1Z87L8

ok i know my circuit board is shot, see
http://forums.corvetteforum.com/show....php?t=1417121
for photos.

well i dont need to test the tach.found this upon further inspection.







so, do you think it can be saved?

ACECO

If the IC chip is still good, the other parts can be replaced. What are the stripe colors on that burnt 2 watt resistor, starting at the red end? I can't tell for sure from the pic.

Kilroy1024

The resistor is a 20 ohm.
The diode to the right of the resistor is probably gone too.
(Black with white stripe).
Replace it with an 18V/1W Zener diode.

ACECO

Do we know what the IC chip is? Based upon the technology back then, perhaps some standard TTL 7400 series or CMOS 4000 series chip?

Kilroy1024

National Semiconductor LM1819 Air Core Meter Dirver.


A copy of the data sheet can be found here:
LM1819.PDF

1Z87L8

im at work now but i will post another photo later as i dont think
it can be saved.

upon closer inspection when it let go it burnt deep in to the circuit board. if one of you EEs would like to take a shot at it after the new photos let me know and i will ship it to you.

gq82

Do yourself a favor....replace the board.

ACECO

PM sent.

ACECO

I received Rich's Tach PCB and have been able to identify a couple of the parts. However as the one corner has been rather toasty at some point, it has heated some of the surrounding components which have lost their colors/markings. Thus if anyone could let me know if my "guesses" are right based upon the following picture, I'd appreciate it.

TIA to all who respond.

66grandsport

I got a tach replacement circuit board from eklers and now my tach works but it is off 2000 rpm when over 1000 rpm. it junk waste of time and money. Can I possibly find what is wrong with old board and replace from new board. Other solutions. My old board is not burnt-it looks fine.

Kilroy1024

Well, you did a pretty good job of reverse engineering the board.
However, I would say part 6 looks like a 5.1V zener diode.
1N4733 or equivelant.

What I've done is cross reference the parts on the LM1819 Schematic to the parts on this PCB image.

I would start by changing R3 and D2.
D2 is a zener diode which typically fail shorted.
This causes R3 to pass close to an amp of current which eventually burns it out.
No other parts may be damaged.

I can give more info, but I'm not sure everyone else wants to wade through it.
PM me if your interested.

Kilroy1024

The LM1819 schematic specifies 470uF for C3.
The boards in the pics are using .1uF, after a 20 ohm resistor.
Its just a hunch, but I'd like to see that .1uF increased to 100uF/35V.

ACECO

Thanks Vern. R3 is obviously shot and D2 basically fell apart when I removed it from the PCB. However R4 was also starting to burn the PCB under itself so I thought as long as I was "in there", I'd change everything in that corner of the board. As I have no good way to test the board w/o setting up some kind of test jig or opening my dashboard, I thought that changing everything couldn't hurt before I send it back to Rich. I have many of the parts already and the others are nickel and dime stuff so why not at this point.

The image you reference shows a 140 ohm resistor as R4. I've seen other images of repro boards w/150 ohms. I know 150 is within the tolarance of a 10% 140 ohm, but is it supposed to be 140 or perhaps just on the 80/81 boards? I see the 1819 typical schematic shows 120 ohm, but also in a 2 watt version. As 120 and 150 ohm are "standard" values, perhaps this is why?

Your thoughts?

Thanks again for everything!

Kilroy1024

The tach uses two coils mounted at a right angle to each other.
A vectored magnetic field is created by varying the voltage across the coils.
Note that if all the connection came out, you would have 4 wires coming out of the tach motor. BUT, you don't you only have three? One end of both coils is joined together to create a common voltage point.

This point is tied to a 5.1 volt zener which sets the common point at approximately 1/2 of 12V. By varying the other end of the coils, you can spin the tach 360 degrees.

Eg: If 10.2 Volts is applied to the other end of the coils, you have will have +5.1V across that coil (10.2V-5.1V). If 0 volts is applied to the same end, you will have -5.1V applied to the coil. This reverses the magnetic field and spins the pointer 180 degrees.

To summarize, the value of the 120/150 ohm resistor isn't really all that important. What is important is that the common voltage be about 5.1 volts. Power it up and measure the voltage at the common. If its 5.1 to 5.5 volts, its ok.

ACECO

OK. I think I get it. R4 is just the current limit resistor for the zener, and a few milliamps either way isn't going to make a difference.

It figures that the two components I don't have are the 20ohm resistor and the 18V zener. I'll get some shipped tommorrow and will report back as to the results.

Another question. Couldn't I test the board by using a signal generator as the coil input and measuring the shift in voltage from one coil to the other? I have an old EICO signal generator I use to align old tube radios. I'm assuming this should work?

Thanks again.

Kilroy1024

Yes, absolutely. RPM = Fin * 15.
Square wave input, 0-5V should do it.

theoUK

I guess you will get a response by using 0-5V square wave, but just out of interest you might like to know that I hooked a scope up to the tach output of my 1980 HEI dizzy, and the waveform was more like 0-30V. From what i recall it was centred about 14V, then would swing from 0 to about 28/30 every pulse. So basically 14V +/- 14
Also a very good way of checking the effectiveness of the tach filter, the signal was much cleaner post filter than pre-filter!

ACECO

So then if the 5.1V zener is the center reference point for the tach coils, couldn't the tach movement be "trimmed" by varing this voltage reference? Now that I type it out, I think this would only effect the zero point, huh?

I'm trying to understand why so many people seem to have issue w/the linear progression of the tach using repro boards. In other words I hear complaints of being OK at one RPM but way off at another. Obviously there must be a way to adjust this? Any thoughts?

Parts for Richs board "resurrection" have been ordered. Hope to be able to bench test it later this week. Will report back then.

ACECO

The parts arrived and I replaced them on Rich's tach board. Before pic is in Rich's post above, and after looks like this:



A few new parts, a little adheasive to seal the open fiberglass strands where the old resistor had burnt into the board and connection wise it LOOKS as good as new. OK well at least perfectly serviceable.



At this point I realized that my signal generator being an older unit for RF work, didn't go much below 100Khz, way to fast for this application. So I whipped together a little square"ish" wave generator to test the boards outputs. Based upon Kilroy1024's formula, the "test jig" should be simulating approx 1200 to 6000 RPM input to the tach board. Here's a pic of the test circuit:





Hooked it all up with a bunch of clip leads and I actually was able to see/measure a swing in the two tach output voltages as I varied the input signal (rpm). At that point I went back and added the proper wires/clips so everything could be easily connected together. Here's a pic of Rich's PCB under test.



The only thing I really can't tell is if the tach is going to be linear, indicating the proper RPM across the full useable range. However Rich will let us know more when he gets it back and tested.



All in all it's been an interesting learning experience. And now that I have the knowledge and parts, if I can help anyone else with a "bad" tach board issue, please don't hesitate to contact me.

Have A Great Weekend!

Kilroy1024

Looks like a great job!

The D.C. voltage measurement is a good way to test.
You should see the SIN and COS voltages swing from 1V to 9V, or so.
(Not at the same time.)

The tach should be as linear as it was before you fixed.
The critical parts are the ones in the tachometer charge pump.
(Datasheet, Figure 7) R1, R2, C1, C2
And of course the IC.

Looks like the only one you changed is C2.
I think its purpose is to reduce gain at high frequency.
Its 20% tolerance anyway.

Good work!