Probe values incorrect

Hello,
I've done some searching on here, but havent been able to find anything. I had my heatermeter build working pretty well, but decided that I should calibrate the probes just to be sure (they were pretty close to my other thermometers, so I should have left well enough alone). I dropped one of the probes in boiling water, and just as it was approaching boiling, the value jumped to the 230s and has been stuck wandering around there ever since.

All probe jacks experience the same thing ... they walk between 200 and 300 with no (actual) temperature fluctuation. I've tried multiple probes too, thats not the issue.

I checked all resistor values, and they all seem good to me. My most likely guess is that I popped a cap somewhere, but they all *look* fine. I'm not sure which one to start with. I don't think it's the IC since the software still seems to work.

Just powered it on again with the thermocouple installed, and thats sitting over 600* ... it's definitely some component that ALL of the probes go through.

Any thoughts on where to start? I appreciate the help!

-Brad

First of all, on the TC, did you change the config to select the TC? Did you remove the 10K pullup resistor behind the TC jack?
In general, I have repaired a couple HM's for someone who was reading wrong temperatures. Resoldering the components that make the RC filters fixed the issue. They would be all the parts that are standing up behind the probe jacks plus the caps back there. Give that a try first before you dig deeper.

Yep and Yep, and already done. I re-soldered the entire board just to be sure. Again 1) it was working (for some time) and then popped and 2) it's not going to be a component in an individual path (unless one component in each series simultaneously popped). There's something (probably a single component) that must be in the bath between ALL of the probes (including the TC) and the IC itself.

if you take a look at the schematic you will see the probe circuits are fairly simple and nearly directly connected to the ATMega through their own individual circuits. The only parts between the probe jacks and the ATMega are the resistor and capacitor of each RC filter (on HMv4.2, not present in older HM's) and the pullup resistor(s) (which is what I directed you to in the first place). So your hypothesis that it is a component in the common path between the probes and the input on the ATMega is off base.

The common factors here would be the 3.3v that is fed to the pullup resistors which also powers the TC amp, and the ground. I would start out by measuring your 3.3v and see what the regulator is putting out, perhaps you popped the 3.3v regulator?

I would also ask if you completely submerged the probes in the water during your boil test (including the braided lead coming out of the end), because that is one sure fire way to kill a probe right quick....

Nope (on the submerge) ... Plus I've tried new probes. I figured there wasn't much else in the path ... Do you have a link to the schematic? I'll check the 3.3v source and ground (again), and if that all checks out I guess I'll try a new ic. It really felt like a software issue at first, but I've reimaged and reconfigured with the same result.

In the HeaterMeter Hardware sticky thread there are links to all of the info about the HM, hardware, software etc.

The HeaterMeter Hardware page has the schematic linked there...

http://capnbry.net/linkmeter/pcb/hm-4.2/HeaterMeterPi.png

I would start out by measuring your 3.3v and verifying 3.3v on the legs of the pullup resistor(s) to make sure it is getting over there. If that is ok and your grounds are OK then it couldn't hurt to remove and reseat the ATMega chip, I've had that clear up odd behavior before. And then flash the ATMega (AVR Firmware) if you think something is haywire there.