IC SN29736P from rev counter pulse shaper board

[spanishfly]

Another day another problem.

Does anyone know if the chip on the tachometer boards normally dies suddenly? Or any other reason for the tach to suddenly stop working? My problem is with 2.1 dj but same chip in the 1.6JX board.

I have a wonderful Chris board that has been working fine for a couple of years but suddenly the tacho stopped.
(I've measured lots of things not that that means I've completely understood the results) 

the voltage entering the board is 12.75v at tick over between gnd and tacho signal wire decreasing as revs rise.
the voltage between tacho signal wire (grn) and +ve 1.04v rising with revs
These are measured using the RPM setting on my old Fluke 88A. 

Also measured all the resistors and diodes and checked the capacitors (a friend did that as I don't have an ESR meter tool).

All these measurements were cross checked with a working tacho and a spare tacho I bought from someone on here that doesn't work either?? 

The one I bought on here might be muddying the water as I've never seen it work and the seller contacted me a few times to ask if I had a working petrol tach for sale after selling me his so perhaps it didn't work when I bought it? The resistor values on board are the same for all 3 so I am assuming the Ti chip might have died? 

Any thoughts much appreciated.

[AngeloEvs]

The IC is a SAK215 for both petrol and diesel tacho. Available on EBAY, I paid £6-00 for one a few weeks ago for a repair that I carried out. The component values are different for diesel tacho’s due to the difference in the type and magnitude of the pulse voltage. Note that there is a series resistor that can go open circuit, particularly if the tacho has been connected incorrectly, if it is open circuit there is a high probalbility that the chip has also failed and will need to be replaced.

Do not disturb the pre set potentiometer as this is used to calibrate the tacho, disturb it and the tacho will need to be re - calibrated.

I will post a picture of a petrol tacho pcb showing the location of the series resistor that needs to be checked.

[spanishfly]

Thanks, very reassuring! Really good of you. I don't think I plugged it in wrongly but guess it's possible? As I built Chris's board from the kit it uses dupont leads rather than the original connector so wrong plugging would be possible. I have: reading left to right, pin 1. -ve, pin 2 blank, pin 3 +ve, pin 4 tacho.
Been trying to work out how the circuit works by tracing tracks and making a drawing... I realise this is far from a proper circuit diagram but it did help a bit. I've also tracked down a new genuine Ti SN29736P chip atleast that's what it says on it...

Is the series resistor you mean, one of the ones on this diagram?

The red circles denote resistors that don't appear to show what I expected by looking up the colours on the internet.....???

The purple squares are values from the one bought on here that doesn't work.

The pencil circles are the ones from mine that stopped working

The plain pencil from the values based on the coloured rings....

[AngeloEvs]

The resistor which can fail is R1 (100ohm).  For anyone who is looking for the IC,  SAK215 is the commercial ID and correct for the VW and easily sourced. 

[spanishfly]

Thanks.
Does that imply my resistor is good? As it measured 104.3 and 105  but that the chip could still be bad? 
 

[AngeloEvs]

I test by injecting a low frequency AC voltage directly onto pin 2 and if the chip is good and resistors measure ok, the needle should deflect. The only other course of action is to replace the chip. There is a long thread that Chris started, who designed a pcb for the VW petrol tacho, that deals with the VW tacho and testing it, etc. It’s around page 7 or 8 in this electrical section, the title is VW Tacho Alternative or something like that. It is a very long thread so you won’t miss it if you scan the pages.

[Robsey]

Probably found it by now, but if not....

Here you go.

viewtopic.php?t=165030

If you don't want to read the preamble, you can cut to the chase on pages 9 and 10 of the thread.

[spanishfly]

Well after all that it was an internal break in the dupont lead for the +ve.
Feeling foolish as I'd checked at one end but not at the other...
Thanks for the help!