic was designed to work in a 6 volt positive ground circuit. Since the original mechanical vibrators are no longer being made new as far as we know some people may be tempted to try a new all electronic version of a vibrator in a Mercotronic that is designed to work with older radios where the vibrator was used along with a transformer to create high voltage AC to power the tubes. I do not think they would work properly in a Mercotronic since most are designed to switch two transistors on and off – one transistor turns on for a certain amount of time and turns off and at the same time the other transistor turns on for a certain amount of time and then off and the cycle repeats. The time period between when the first transistor turns on and off and the second transistor turns on and off is so instantaneous that there is very little if any off time between when the first transistor is on and the second transistor is on in most electronic vibrator versions designed for radios from what I have been able to find out. This off time is needed in the Mercotronic to allow the ignition coil to actually have time enough to fire and oscillate properly before the primary windings begin to have current supplied to them again which is why we used a 555 timer circuit to power a single Mosfet transistor on and off – this allows for the proper off time the way the original mechanical vibrator contacts provided.
Hopefully this post does not bore readers but I thought I would provide a bit more insight into the original vibrators purpose which is slightly different that what a vibrator in a radio does.