[Stuart]

p/n 5657922 fits a 1972 A body car.

[Shiny]

Stuart - thank you for finding out what year the tach was used

George - I may need your help with the parts values, and definitely need more advice.

I repaired the broken cap lead.

I attempted to check out the tach, but did not get far. With engine off, I hooked up the 12V at battery +, attached the input lead (brown wire) to the negative terminal on my coil, and when I started to ground the case, I again got a lot of sparking that seemed more like too much current flowing. Without the car running, I suppose I could still be getting an inductive "kick" when grounding, but this seemed like a lot of current.

I am reluctant to simply attach the ground for fear of burning up components or wires if my tach has a short.

I will check resistance to ground on the 12V terminal, and if I understand your schematic correctly, should not see low resistance unless the engine is running and "triggering" the transistor.

What should I measure on my ohmeter between the brown lead, the red lead, and ground?

Thanks again

[Peter Serio]

Those tiny disc circuit boards were never meant to be serviced, they were a throw away item back when the cars were in regular use.
That is why the wires plug in instead of being hard-wired (soldered). Be really careful soldering, those cannot take the heat! As you have seen they have many surface printed resistors, to save money, = manufacturing costs. They also can make the board smaller and lighter. After all these years overheating a "leg" can easily cook one of the micro-thin resistors.

[Shiny]

Thanks Peter... all the resistors appear OK but I will check them tomorrow.

I did quickly check between the 12V and ground using my meter on "diode" setting. It beeps with either polarity, and to me, this means either the transistor is shorted or the zener diode. I don't know what the breakdown voltage on the diode would be, but I doubt my meter applies much voltage.

Because this ceramic "board" has holes for the components, it is pretty easy to replace parts.

If someone can confirm I've got a bad component, and tell me what to replace it with, I'll give it a try.

[george kujanski]

Use the ohmmeter function and measure between red and black. You should see some measurable resistance because R2 is in the circuit. Even if the transistor or zener is shorted you should still read the R2 Resistance. I'd say it should be in the range of 200 to 300 ohms. If the resistor is intact, connecting the tach to a battery supply you will get about 70 milliamps going into the circuit if the transistor or diode is shorted.

You could jumper across the zener to remove it from the circuit and then you would measure R2 directly, since it'll be the only component between red and black.

If R2 is good, have no fear of connecting the tach to the battery supply.

George

[Shiny]

Minor victory, but still losing this battle.

I opened the case, confirmed R2 was good, reading 700ohms.

Still showed a short from 12V to ground at the external connector....

Yup, I had managed to have the connector post shorted to the case. The round white plastic spacer was not centered in the hole... doh !!

So thinking I had this thing beat, I hooked it up, confirming 12V and ground, and NO TACH.

Step at a time, I guess.... any advice on what to do next? What is the max current range for the meter? I can verify that works pretty easily, then try to check the other components ??

[george kujanski]

The meters used back then were probably 100 uA (microamps) or 1 mA (milliamps).

Get a 15k ohm approx. resistor, connect it in series with a 1.5 volt battery to the meter. If it goes full scale, it's a 100uA full scale. if it goes one tenth of full scale, it's a 1 mA meter. Confirm the 1 mA with a 1.5k resistor.

I had one once where some little critter set up a home inside the center hole of the magnet, preventing it from moving.

George

[Shiny]

George,

Looks like game over for me..

The meter is not working. It moved a little with a 15Kohm resistor and a battery, so I dropped to a 1K ohm resistor (what I had) and it moved the same amount.

I verified 1.5ma through the resistor alone, but with the tach meter in series, the current flow was almost nothing. My conclusion is the meter got overloaded and is open or at least has a very high resistance.

By the way, I have checked this several times, and the transistor on my board is not wired per your schematic. The middle lead, which I assume is the base, is tied to 12V through R2 and not to the brown input lead as expected. Maybe someone tried to repair before and wired it wrong or maybe the transistor is weird being so old ?

I have no idea of the history on this thing, but I think it is time to move on unless there is a way to fix the meter...

Oh well...guess I know why it was given to me but I gave it a shot.

Thanks again for all your help and let me know if you can think of a way to salvage it.

[george kujanski]

Does the pointer move easily? check if there are foreign substances in the movement. I would expect the meter to require no more than 1 mA full scale, at least that's what I've seen in the past.

The pinout on those plastic transistors is not always the EBC, left-to right when looking at the flat side like on a standard TO-92.

George

[Shiny]

Yes, in general the pointer moves easily. It has a couple "rough" spots, but they are not the problem.

I think the coil in the meter has been blown open or something similar. When I use the ammeter in my multimeter to monitor current in series with the battery and current-limiting resistor, the tach meter is clearly acting like a large resistance. I would expect an ammeter to have very low resistance.

Have you ever taken one of these meters apart and put them back together successfully? I suspect it may be beyond my abilities, but I may have nothing to lose at this point.

Thanks

[george kujanski]

The movement should have no rough spots, but they shouldn't have hi resistance either.

The std meter has a squareish frame onto which fine wire is wound and it turns inside the magnetic core. The connections to the coil are made at the front and rear pivots; perhaps there is a poor connection there? There are also mechanical tweaks so there is a minimum play at the axle ends.

George

[Shiny]

George,

You are on target again. Go buy a lottery ticket. To quote Winston in Ghostbusters, "If anyone asks if you are a god, just say "yes" !"

I pulled the face off to get a look and could see nothing obvious. It was obvious, however, it was not going to come apart without unsoldering and resoldering some very delicate clock springs and there is no way I am equipped to do that.

Started probing with ohmeter and found high resistance in current path on white-wire input to top of movement. The offending connection was a clamped contact that feeds the current to the clock-spring, which feeds the rotating coil (see picture). This contact is between a brass washer and the pot-metal casting, so was just oxidized as it is not gas-tight.

The connection on the back side, with black wire, does not have this type of connection, so has no chance to degrade. It measured fine up to the clock spring.

I rotated the arm that the clock spring is soldered to, and realized it is the adjustment to zero the needle. Rotating this zero adjustment back and forth scrubs the evil contact and was enough to restore the connection !! I put the battery and 15K ohm resistor on the inputs and the needle went about 10%. I put the 1K ohm resistor on and pegged the needle !!

Other good news for me is the "rough" spots in the needle movement disappeared by loosening the bearing adjust screw on top just a little.

I will put it together and try it on the car again tomorrow.

[george kujanski]

Good going! You're going to be in competition with Peter Serio B4 long

George

[Shiny]

Actually I think I am competing with the 3 stooges...

Hooked it up to car, started, and the needle stood still and laughed at me.

This is starting to feel like it's getting personal.

So if I want to replace components on the board, just because nothing else has fixed this thing, can you recommend parts from radio shack or ??

Is there a source for replacement boards? If there is, I would guess I can't afford them. I have seen replacement boards for Mopar tachs advertised by a company called Real-Time Engineering, but they get $75 per board... I am waaay cheaper than that but don't mind soldering.

Also I spoke too soon on freeing up the needle motion. The needle sticks on its way back to zero and the adjusting screw has little effect escept to bump it loose. Is the adjusting screw on top for a jewel bearing? If so, and loosening the screw doesn't help, what else could be causing the binding? I will look in between the moving coil and housing but am now curious about the purpose of the screw.

[george kujanski]

I'm fairly certain the screw is used to adjust clearance of the bearing to the axle shaft.

If you need to make a board: experimenters use "perf board", essentially a fiberglass bd. with a lot of holes in it. You stick the component leads thru the holse and solder the leads together on the bottom side. Another version of that has the back side copper clad, so by cutting away copper cladding you can make traces on the back side and solder the leads to the traces. You would need to use actual resistors, of course.

You used to buy simple stuff like that a Radio Shack, but if you can't get it locally, go to www.digikey.com , they have all kinds of electronic stuff and ship quickly.

It's probably a just a bad component or bad connection tho.

I'll see if I can get some values for you. Measure the resistors if you have the time, and get the value markings from the large cap. Th smaller one is a .068 uF from the pic. The large one may be a .27uf if I remember correctly.

George

[Shiny]

What I measured:

R1 4.20K
R2 709
R3 1.93K
R4 4.07K
R5 411

Labeled:

C1 .068uF
C2 .39uF (picture attached)

I believe it is a bad component, as I have been over every connection now at least 3 times with my ohmeter. I also just checked the meter again with the battery and it is definitely working now. My engine runs, so there is voltage going up and down on the input terminal and the tach was getting 12V and ground direct from the battery.

I will chase the other components down if you can describe generic specs. For example, what breakdown voltage for the zener ? Any NPN transistor OK? What current ratings for the diodes? I assume the voltage ratings for the caps are the second part of the label ?

I would prefer to replace components on the ceramic board, as I have verified all the connections and resistors. I was able to solder it when I fixed the broken lead on C2, so this would be faster than starting from scratch with a perf board and I can use all the existing posts for the wired connections.

Thanks again

[george kujanski]

I'm taking this discussion offline, but if anyone is interested, I'll respond privately or back in this thread if there's a lot of interest.

George

[Held for Ransom]

Yours looks a little different than mine. And, yes it's a '72 A-body tach.

[george kujanski]

HFR: yours is very similar except it uses a smaller-case tantalum capacitor instead of the large metallized capacitor.

Perhaps they changed components because the large capacitor is massive, and may tend to fracture at the leads due to vibration, as Shiny found in his unit.

The tantalum cap functions the same, but is less massive. Yours was probably mfgr'ed later.

George

[Shiny]

George is the man !!

He was able to guide me through a zillion questions (and maybe a few blunders), but I was able to get this working. I ended up having to build a replacement board using George's schematic, but I had no problem finding the parts.

And thanks to everyone else who contributed info and advice.