Tachometer kaput, how to fix?

[Zendalar]

My tacho was working just fine after i installed new ignition system, but after a week, it just died. Now it has been almost a year since i saw it working and today I decided to open it:





Sometimes when i shut off the engine, the tacho just slightly bounces... Should I just change the transistors and check the diodes? All 3 transistors are BC 170 C, this common today?

Is there any way to test this system to check out which component to change?

[jstobo]

Send to North Hollywood Speedometer.

[Zendalar]

Hehe...I live in Finland, and merely a postage to one way is 40 bucks minimum..And I recall that there were some people here the board that have fixed these and i was just wondering if there was a common thing that usually breaks.

[jluetjen]

You can get an S tach off of eBay for a little more the $100 + shipping from the seller. In fact I just sold one 2 weeks ago. There's another up there now. Unfortunately, all that I've got now are 914-4 tachs. I would bet you that a T or E Tach goes for quite a bit less then that. If you ask around I bet you someone on this BBS will have a nice one that they'll sell you.

As far as that transistor that you mentioned, I did a quick search on the web and it didn't turn up anything. I tried checking in one of the distributor web sites (www.digikey.com) and still came up with nothing. Given the trouble involved in trying to fix it, I'd just buy another.

[POKDUB]

I would do the following checks to isolate the problem.
1. Is there a signal feed coming from the engine (coil, scr, distributor, where ever connected) ?

2. Check the working of the meter coil only, disconnect greens wires from PCB and run 0mA to 10mA through it (through battery source and variable resitor). V=IR. See if needle moves.

I am in the process of fixing my tach as well. It looks similar but uses a IC chip instead of transistors. I could not find a supplier of chip.

Look at this link for possible alternate circuit. I plan to build it if I cant fix my pcb.

http://www.rgp.nl/bird/BritishWheels.htm

[Zendalar]

Thanks.

Seems to me that I can't get the needle off the plate, so i will just change whatever i can from the circuitboard..that being the electrolytic capacitor. These have tendency to dry. If this does not work, then it is to manufacture a new tacho.

[Zendalar]

Oh yea, i thought i would add this:

http://www.niksula.cs.hut.fi/~mdobruck/siililand/mini/diy/alien/tacho/tacho.html

[axel mtz]

hi to everyone, I have a question that is possibly already under discussion, I have a tachometer from a 911 from 1970 and I am trying to find a replacement for the bcz10 and ac128 transistors, has anyone been successful in finding the correct replacement or is there an equivalent number, thanks

[Walt Fricke]

Axel
Have you tested these transistors and found them wanting?

My take on repairing electronic stuff with some age on it is to replace all electrolytic capacitors. They are far more likely to fail than a transistor. Some you can pretty well guess have failed, because an end has bulged out, but it seems best, unless you want to remove and test each one with your capacitance meter, just to replace all of them. Can't be too many in this application.

50 years is a lot to ask of an electrolytic, though they can last longer than that also. My homemade power supply has a very large capacitor about that old, and it still works fine. But it is in a much thicker case, with screw terminals, than the small caps which would be in a tach.

I have a computer monitor which failed. Without benefit of a schematic or real knowledge of how it works I replaced the caps, and it went back to working fine.

As to the transistors, I assume a Google or the like search didn't turn up anything? You might try some other search terms, like obsolete transistors or obsolete transistor replacement or other variations - might turn up a document which Google directly can't find - a list in picture format, or some other format Google can't word search other than a title.

[mysocal911]

Originally Posted by Walt Fricke

Axel
Have you tested these transistors and found them wanting?

My take on repairing electronic stuff with some age on it is to replace all electrolytic capacitors. They are far more likely to fail than a transistor. Some you can pretty well guess have failed, because an end has bulged out, but it seems best, unless you want to remove and test each one with your capacitance meter, just to replace all of them. Can't be too many in this application.

50 years is a lot to ask of an electrolytic, though they can last longer than that also. My homemade power supply has a very large capacitor about that old, and it still works fine. But it is in a much thicker case, with screw terminals, than the small caps which would be in a tach.

I have a computer monitor which failed. Without benefit of a schematic or real knowledge of how it works I replaced the caps, and it went back to working fine.

As to the transistors, I assume a Google or the like search didn't turn up anything? You might try some other search terms, like obsolete transistors or obsolete transistor replacement or other variations - might turn up a document which Google directly can't find - a list in picture format, or some other format Google can't word search other than a title.

The original transistors were mostly Germanium types, and can be replaced with silicon PNP types, e.g. 2N2907A.

[911obgyn]

Capacitors!

[mysocal911]

Originally Posted by 911obgyn

Capacitors!

Or guess again, right?

[Mark Henry]

Originally Posted by Zendalar

Oh yea, i thought i would add this:

http://www.niksula.cs.hut.fi/~mdobruck/siililand/mini/diy/alien/tacho/tacho.html

dead link.