I successfully repaired the Tach amp!!!
 

Not being able to read the i.d.on the burned component, and since it is in series with +12V, I replaced it with a blocking diode, re-soldered the failed solder joints at the burned out component, I measured all the pins to make sure there are no shorts, plugged it into the socket, started the engine, revved it and the Tach works!!! Yeak baby! Here's a pic of it during testing. When I put it back together, I am not gonna pot it with silicone since it will run cooler without it.

http://i31.photobucket.com/albums/c3...052807_001.jpg

Just for the hell of it I dug out my old Tektronix scope and looked at the output of the Tach amp

Here's a pic but I don't think you can see the 2 pulses. It is a negative going pulse approx 1 ms wide at &pprox 6V p-p. Amplitude is contant with rpm change. At idle, the period measured was 82 milli-sec, which works out to around 731 rpm. (1/0.084 *60). The scope is probably not well calibrated but I'm gonna raise
the idle just a bit since it sounds low. I took a short video clip also. I'll see if I can upload it Photobucket and post a link later

I don't see it.

I'd be very interested in both, the signal at the sensor and the output to the tach.

Nice job on the repair, finally getting somewhere on this long running project.

Still interested in seeing a full on remap/schematic. I suspect your burned up component may be an isolated incident whereas many of the failures are due to a loose component that is temporarily fixed by the now infamous cigarette butt trick.

You electrical geeks are blowing my mind with this tech talk (obvious capacitor problem? blocking diode! WTF?!). Who's going to explain the concept of the square root of -1 next?

It's kinda fun for us tech geeks and I'm one of the worst! I still live in the old vacuum tube era and all my equipment and even my stereo is full of tubes! 7868 linear power pentodes in class AB1 with 9.41% inverse feedback error correction to be exact! 50 Watts per channel that can be heard down the end of the block and whe it's cold outside it will heat the house!:D I'm still working on the schematic end of this project and i hope to have it out on a post soon as I found the typical operation analog service schematic for the LM1815N semiconductor and it's operating characteristics. I'm hoping to come up with a more rugged and durable pulse controlled modulation inverter assembly.:book:

"you mean this sucker is nuclear?"

"no marty, this sucker's electrical, but I need a nuclear reaction to generate the 1.21Gigawats for the Flux Capacitor"

...
John

That's great! I sometimes forget how this sounds, but posts like this just drive me onwards!

I have a 4CX15000 tube laying around. :eek2: Yes 15 THOUSAND.


Now would THAT make for a great LOUD amp!:flamethro

NC Benz
1983 300D turbo
225,000
Ham radio
and
Broadcast Engineer (I play with the BIG power!!)

They're around $60 on ebay, new.

I haven't seen a tube that large since I changed the broadcast power output tubes at KIRO 710 AM in Seattle!:eek: I need a nuclear pulse to generate 1.21 gigawatts for the flux capacitor! :D Have you seen the schematic for the LM1815N IC yet? I :Dbelieve it's on this post. What are you going to do with this tube?

So what was the end result of this? Was a new, better engineered componant designed? Was a consistent repair method developed?

Well, I'm still working on it. I'm looking to better build a new piece. Consistent repair methods are difficult for this device because of the variables I'm trying to engineer out.:D

End result is that I can repair the Tach amp easily. All the components are available. I have a way to test it in and out of the car. The design is fine. It's the packaging that is the problem IMO. Is there demand for replacement Tach amps? If there is, I can look into building them.

The Tach amp I repaired is still running and being tested in my 300DT without problems. Here's a better pic with a digi cam:

http://i31.photobucket.com/albums/c3...a/DSC00092.jpg

Here's a video in .mpg but when I uploaded to Photobucket it was changed to .flv (Flash) so I am not sure if it will play:

http://s31.photobucket.com/albums/c397/funola/?action=view&current=MOV00093.flv

it's choppy, and pililated, but it plays fine.
John

Thanks for the confirmation. I don't have Flash so no way to tell.

What do you think is a reasonable price to charge for the Tach Amp that I repaired (with a 30 day guarantee)?