Thermocouple reading WAY too high


 
S

SJWebb

New member
Hi all.

I finished my first HM build this evening, which is pretty exciting. Everything seems to be working correctly (except the LCD, which I'm pretty sure I screwed up), but that doesn't bother me much, I am more interested in the web accessibility of the device (which I also need help with, but don't have the time to try and figure that out tonight). However, the thermocouple is reading about 575 degF. Which is obviously wrong. My maverick et-732 probe in probe 1 slot is reading the correct room temp (about 64 degF). I have tried a reboot. Does anyone have any ideas about what the problem is? Anyone seen or heard of this before? Thanks in advance for any help, and thanks for all the hard work so many have done on this project. I can't wait to get cooking!
 
Do you have any flux cleaner and a toothbrush? Give the TC amp area a good scrub. 99% isopropyl alcohol will also work fine.
 
Did you go into the config page and change the pit probe to thermocouple? That would do it.
 
Hi guys,

I don't have flux cleaner and only 70% isopropyl. I will get some of the 99% stuff and clean the TC area and report back. I HAVE changed it in the configuration to the thermocouple. That is the setting under which it reads 570. Any other thoughts while I investigate cleaning the TC part of the PCB?
 
I did the TC tests when assembling the board and everything checked out. After cleaning, now the TC won't read anything. I tried the continuity tests again, which work. The voltage tests give me 3.26V when NOT shorting the TC inputs together, and 1.48 when shorting them together.

I am not super familiar with EE, so THANK YOU for all your help!
 
When shorting the inputs at room temp ( ~20C ) you should see about 100mv ( 20C * .005v = .1v )

If you're seeing 1.48 volts, that would be calculated as ( 1.48 / .005 ) 296C / 565F

Any chance you can take some clear and close up images of the TC amp section?
 
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I've included some images below. I am still not getting a reading from the TC, too high or otherwise. Could I have shorted out the actual thermocouple? Or potentially screwed up the actual TC jack? I have a backup of that I can replace it with. Let me know if you need a different angle, but that is as close as I can get and focus properly.

16230192306_1f8051d52c_o.jpg

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Update: I am back to 500+ degF on the TC reading instead of no reading, just gotta figure out this voltage thing.
 
Im on my phone, so pardon if you already answered this.

Make sure you have it switched in the settings to thermocouple on the pit snd also make sure its set to 5.


Also, there could be a chance, as it is kimda confusing to mix up the caps on the board. Your amp soldering looks ok. So the only other thing it might be is a wrong cap in the wrong place.
 
Hi John,

I did swith the probe to thermocouple in the settings. Could you be more specific what you mean by "cap". I'm new to soldering and PCBs so sorry for the ignorance. Thanks for your help!
 
The tan little SMD boxes are caps (capacitors). Unfortunately they do not have a number on them like the resistors, so we can't tell if you have the right values in the right places.....
 
Are you sure your thermocouple itself is ok? Check the wires inside the plug to make sure they are screwed down tight... After that I would carefully reflow the solder on all the SMD stuff to make sure they're making good connections with the board. It helps to use a magnifier of some sort when doing this...
 
The 0.1u ( 100n ) cap is a little bigger than the 2 x 1n and 1 x 10n caps, and looks to be in the right spot. I'm not sure and perhaps Bryan might chime in, but I doubt it would be too harmful to have a 1n cap in the spot where a 10n cap should be. Worst case it would short out, which would cause the TC amp inputs to be shorted together, which won't harm anything.

The PCB still looks like it's got a lot of flux everywhere around the TC amp circuit and could use a cleaning. Shorting out the preamp inputs by putting a small wire or a small paperclip into the TC jack openings should cause the TC to report ambient temperature without the need for an actual thermocouple probe.

With 500F being reported ( 2.5v output ) I would start looking for some sort of short from the 3.3v line on pin 7 to the output on pins 5 & 6

In this diagram, 3.3v is labelled as 5 and outputs are labelled as 4.

 
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Hi all,

As far as the caps go, I will double check, but I only pulled them out of the package one at a time. However, if they were mislabeled at the shipping end I may have an issue.

I did check for shorts with between pins 5/6 and 7 when doing the continuity testing, but will revisit this. I will also try reflowing and cleaning again and report back. As always, thanks for the suggestions.
 
You guys are awesome! Thank you Steve, Bryan, John and Ralph for your time and thoughts on this issue. After cleaning and reflowing everything, my HM is reading 65 degF. I cleaned and reflowed at the same time, so Im not sure which did the trick, but it's working. Sorry for not cleaning better the first time, I was a little tentative and didn't want break anything. I can't wait to smoke some meat this week!
 
SJWebb said:
You guys are awesome! Thank you Steve, Bryan, John and Ralph for your time and thoughts on this issue. After cleaning and reflowing everything, my HM is reading 65 degF. I cleaned and reflowed at the same time, so Im not sure which did the trick, but it's working. Sorry for not cleaning better the first time, I was a little tentative and didn't want break anything. I can't wait to smoke some meat this week!
Click to expand...

Glad you got it.

If you decide to build another, which most of us do, You can get the expensive pcb cleaning stuff from RS or I what I do, when cleaning the board, is I buy a bottle of, atleast 90%(99% much better) and a toothbrush. Pour some on the board and brush away. Sooner you do it after soldering the better.
 
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