70 LS1

What does the circuit board on the back of the tach actually do?

Is it a frequency to voltage converter? If so does anyone know what voltages would be full range on the tach?

The reason I ask is if it is indeed a F to V converter then I will probably get rid of it and make my own. This would allow me to use a tach with a speedo face on it to make an electric speedo that would be run by the Vehicle Speed Sensor in the transmission.

Thanks

Bullshark

Jeff, It's not as simple as what you are thinkiing. The Tach meter movement consists of two windings which are driven by sine and cosine generator circuits. Those circuits are driven by another circuit which integrates the amplified pulses off the coil. The original GM design used op amp IC's which are hard to come by today, so repair can be a challenge if its one of those that failed. Most of the time it's a resistor or diode that goes along with a PC board track. The repacement boards on the market today are a redesign using available parts. I have the old GM schematic for what it's worth.

Bullshark

ACECO

I'd like to have a copy of the schematic if possible.

Thanks!

70 LS1

Me too. That would be helpful.

Kilroy1024

Me 3 on the schematic.
I have schematics for the later versions which used an LM18919, but not the original circuit which used two IC's and discrete transistors.

As 70 LS1 suggested, the tach circuit is essentially a frequency to voltage convertor - actually its two F-V converter that are out of phase. (SIN and COSINE). I'm thinking the original circuits used two F to V chips, but I'd like to see the schematic.

If you want to build your own tach circuit, I would suggest using the CS8190.
You can buy them through Digikey - P.N. CS8190ENF16GOS-ND

70 LS1

What I need is to build an adjustable circuit that will drive the tach. That will allow me to use it as a speedometer and adjust it for different tire sizes and rear ratios.

Kilroy1024

"The CS8190 is specifically designed for use with air−core meter
movements. The IC provides all the functions necessary for an analog
tachometer or speedometer. The CS8190 takes a speed sensor input
and generates sine and cosine related output signals to differentially
drive an air−core meter."

CS8190

Bullshark

Vern, I took a look at the CS8190 earlier today and you are right, that thing looks like it can do the job pretty easy. On semiconductor even gives you the schematic. Thier design looks to be capable of driving the Corvette air core meter movements (225 ohm). Good put
The old GM design used two quad National Semiconductor op amps, The schematic doesn't call out the old obsolete IC part # and I forgot which one it was now. I would have to try to match it via the 14-pin DIP pinout. Probably not worth it since I have been told they are very hard to find.
I told Jeff I will try to scan the old schematic and PM you a copy, problem is it is in very small print and may be hard to read.

Bullshark

Kilroy1024

I received the schematic and can read it fine -
Thank you very much!
Its a good addition to my collection of tach info.

The footprint on this op-amp is just wierd - its no wonder replacement parts are hard to find.

The CS8190 should drive a 225 ohm coil with no problem.
However, its designed to drive both ends of both coils.
Thats 4 contact points as opposed to 3
A standard C3 tach has only 3 terminals.
One end of each coil is joined together into a common.

Bullshark

Yes, I did notice that, I wonder how hard it would be to seperate the windings at that common connection internaly. I haven't had to go inside that deep on these.

Bullshark

70 LS1

I think you could use that, but you would have to take the outputs of the CS8190 and run them through another circui. The stock tach outputs are 0-10V where the two outputs fron the CS8190 go from -7V to +7V. Once you condition the signal, you should be able to tie the two (-)'s together and feed the tach.

I think it may be easier to use a stock or replacement board and just modify the frequency coming from the PCM to calibrate it.

I actually think I can place a speedo face on a second tach and use the PCM to drive that as well.

Bullshark

Haven't sat down to analyze in detail,but It looks to me like the CS8190's outputs could provide the required current thru the two 225 ohm meter windings that would cause required movement deflection if it could drive the windings independently. I do agree though that some output conditioning is needed to enable the two (-)'s to be tied together. Using the CS8190 isn't as easy as it first sounded given the meter winding configuration.

Bullshark