Tach problem?

Maybe someone can shed some light on this or give me a starting point.
To start: steel dash 1970 GT, new dist with Petronics ignition, new coil, car runs great. I didn’t install anything, I got it this way.
Problem, tach is not accurate at higher RPM’s. Tach also pins the needle when ignition is on but when engine starts it works as it should only inaccurate at higher RPM’s. (read low) and slightly unstable.
I just noticed that when I blow the horn the needle jumps. I’m stuck and don’t know where to start.
Any ideas or comments will be appreciated? Thanks in advance.
LHH

Is it the original tach from the steal dash? As they were set up for positive ground this may be effecting the tach now. I'm sure there's a way to wire it so it will work properly but possibly PO installed incorectly. 68 and up were (are) negitive ground.

Brian,
Good point , The car is neg earth from the factory and I'm pretty sure that it's the original gage

I've been doing some work in this area. These tach's are not especially accurate. They are also sensitive to the waveform of the ignition. I think they are a little tempermental with electronic ignitions. They will read high, low or bounce if the waveform isn't to their tastes.

There is a capacitor inside that can drift in capacitance. That doesn't help and it can be changed (the value is 2.5 uF). The transistor is germanium, so don't blow it.

There is an adjustment screw that can trim it - but I found you have to pick whether you want it to be accurate high or low - you may not get perfection at all RPMs.

I posted what I thought was the schematic a few weeks back. You can look at it if you like - I'm not sure it is perfect. Tracing PC boards is confusing and it was late.

Terry,
Thanks for the insight. I'm sure the tachs are not accurate although the tach on my 74 isn't too bad. This one is way off and never reaches over 3000, 3,500 with bounce. The jump when the horn blows is understandable from a spike. The needle 'pinning', with authority, when the ignition is turned on makes me believe that something isn't wired right. The coil was changed and I'm not sure it's the right value. I've no experence with trouble shooting this part of the ignition.
"The transistor is germanium". Transistor there's transistor in there? or do you mean diode.
Larry

Larry,
There were two types of tachs. The later ones (probably including your 1974) is voltage triggered. Basically a wire runs to the distributor/coil and picks up the voltage spike. The earlier tachs are current triggered. The entire current of the ignition coil runs through the tachometer. Changing the coil might affect it, but I doubt it would make that much difference.

Either tach can be sensitive to voltage spikes (since that is what they detect). A bad ground or bad power lead may cause jitteryness.

There is (at least in my early 1971 tach) a germanium transistor. The circuit is a monostable multivibrator. Basically an ignition pulse generates a fixed width pulse at the meter coil. The instrument integrates the pulses and drives the needle.

The circuit is a bit mysterious. Some clever engineer seemed to find a way to make one transistor do several things. I'm an electronics buff and still haven't figured it out completely.

And there may be several different circuits, so no guarantee that my tach looks like your tach internally.