Re: Ignition Waveform
Posted: Wed Oct 21, 2015 2:13 pm
First, there are early and late Smith's tachs, that have two transistors or just one respectively. Here is the circuit diagram for the "late" with one, which is what I have:
I replaced the capacitor, and the needle is steady but reading way too low (like maybe 1/3, I have an external meter I can calibrate against). Now I have to calibrate, and looking I see two adjustment screws:
(WHY didn't I trace circuits before putting it back together, you ask?) I only see one variable (plus the thermistor) in the diagram - can somebody tell me how to properly calibrate?
I replaced the capacitor, and the needle is steady but reading way too low (like maybe 1/3, I have an external meter I can calibrate against). Now I have to calibrate, and looking I see two adjustment screws:
(WHY didn't I trace circuits before putting it back together, you ask?) I only see one variable (plus the thermistor) in the diagram - can somebody tell me how to properly calibrate?
JGeezer wrote:bob_rich wrote:Hi JG
I think there are a number of reasons why your waveforms look odd and suspect there is not a real problem ( with the ignition at any rate ) when the points open , close, and the spark occurs at these 3 time points current around the circuit changes rapidly -- this can induce voltages in what might be thought of as "0V or earth" connections. Depending where the scope probe earth or 0V is connected this can give the appearance of some ringing as well as appearing to produce a voltage that seems to go in the other direction. the leading edge of the first pulse seems to show this effect.
The capacitance of the hIgh voltage plug leads determined partly by how long they are and how close to earthy stuff, it is not unusual to see each cylinders pulse looking a bit different. Rarely are all four plug lead identical in layout. Also in the dizzy the volts have to jump the dizzy gap as well and they may be variation here
Not sure what coil you have but they typically step up the voltage around 50 to 60 times so to get say 10000 volts, (probably enough volts at fast tick over with a high vacuum) at the secondary probably means around 180V at the primary. So your scope vertical amplifier system would be overloaded. Depending upon the scope how it responds to this overload may give false readings AFTER the high voltage pulse has occurred.
From your waveform it looks though the points closed period is around 2/3 of the time between pulses and this seems about right for a 60% or so dwell.
Regarding the tacho if you have an RVI type with the inner trigger loop ( when you open it up if there is only a single transistor then it is) then it is probably the capacitor across the base - emitter of the transistor. I seem to recall a 2.5uF at 10V tantalum or electrolytic will substitute for it OK. When mine went iffy I changed this capacitor and it is now OK.
Hope this helps best of luck
Bob
Thank you Bob, very clear explanation. Very cheap little scope (half the size of a phone), so I do only take it's waveforms to be a very general guide. I will be pulling the tach tonight.
This tread turned out to be very informative for me, I never thought about the capacitor interacting with the inductance of the coil on the rising signal, makes great sense.