Can Anyone Explain This Tachometer Quirk?

DANG, Troy! I knew you were good at these things, but I didn't know you were MR. Science as well. Great troubleshooting skills you got there! 

It was just total luck.  If my finger didn't happen to also touch the case while I was wiggling the terminal connection I would have never discovered the fix.

I am however, very proud of winning the Golden Shield award at Marina High School for excellence in Industrial Arts during my sophomore year in 1965.  That accomplishment solidified my nerd status for the rest of my high school days, but for some reason, the babes just couldn't appreciate it!  

Troy, that's interesting juju.

I have a neighbor with a 5-year old Vintage.  His tach worked fine on the mechanical points distributor.  But when he switched to Pertronix (or Compufire, I don't remember which) electronic "points" his tach stopped working  --  sort of.  The needle moves, but maybe about 1/4th of what it should.

So what is the science behind grounding a tach terminal through a resistor to suddenly make the tach work?

"If I touched the case with one hand and the terminal with the other, the tach worked, but I also got zapped!"

Can you tell me if the zapping pulsed approximately to the frequency of spark pulses?

If so (and I'm guessing, here) the signal coming from the electronic ignition is too much for the input of the Brazilian tach's meter driving PCB, driving it to saturation and the output (to the meter) goes to zero.  When you touch the case (ground) and the input lead (the center tab in the back) you're bleeding off some of the input signal power (current, in this case) thereby making a weaker signal that doesn't overload the circuit and allows it to drive the tach meter movement properly.

Pure friggin luck that you discovered that, and I was thinking the same thing - The connection provided by your fingers is in the 100K to 500K ohm range at that distance so you lucked out in resistor value selection (probably from your outstanding geek work in high school, no doubt).  

So why don't you get zapped from one hand but feel the current when using two hands?  Well, more nerve endings (driven by electricity) are in the circuit between two hands versus two fingers of the same hand (that's the best I can come up with).

I love this story. 

Yes Gordon it was a pulse.  Not nearly as strong as what you feel when you get zapped by a spark plug wire.

Before I headed to Radio Shack, I used my multimeter to measure the impedance across the tip of my finger and came up with about 150k ohms, so I had a pretty good idea of what value I needed and bought several above and below that. They were 90% off and only cost about 35¢ for a pack of 5! 

The smaller values got a little warm, but the 100k resistors stayed cool to the touch. 

I need to try this on one of those twitchy Chinese tachs! 

There are a lot of old posts about fixing tach issues with a diode placed in line with the terminal and the orange coil wire, but this is different. 

You're still lucky in one regard - We're down to the last two Radio Shack stores in the state and they'll be gone in a few months.  When I was in high school, abot the same time as you, "You Do It!" Electronics just outside of Boston was the go-to place for electronic components.  Believe it or not, they're still there, and online, too.  Of course, it helps having a couple of dozen Engineering schools within a 30 mile radius!  

http://www.youdoitelectronics.com

Built my first electronics Science Fair project with their stuff - a small demo of dual J-K Flipflops (the cornerstone of computer hardware logic) - back in 1960.  NOBODY on the judging panel knew what the heck they demonstrated and I didn't even place.  Went away grumpy for a couple of years, then came back with a 400-watt Tesla Coil that would light any flourescent light bulb within 50 feet...Whether or not it was connected to anything.  THAT got their attention!

What Troy's not telling us is that his finger looks like this:

LOL.  Actually, I'm color blind and could never read those color codes. 

Color code goes something like this...."Bad Boys Rape Our Young Girls But Violet Gives Willingly, Get Some Now = Black Brown Red Orange Yellow Green Blue Violet Grey White, Gold Silver None = 0-1-2-3-4-5-6-7-8-9, 5% 10% 20% accuracy tolerances + or -

I had to reach back deep to remember this...Navy Aviation Electrician school, 1960

There's also "Batman Blows Robin (sorry) On Yonder Gotham Bridge.  Very Good, Will Get Superman Next."

Troy...Love your "horseback engineering" !  100% solid logic, measuring finger resistance to determine a ballpark value in order to select a resistor. That was priceless !..............Bruce

The other thing I may have lucked out on was the 1/2 watt rating.  Some of the resistors I bought were 1/4 watt and one great big one was a 10 watt.  I had no idea what wattage I really needed and just bought what they had in their very limited selection that was jumbled together in a big 90% off box full of all kinds of switches, knobs, thermocouples and other small Radio Shack do-dads. 

I would have bought the whole box......Nevah know when it'll come in handy.

That's why I have unused stuff I bought in 1970 still kicking around.  

Basic EE. My bet is that Gordon has it about figured.  If one were to inspect the Hollywood model's innards,, one might find a shunt resistor to ground in there that does the trick.  Or possibly a Zener or two and some other more sophisticated circuitry to take control of that input signal.

The tach was working fine in a Beck before I acquired it.

The tach doesn't care if it's in a Beck, VS or CMC.  All it cares about is 12 volts, a ground to complete the circuit and an input trigger signal (within expected limits).  Give it that and it's happy in whatever language it was created in.  

El Frazoo posted:

Basic EE. My bet is that Gordon has it about figured. 

I think you meant Fingered

Gordon Nichols posted:

The tach doesn't care if it's in a Beck, VS or CMC.  All it cares about is 12 volts, a ground to complete the circuit and an input trigger signal (within expected limits).  Give it that and it's happy in whatever language it was created in.  

Of course.  Then I guess there must be something different between the input trigger signal from the coil in the Beck and from the the coil in the VS.

Thanks Troy, Gordon;  

So you're saying to put a 100 ohm - 1/4 watt resistor between the center tach terminal and ground.  Nothing bad will happen and the tach in my post above might start working.

Sounds worth a try.  Thanks!

PS:  Color coded fingers -- LOL.  

No, I think Troy used a 100K (100,000) ohm resistor, NOT a 100 ohm.

1/4 watt is fine, 1/2 watt will work, too (it's just slightly bigger).

Just to be clear, the orange wire remains connected directly to the center terminal and the resistor is also connected to the center terminal at one end and the other end goes to the tach case, which is grounded.

My problem was not the typical "twitchy tach" problem we talk about all the time, but it's worth a try. 

OK, got it, thanks!  

Wife and I are taking off tomorrow morning for a month-long trip with our little travel trailer (Casita) through southern UT, northern NM and AZ , then back up eastern Sierras.  I'll get together with neighbor and a 100K resistor when we get back.

Thanks again!

@RS-60 mark sounds like you're off to a great adventure in the Casita! Safe travels, my friend.  

MusbJim posted:

DANG, Troy! I knew you were good at these things, but I didn't know you were MR. Science as well. Great troubleshooting skills you got there! 

Well:  

I would equate his brilliance to that of this well know person in the scientific community :

Quite the resemblance as well !!!

Tebs

You'll pay Teby, trust me...you'll pay BIGLY!

Flashback to Basic Electricity. 

I finished HS in 3 years but still had to go for at least 4 classes as a Senior. I wound up in a couple of Freshman classes including Basic Electricity. I think I forgot almost all of it except for the resistor color code. Amazing that I remember that. I think it was the part about Violet.