try this for a test. plug the thermocouple into the board. measure between shield and one leg of the thermocouple on the board. It should read greater than 10 megs. The higher the better. Do this for the other lead to shield. measure across the thermocouple leads on the board. should measure 0 ohms. Last measurements will be between each thermocouple solder point and power ground. you should see 1k on each lead to power supply common. Do this again for shield. It should read greater than 10 megaohm. When it comes to noise, you need to make sure you have plenty of current when using a wall mounted power supply and also since we use switchers, the quality of the filtering can be very marginal. I always measure for ac ripple when I use wall plug power supplies. I first measure for ac ripple with my meter and if it looks high, then I look with scope. I do this open circuit and then with the supply connected to a load. I personally use only battery power with my heater meter. I do have a go to wall 3ADC supply that measures low for ripple and noise and will use that sometimes, but I mainly use battery power. C8,C9, and C11 must be correct values and have good solder joints. These components decouple the noise along with the 3 input resistors like Brian said. I am still suspicious of the thermocouple probe and power source. I use Thermoworks probes and have had no issues. Probes I have purchased off of e-bay have given me fits, mainly because of origin and they do not come with proof of performance. If you have a scope, repeat the same measurements and this time touch the shield while looking at the thermocouple inputs. If you see noise, I would find a new probe.
Yeah it depends on if anything is in parallel with the resistor. The 10k and 1k resistors in the thermocouple circuit are isolated as long as no thermocouple is plugged in so that should be fine. The capacitors there's no point to measuring their resistance since it changes depending on how much charge they have (and the multimeter will charge them). I know that's not very helpful in this instance because we still don't know what's causing the issue but I really don't know what it could be and where this noise comes from.
Ok, today I removed a couple or resistors and measured them clean and they were correct.
I also added some solder to the amp legs and removed it again and touched all the soldering points of the resistors to ensure all was connected.
Although I did this 2 times before, I managed to fix all my remaining heatermeters this way!
I have no clue what it actually was, especially since the problem was identical with 3 of them but im putting my money on the amp since that is the most tricky to get right.
I need to do some cleaning on the units now due to excessive flux but that should not change anything anymore.
All is reading fine and touching the wire does not change the temp (is visible on the graph but has no real life effect as it shouldnt)
Thanks to all you guys for the help and suggestions!!!!!!
Aww heck yeah! Well I am going to put that in my databank that resoldering a couple of extra times can possibly clear up noise. I wonder which leg / resistor makes the difference. So you completely removed some resistors and replaced them and also resoldered/wicked the amp? All the resistors? I might try to see if I can reproduce this with an old HeaterMeter by pulling some parts off and seeing if I can get an intermittent or high resistance connection. Interesting that it followed the mains signal kinda.
You're going to be a pro troubleshooter by the time your club all has HeaterMeters!
I removed the ones that gave a different reading than when disconnected just as a start.
As soon as I removed them and measured them, they were all fine so I cleaned the solder off and reinstalled them as clean as possible.
I then touched all the soldering points of the other resistors I didnt remove in order to check all connections were ok.
I figured I would do the amps as well so I put more soldering on them, wicked it off and then with a clean tip touched the legs in order to try to make them proper.
This combined worked and the graphs show the same as the correct ones and temperatures didnt change anymore when touching the wire.
It does create noise off course but not in the amount the readings change