Tachometer repair?

[sam am I]

Grasshopper,

Refer to included pic and drawing below (incomplete BUT most likely enough) ....

By Inspection and your measurements/statement, Q1 (MPS2222) smoked! If this transistor ("xster" for short) opens from emitter to collector (the xster blows up in such a way and doesn't function properly), your tach will never come on, e.g., it's dead because it's not getting power on pin 13.......Q1 job is to turn on pin 13.

How does it do that? I'm glad you asked......

Evil zener diode(EZD) D1 (18V?) and its evil counter part R1 (xx Ohms) sets up a base voltage on Q1 provided by the +12V coming in from your boat. Q1 is configured as a "emitter follower". Sooooo, the voltage the emitter will follow the base voltage less 0.5V.

Ve = Vb - 0.5V (eq. 1)

EZD "clamps" the voltage on the base to 18V or less and its evil partner R1 limits the current. This in-turn protects IC1 from seeing too high of voltage on its pin 13 in the event your boat 12V get crazy high somehow(more on that later).

Why did this xster blow up? I'm glad you asked........

The current path that would blow it up is into the emitter out the collector and to the battery (electron flow for the techies, more on that later) ......If C1 and IC1(and its external components), either of those items short or goes near ground, that xster will get hot and/or pop(explode).......

Why is the copper trace burn't? I'm glad you asked........

That current path to heat that trace up is from ground through EZD D1 and into the base and out collector of Q1 and/or in to and out of R1 back to the battery (electron flow for the techies), the ONLY way that burn't is ...

1) The 12V went waaaaaay up past 18V and R1 was the only thing limiting the current.

2) The 12V went up past 18V and Q1 shorted b to c. (R1 won't be able limit ANY current with a short in parallel)

3) EZD shorted or went down in value, say to 5V or something. (recall I/we never trust them, my gut says we'll be back here)

Next?

Replace the xster, AND add a temp inline 1 amp fuse into the 12V feed BEFORE you apply power to test this thing.

Once the temp fuse is inline, apply power and ground to the tach and if the fuse stays good, test pin 13 of IC1 for around *11.3V. If all is well, hook up the sig and test.

If the fuse blows, IC1 and/or EZD are probably close to if not shorted, we'll address this if need be.


* See equation 1.

 

[esanford]

Alright, I finally found time to replace xster Q1. I powered up the gauge with a 1A fuse, and it did not blow I now have 12V on pin 1, but nothing on pins 2 or 3. I also have nothing on pin 13 of IC1. I see a small brown mark on R1. I'm suspicious that this resister is open. I've read a little about emitter followers, but I'm not clear on how to verify my suspicion.

 

[sam am I]

Not sure how you're getting 12V on pin 1 w/o anything any voltage on pin 13? (See below pin-out of IC1 to re-verify pin-outs), However...........With 12V applied

1) Check for 12V on collector ("c") of Q1 (See below for pin-out Q1)

Vc good? Go to 2.

Vc not good? Burn't open copper trace? Check path from 12V stud to collector ("c") of Q1


2) Check for 12V on base ("b") of Q1(See below for pin-out Q1)

(R1 and D1 just form a voltage divider of sorts.... So with 12V on one side of R1 (15 ohms?) and its other side connected the cathode of a grounded 18V zener (D1) , Vb of the xster Q1 should equal to around 12V)

Vb good? Go to 3.

Vb not good? Power down, check ohm value of R1, should be around 15 ohms (guessing and as per chip manu)

R1 good? Go to 4 and 4a.



3) Recall Ve = Vb - 0.5V........Check Ve for around 11.5V

Good? Re-check pin 13 for 11.5V (See below for pin-out IC1)



4) Power down and follow 1st half of this video, e.g. With meter on "diode check" check diode's D1 PN junction.

4a) Power up and follow 2nd half of this video, e.g. With meter of DC voltage, check Vz (meter + from cathode, meter - to ground) of D1.

 

[sam am I]

Addenda ......(Red)

Vb good? Go to 3.

Vb not good? Power down...

1. Check ohm value of R1, should be around 15 ohms (guessing and as per chip manu)

R1 good?

1. Check continuity between base ("b") Q1 to proper side (NOT 12V/Vcc side) of R1 and to D1's cathode (striped side) as there is that burn't trace.......did it open?

Continuity base Q1 to R1 and to D1 cathode all good? Zero'ish ohms....

Go to 4 and 4a.

 

[esanford]

sam am I thank you for the detailed troubleshooting steps. You're right, I misspoke, I have power on pin 3 (C) of Q1, not pin 1- I was looking at it backward. No voltage on B or E of Q1. The resistor tested out at the nominal 470 ohms (which is good, because I ordered 4.7k ohm resistors- that brown multiplier band sure looked red to me!). So I disconnected the evil zener diode, and it reads shorted. The markings look to read 1N4741A, which comes up as an 11V zener. I'll order some up HERE unless you think there's anything else I should check. I see you mentioned that a shorted D1 would blow my 1A fuse, which it is not.

 

[sam am I]

thank you for the detailed troubleshooting steps.

Np, it's my pleasure, being cheap that is

disconnected the evil zener diode, and it reads shorted.

Sorta thought we'd be back here.

The markings look to read 1N4741A, which comes up as an 11V zener. I'll order some up unless you think there's anything else I should check.

Works for me and no, well yes!!.........Check D2 (use meter's diode check function, test its PN junction), it should be okay since R1 was good, best check it though. So if that checks out, think you'll have it licked with this.

I see you mentioned that a shorted D1 would blow my 1A fuse, which it is not.

R1 being good and its high'ish value of 470 ohms saved our butts by limiting the current

12V/470 ohms = 25mA


So with a (working) 11V D1, pin 13 on IC1 should run at around 10.5V. Also, once D1 is up, go back and check Pin 1's EZD running voltage, the data sheet says should be at around 5.4V.

 

[esanford]

sam am I D2 reads shorted as well. That's a 1N4733 5.1V zener. I unsoldered one end of each of these and am checking with the diode / 2k ohm setting on my DMM, I hope that's right. Any further suggestions knowing that?

 

[sam am I]

Interesting why and how diode D2 went, hmmmm....D1 went because Q1 shorted b-->c, this effectively put a wire across limiting resistor R1. So (12V'ish-11V)/0.001 ohms = WAAAAY over current = POOF!

However D2 shorted could mean bad news bears for IC1..........If I were to hazard a guess this far in and being a forensic failure kinda guy when I'm not dodging forest fires and GIANT smoke plumes, did perhaps power and ground to the tach get reversed?....... This comes by the nature of what, how and which components all have blown up.

At any rate, IC1 could be smoked as well being D2 is shorted and R2 is okay?

Sooooo,

It looks so far like the passive components (resistors) are all checking ok, so you might have to replace IC1....

Best you can do at this point is replace D2 (also D1 and Q1 of course) and since this covers all the active components in the power circuit, is to power it up (with inline fused still) with the 3 new active parts and make a few measurements.........

Once powered up and IF, you have power where it's suppose to be and the power/s are all at the levels they're suppose to be AND, if the tach is dead (we know the meter movement is good, you proved it was [see post #10]) AND, if we're still firm that the passives all are ok, you might have to replace IC1.

Lets get'er powered up though first eh?.............


If need be, IC1 and its cross are both obsolete BUT, you can still buy the cross here

 

[esanford]

sam am I, alright I replaced both zeners. Now I have 13.4 on Q1 c (pin 3), and 12.8 on Q1 b (pin 2), but still nothing on Q1 e (pin 1), so of course nothing at pin 13 of IC1. I'm 99% sure I have Q1 in there correctly, and pretty sure I put my zeners in the right way based on which end the stripe was on on the old ones, double checked with the pics. Is my new MPS2222 bad?

 

[sam am I]

but still nothing on Q1 e (pin 1)

Hmmm....

Now I have 13.4 on Q1 c (pin 3),

I assume your in your boat and it's running with the 13.4V?

and 12.8 on Q1 b (pin 2),

Should be 11V tops if D1 is a 11V zener and R1 is anywhere near 470 ohms.

Is my new MPS2222 bad?

Not sure...........


With the 1 amp inline fuse still in place with the tach's battery +12 stud , short out (use a jumper wire, etc) Q1 e--->c (pin 1 to 3) and fire up the boat/13.4V....



Fuse still okay?

Pin 13 have 13.4V on it?

Pin 1 have 5.1'ish V on it?

Tach work?