56 minutes ago, crayZlair said:

Sorry, I was being rather facetious, thus my comment "Easy - Peasy 😄"

Proof is left to the student.        Haha!!

Edited November 30, 20204 yr by Captain Obvious

Many years ago, a friend sent his 260Z tachometer to Bob's Speedometer for repair. He was pleased with results.

https://www.bobsspeedometer.com/1/120/index.asp

There's also this one in my neck of the woods: http://www.hampspeedometer.com/

2 hours ago, Zed Head said:

Didn't I already say that, essentially?  It's in the comments about using the DMM to calibrate my tach and my simpleton remarks.  Kind of buried.

Actually, heyitsrama still has that damaged component to worry about.  Whatever it is.  Let's figure that one out.

And, on the capacitor question, he showed a nice shiny blue electrolytic capacitor in one of his pictures.  The side view.

Yeah! i figured i had it all apart, and I had some of the correct caps in my parts drawer, the original one seemed like it was working correctly. The varistor is tucked nice and cozy inside the tach in the dash now. I don't see it used on the 260z tacho that is on the bench. But I still need to get the the 280z tacho tuned with a known good RPM source as i messed with the potentiometer. I asked the neighbor if he had a wave form generator, he did not but he was able to make something with an arduino and a potentiometer, we tested 0-300hz @ 5v and the tacho did not respond. I think it might have been an issue with the waveform that we used, i did not ask him which one it was, ill ask him when i see him. I dont have a 12v transformer that i can use to test it out, that might be another option.

Are you using a 280z tacho with the GM HEI module?

 

1 hour ago, Captain Obvious said:

Crap. I was worried about that. I found nothing on the internets for either of those numbers. It could be as simple as a resistor or capacitor array, but I suspect it's not.

The reason I was worried about them is that I don't see any transistors on the board. And I'm sure there are some transistors at play somewhere in the circuit. They aren't driving the tach completely with just resistors and capacitors. Problem is if you can't see the transistors, then they're probably buried inside those packs. One could potentially infer the function by reverse engineering the rest of the circuit and try to figure out what is inside those packages. Glad it won't be me.  LOL.

In any event, it sounds like you're out of the woods and that effort is unnecessary.   

This thread says that a National LM2907 might be a good place to start. 

Here is a manual for that chipset https://www.ti.com/lit/ds/symlink/lm2907-n.pdf

Yeah super glad this thing is fixed now (thx to ZedHead..... again :p)

Spidya have a good selection of wav files for tuning their replacement electronics for our tacs.

Download here


https://www.spiyda.com/tacho-calibration-information

Use your laptop or other device audio out to connect to your tac - not sure if our tacs are as sensitive but it’s worth a shot.

And of course there is this ....


https://onlinetonegenerator.com/

Ps. I have a tac that behaves perfectly until it hits 25-30 degrees C ambient temp, then the needle goes stiff. I think as they wear they need a bit of lube where the movement is. The rolling road guy told me it didn’t go over 5k rpm but in fact when the tac was cold and the throttle stuck on my old engine, it would happily find 8k! Strangely that engine produced lots of healthy torque after a few such events ;)

Another option would be to attach the tach to your car and use some other device as a "reference" to calibrate the one you messed with. If you have a timing light with an RPM display, or an oscilloscope,  or even meter that will read frequency. You could use one of those to monitor the engine, hold the RPM steady at some level and then compare what you get on your tach to what you see on the reference. Adjust the pot until the two read the same.

Not as elegant as calibrating it on the bench with a signal generator, but if you don't have a sig-gen, then you do what you have to do.

2 hours ago, Captain Obvious said:

Another option would be to attach the tach to your car and use some other device as a "reference" to calibrate the one you messed with. If you have a timing light with an RPM display, or an oscilloscope,  or even meter that will read frequency. You could use one of those to monitor the engine, hold the RPM steady at some level and then compare what you get on your tach to what you see on the reference. Adjust the pot until the two read the same.

Not as elegant as calibrating it on the bench with a signal generator, but if you don't have a sig-gen, then you do what you have to do.

I think some ignition timing lights have RPM functionality... my old snap-on one does not.

@AK260 well when you open it up maybe you can check out that green/real varistor, take some pictures to continue this thread 🙂

You can use the AC Volts (Hz) measurement function on your meter if it has it to count coil discharges.  Not typical alternating current but the meter doesn't know that.  I'd guess that's what the automotive meters do, but with internal software to do the math and convert to RPM.

I use one of these when I'm balancing carburetors and setting the idle. https://www.amazon.com/gp/product/B0002LZU7K

It measures RPM and dwell, though I haven't played with the dwell function (despite the fact I should).

There are lots of other meters that can perform these tests. 

[mention=32052]AK260[/mention] well when you open it up maybe you can check out that green/real varistor, take some pictures to continue this thread [emoji846]

Will do. Been a tad hectic at home recently but will see if I can do it this week.

Back to how it works.  The signal from the coil is just a voltage pulse.  The capacitor(s) just "collect" those pulses and convert them to some sort of steady voltage that creates a magnetic field.  More pulses equals stronger field.  So, what components fail in a way that makes them fail sporadically, to make the needle jump then fall back like yours does?  Sometimes they work and sometimes they don't.  I assume that there are diodes involved also.  
Not an electronics guy, just like to dig as deep as I can.  The above is a simpleton's guess at how the tach works.  Feel free to correct.

OK, let me try to put my parents’ investment in my electronics degree to some use: you are basically correct even if you are saying it in an unconventional way. The circuit diagram I shared, while for a ‘70-‘73 model is basically the same in principle for all our “Smiths based” tacs (barring some small differences on the pickup side between the years).

The inductive loop coil “pickup” (looks like a mini transformer) gets energised by ign coil pulses and creates a small current. The transistors you see are in what is typically called a “mono-stable flip flop” configuration. Let’s not get our minds twisted as to how the circuit works but in short, a positive pulse at the base of one PNP transistor turns it off which momentarily turns the other on - and so on - so it flips and flops from one state to another driven by the signal going high and low from the ign coil. But why do we need it you may ask?

Well, the signal coming off the pickup coil is very weak and shaped like spikes. The flip flop circuit turns it into a square wave and it is effectively “amplifying” the current. That is a very crude explanation for the sake of brevity.

The signal from the transistors provides a current strong enough to drive the coil of the needle. The electrolytic cap (the one with a + sign) is “smoothing” the pulses and preventing the needle twitching with each ignition event. You may know that every time you turn off power to an inductor, it flips it’s voltage to keep the current going the same direction before the magnetic field collapses - hence the configuration of that capacitor.

I have to say, I’m quite surprised that a Zenner diode hasn’t been used to stabilise the 12v rail. Maybe it’s simply not been shown in that diagram.

CO is spot on, those multi-legged components are packaged electronics.

Right, I will shut up now and try to go to sleep! Insomnia sucks.

AK260, Isn't that what I said ? 😀😀😀

On 11/29/2020 at 9:01 PM, crayZlair said:

Basic tachometer theory: Digital to analog.

You take a digital signal (RPM, converted to PPS, pulses per second, also known as Hertz). You then convert that to a current to operate a meter movement from zero to full scale.

(I have a Degree in Electronics also)

Edited December 1, 20204 yr by crayZlair
added more for clarity

AK260, Isn't that what I said ? [emoji3][emoji3][emoji3]

I just scrolled up and saw it.

You did indeed - and in a far more eloquent / concise fashion.

(I have a Degree in Electronics also)

Geek alert!!!!! :p




Ps. Insomnia is still sucking, large!