76REDSTINGER

is a corvette tach filter different from other GM tach filters?

the is a schematic for a gm tach filter the is 3 component
in stead of the 4 component one i found on this site

looks like this:

3k ohm 10k ohm
(in) -----^^^^-------^^^^^------(out)
!
!
# 47nF
!
!
(GND)

Radio Smack has these parts........

R.


sorry cant get this to stay lined uo so it makes sence

R.

mrvette

It should ot be all that criticality ONE.....be fine, IMO....

GENE

Bullshark

WTFO Why do you need two resistors?? If this circuit is correct (hard to tell cause I don't know the source or input impedance) couldn't you just use one ~13k ohm resistor? Dosen't look like sound circuit design to me. IMO

Bullshark

NHvette

The 47nF cap to GND connects between the 3k/10k.
As he stated - he can't get the text to line up.

My only guess here is that the tach for this filter may have a
significant input capacitance ... effectively adding another cap
to ground at the end. The 10k series resistor doesn't do any
filtering without a capacitor following it. I can't find the tach
filter schematic that I've seen before, else I would compare the
two for you.

ACECO

I think you answered your own question. Three is not the same as Four.

Here's the schematic of a vette tach filter.



Radio Shack has these parts too. If your going to build one anyways, why not just use what was there originally? FWIW.

Ybnormal

Just curious "while your at it", what is the purpose of the filter? I'm running my tach straight off the MSD box and seems to do just fine without a filter. I do notice a twitch in the tach every once in a while, will the filter fix this for me?

YBnormal...drive a vette

76REDSTINGER

no radio shack does not have some of these parts and cant get them
or else i would have built one already

ACECO

5.6K - 1/2W Resistor - RS Part # 271-1125
10K - 1/2W Resistor - RS Part # 271-1126
.1 uf ceramic disc capacitor - RS Part # 272-135
100 pf ceramic disc capacitor - RS Part # 272-123



If you want to e-mail me your address, I'll be glad to send you the components so you can build what you need.

Good Luck!

76REDSTINGER

i stand corrected

maybe just my local store sucks ?

thanks

R.

joe73vette

Isn't that supposed to be a .1 millifarad cap, not .1 micro farad?

Isn't .1 microfarad = 100 pf?


Joe

Eddie 70

Click on quote and you can see his original post with everything lined up. This should help with building this tach filter.

Eddie 70

A buddy of mine has a 75 and the tach is not currently working, is this the same filter for his 75. If so, I think I will try and build him one and see if we can get the tach working correctly. Can someone show me or tell me where the tach filter should be located so I can remove the old one and try the new one?

NHvette

.1 MFD is 'old school' terminology. It is really .1 uF
Musta been drawn before we started using metric system in US.

Eddie - it goes in series with the tach wire. If there is not one there,
cut the tach wire and attach as show in the schematic.

ACECO

MANY years ago Radio Shack used to have people that knew what they were talking about. Of course they used to be based around electronic parts too. Nowdays they sell so much consumer electronic stuff and gadgets, they have forgot what electronic parts are. And the help..... (at least around here) don't get me started. They don't even know what is in their own catalog.


Nope. mfd is microfarad. mfd = uf and is (if I remeber correctly as it's been a few years since I've done electronics breadboarding regularly) 6 digits to the right of the decimal point. Thus 1uf would be .000001 farad and .1uf = .0000001 farad. pf is 12 places to the right so 1 pf is .000000000001 farad and 100 pf is .0000000001 farad. Hope that helps.


Yes, should be the same for all C3 electronic tachs. The filter is located at the rear of the intake manifold, bolted to one of the rear bolts, drivers side. (at least on mine) It looks like a typical round metal can filter cap with a wire at each end and a tab welded to the side which is used as the mounting point. Simply jump across the filter w/a wire to test if it is bad. Assuming everything else is OK, the tach should begin to work if the filter is bad. However it should not be left like this as electrical spikes could damage the tach PCB.


It's there to filter out random electrical spikes from doing damage to the tach circuit board. It will work w/o a filter but low voltage electronics doesn't like spikes. That very well could be the "twitch" your seeing at the tach.


I'll make the same offer I made to 76RedStinger for anyone who needs the parts. IM or e-mail me your mailing address and I'll send you out, no charge, a set of parts and a piece of perf board to assemble what you need. All I ask is that you "Pay It Forward" and help out the next guy that you can on the forum that needs help.

Good Luck to all!

NHvette

I haven't been to RS since I was a kid.
Locally, we have a few places that would have these parts.
U-Do-IT Electronics is the one I remember - in the Needham MA
area.
There are good places online to order. I like Digikey - but they
have a minimum order plus shipping. Hardly worth it for a few
parts. Digikey also has the LED 1157 replacements at a good price.

So, I analyed both of the filter circuits. The results are in the
PDF file below. In a nutshell: use the original circuit and values.

http://NHvette.com/ftp/in/NHvette_filter_comp.pdf

ACECO

Hi Dave,

Thanks for the accurate technical input. I suspected the original circuit would do a better job but had no hard core evidence to prove so. Thanks for providing it.

I also use Mouser Electronics to get misc components when needed. They used to have a warehouse about 3-4 miles from my house but they closed that one a couple years ago and now I have to do mail order out of Texas. However they don't have a minimum so if you just need a couple parts, they will sell them to you. Of course shipping offsets most savings at small quantities, but at least the parts can be had if needed.

Thanks again for your input. Have A Great Day!

76REDSTINGER

excellent work !!!!!

thanks for the effort

R.

76REDSTINGER

dave

why the second (50pF) cap on the second circuit?

R.

NHvette

Rob,

It simulates the stray capacitance of the wire and the input
capacitance of the tach circuit board. I could only reason the 10k
resistor in the second circuit, if there is a cap following it.
Other wise, it does close to nothing. So, I came up with a number. 50pF
isn't much at all.
You'll notice I added this to both outputs. It could be more - have to measure it.

-Dave

76REDSTINGER

yes your right

i didnt think of that

R.

thats why im not an EE :o