Strange HM 4.1 Probe Readings


 
C

CJ Barnes

New member
Well, I've assembled my first HM4.1, and it fired up no problem. Using Maverick 72/73 probes, and an 18" WSM. However, I'm getting strange readings on probes 0, 1, and 2. I've checked the resistors on the board and they read as follows (installed):

Probe 0 - 7.6k
Probe 1 - 6.7k
Probe 2 - 7.8k
Probe 3 - 8.0k

Every other resistor on the board measures what it should, apart from these four.

Probe 3 pretty much reads accurately, but the other three are odd. With nothing plugged into probes 1 or 2, I will get temperature indicated. Probe 1 will follow probe 0 to an extent, but reads lower.

Probe 0 seems to read about 40-50deg lower than the reading I get from a separate Maverick therm in the same spot.

Example reading right now with pit probe in and nothing plugged into probe 1 or 2

Pit - 288
Probe 1 - 197
Probe 2 - 16

I'm so confused, I can't see anything wrong on the board physically, has anyone ever had a problem like this before?

It looks like there is a 90 degree decrease from the pit reading to probe 1, and then a 180 degree decrease to probe 2. This seems like it's telling me something, but I can't find where it's pointing.

Any ideas? Thanks in advance.

CJ
 
If you're getting readings with nothing plugged in then there's definitely two pins bridged together, or something floating. The fact that you're seeing non-10k resistance on the resistors means it is probably the former.

With no probes plugged in, try measuring the resistance and it should be within 1% of 10k ohms for each of them. The voltage on both sides of the resistor with respect to ground should measure ~3.30V. Check to make sure the top of all three resistors have continuity to the VCC pin on the FTDI connector. Check the bottoms to make sure none has continuity to each other.
 
Bryan gave you pointed advice, I would add that you might want to take a magnifying glass and look closely at the solder work for bad connections and bridges, or take a closeup digital picture and zoom in on your computer screen to do the same...
 
Thanks Bryan and Ralph for the quick responses. I will have a hunt around for a magnifying glass and look closely at the board. I thought that may be the problem, but I haven't found a smoking gun yet. It's odd because it isn't consistent now. The resistance changes each time I check it.

OK, first things first, some serious close up inspecting. I'll post what I find. Thanks again gents.

CJ
 
Well, I can't find any bridged joints, and nothing looks to be a bad joint. And now, for no apparent reason, the resistors are now reading differently (readings below). This is really frustrating because the first time I used it, the pit probe and food 3 worked fine, and both read around 9.7k IRRC. I've always had trouble with 1 & 2, but now it's all of them.

I checked voltage and it's 3.29VDC on all four, and all four have good continuity to the VCC pin on the FTDI connector.

When I checked the bottoms, there is not continuity between the resistors. It does however read the combined values of the two resistors I'm ohm'ing through (as it should).

Probe readings now:
0 - 8.59
1 - 8.00
2 - 9.37
3 - 7.13

Bryan, do you have a schematic for this board that would allow me to check voltages and continuity to try and locate my problem? Thanks fellas.

CJ
 
Floating meaning disconnected, specifically when an ADC pin doesn't have a good connection it "floats" close to another ADC's value.
 
Well, after much troubleshooting, soldering and desoldering, I finally broke a solder pad off and cut a trace. Still couldn't find anything clearly wrong on the board. Soooo....

I have a spare board fortunately, so I'll give that one a go and see what happens. Wonder if there's a component that could cause this?

Strangely, I removed the #3 probe's resistor and was reading ~43k ohms across the board terminals for it. ATMega was removed, so it should have gone to an open on the bottom side, and therefore given an open reading, right?
 
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