JimStim troubleshoot - resistance from 3.3v to ground?

[TheMonkey]

jimstim has worked great for a couple years.

my jimstim has been in a drawer, and when i just fired it up, it was not giving a tach signal - just a constant full voltage of ~15v. nothing ever got hot when leaving power on.

i checked the 3.3v post had no voltage. i noticed that the voltage regulator had been bent back and forth a lot so i checked it like a loose tooth and it came off. i soldered it back down.

when i powered back up, i checked voltage at the unregulated voltage input pin and saw that it was only ~7v. at the same moment, i noticed that regulator was getting super hot, so i immediately pulled the power out. with regulator still in, i checked resistance between ground and voltage output (3.3v) and saw that it was only 1.4ohm. i thought i must have bridged the solder, so i pulled the regulator, and wicked out all the solder from the board - looks clean.

with the regulator out, the resistance between 3.3v post and ground is still only 1.3ohm. is this right? the regulator stand alone has 6.9k ohm between ground pin and volt out pin.

can someone confirm the proper resistance or perhaps i have short somewhere (less likely since it previously worked).

thx, Scott.

** edited to add: this is board 1.3, with the cpu on board. it's soldered in, so can't easily pull it for testing.

[TheMonkey]

i just tested the regulator directly off of wall wart. 16.9v in, and 16.1v out.

should i just put a replacement regulator on, or is there anything on the board i should troubleshoot first?

also- does radioshack have any suitable replacements for the regulator?

[Peter Florance]

i just tested the regulator directly off of wall wart. 16.9v in, and 16.1v out.

should i just put a replacement regulator on, or is there anything on the board i should troubleshoot first?

also- does radioshack have any suitable replacements for the regulator?

Mine's 1K

I'd just replace the regulator. I don't think RS has anything close to that.
Most likely the processor is bad?

The regulator is probably good.
Maybe Jean can sell you a regulator along with a replacement processor

[Peter Florance]

I suppose C1 could be shorted

[racingmini_mtl]

On the 1.3, there aren't that many things that could go wrong other than the CPU itself. If the regulator allowed more than about 4V to get to the CPU at any time then the CPU has very likely failed. Unfortunately, the only way to know for sure is to remove the CPU but it difficult to remove a soldered one without hurting either the chip or the board.

I would recommend that, unless you're used to doing such a job, you clip off the legs of the chip and remove them one by one from the board. This will destroy the chip but it will be the best way to make sure the board doesn't suffer in the process.

I have replacement CPUs but not the regulator. You can get the CPU and the regulator from the MS vendors. And I would suggest you use a socket when you replace the CPU.

Jean

[TheMonkey]

thanks guys.

i'll pull the CPU. i'm not too bad at wicking solder without getting too hot.

based on my test of the voltage regulator off the wall wart, it was definitely putting out very high voltage. i just wish i knew what caused what failure.

no socket was my very first MS mistake a few years ago... there were more...

when CPU comes out, what should the resistance be between 3.3v pin and ground at that point?

[Peter Florance]

On the 1.3, there aren't that many things that could go wrong other than the CPU itself. If the regulator allowed more than about 4V to get to the CPU at any time then the CPU has very likely failed. Unfortunately, the only way to know for sure is to remove the CPU but it difficult to remove a soldered one without hurting either the chip or the board.

I would recommend that, unless you're used to doing such a job, you clip off the legs of the chip and remove them one by one from the board. This will destroy the chip but it will be the best way to make sure the board doesn't suffer in the process.

I have replacement CPUs but not the regulator. You can get the CPU and the regulator from the MS vendors. And I would suggest you use a socket when you replace the CPU.

Jean

The only alternative to clipping the legs is to us ChipQuik. but that takes some practice and I usually have to pre-heat the board to keep it molten long enough to extract

I have vacuum guns so it's not an issue for me

[Peter Florance]

resistance probably infinite depending on polarity of ohm meter leads

You might see C1 charging

[racingmini_mtl]

You also have the coarse and fine RPM pots between the 3.3V and ground so with those in parallel it would make it 5K Ohm. The only 2 other components on the 3.3V rail besides the CPU are C1 and C2.

Jean

[TheMonkey]

i pulled cpu. sufficiently destroyed it like you suggested. just too many pins to try to salvage.

anyway, the resistance between 3.3v and ground is behaving as expected, and the cpu pins 3 & 4 have 1.3 ohm between them.

so, i'll replace cpu with socket and regulator. prob get an extra stim too.

thx.

[TheMonkey]

okay, back in bidness.

i did figure out that the regulator cannot be tested directly on a wall wart. i think it must need a bit of resistance to output 3.3 volts, because by itself it output barely less than input voltage ~15v, but on the board it worked fine.