Voltage checks

[Wragie]

Just wondering if someone can verify something for me. I was asked to get a V4 board fixed
and started off with it running but not responsive. I swapped out the max232 and no change.
I rechecked voltages and found 7.5 volts across the oscillator. I was seeing 4.99 everywhere
else it should be. The soldering on this was somewhere about horrible so I am suspecting
the oscillator is toast and perhaps one of the schottkys next to it.

Anyone think of something else to check/replace?

Thanks

Dave

[NITROPIXIE]

Hi Wragie and welcome to the forum.

Is the mjlj not talking with the computer, the EDIS or both??

It seems odd to have 7.5V, when as you say, everything else is at 5V. What are you powering it with??

Can you perform continuity and insulation checks between:-

12V at molex and 5V at oscillator
Pin 1 (nearest C1) of U3 (voltage regulator) and 5V at oscillator
Pin 3 of U3 (voltage regulator) and 5V at oscillator
then each 5V point and the 5V at oscillator

Can you also test the voltage at Pin 4 of U1 (processor)

[Wragie]

Hi

I printed out the schematic and followed the volts through. Everywhere was 4.99 other than the crystal.
I would have expected 1-2volts or so on a duff one so 7ish was rather strange to see. And I did measure
that one several times and changed ground points just to see where I was screwing up. It never changed.
I'm using my good fluke meter so I know its not that end. It is either intermittent operator failure or
the board/parts ;-]

Other than one of the schottkys being suspicious measurement wise I can't find anything else off.

I was using a 9volt cell to test. Just occurred to me that the drop across the diodes and a dead short
to the battery should get me about 7.2. I wonder if the 'plumber' who solder this one up shorted something
underneath. Time to get out the schematic again.

I will remeasure everything according to what you asked and post that asap.

Thanks. Appreciate having someone to bounce things off of.

Cheers

Dave

[NITROPIXIE]

The voltage just across 2 diodes would give you 0.7v x 2 = 1.4v across the diodes. So i don't think your sums are quite right. Plus the rest of the circuit wouldn't be getting the correct voltages.

The xtal will take the same 5v supply as the max232, processor chip and the MAP sensor. I would check the continuity between each of these points and the xtal. I would also do an insulation test between the xtal power supply and the Safe 12 v (power supply to ULN2003).

It sounds like a duff oscillator to me at the moment or something isn't quite right in this area. What voltage do you get between ground and pin 4 of the processor (output of the xtal to the processor) when its powered up?? with and without the processor.