Help! Extruder Thermistors not working

[brianmcin]

Help, Thermistors not working.

I have a single extruder Ridigbot, yesterday I drilled out a hole in the aluminum extruder plate for a new belt connector. Unknown to me an aluminum shaving fell into the center of the extruder "B" connector on the mainboard.

So now the Rigidbot always reports a temperature of about 12C for thermistor on extruder “A”. This reading does not change if I disconnect the thermistor at the breakout or even if I disconnect the extruder “A” ribbon cable from the mainboard.

I though I might be able to solve the problem by enabling duel extruders and connecting my only extruder into the extruder “B” connector on the mainboard. But atlas no….. Extruder “B” always reports a temperature of 100C whether the ribbon cable is connected or not.

I troubleshot every thing beyond the mainboard
-I have spare thermistor, swapping did not change anything.
-I have tested the ribbon cable (pins 5 & 6) with an ohm meter, it is OK

I am guessing that the aluminum shaving must have shorted the extruder “B” 24v fan power to the thermistor input and fried something. The thermistor for the heated bed is still working correctly. In fact everything but the two extruder thermistors is working correctly.

I would like to repair the board if possible, I have the schematic but don’t know where / how to begin a diagnosis. Rigidbot will sell a replacement board for $130 which seems fairly expensive for what it is.

[brianmcin]

SUMMARY
Both extruder thermistor inputs are dead
As far as I can tell the thermistor input for the heated bed is OK

RESISTORS
On the board I tested R48 & R43, Both failed neither read anywhere close to 4.7k. I also tested R62 (for the heated bed) it meter perfectly.

CAPACITORS
I tested C42 and C46 (still mounted to the board) and my results are inconclusive. I am using an older fluke meter that that can do a basic test on capacitors using resistance mode.

I also tested C45 the 10uF capacitor for the heated bed, again with inconclusive results.
To validate my testing method I also checked a larger servo capacitor a C33 47uF and got good result.


NEXT
Rigidbot is selling "Dumpster Dive MainBoards" for $5 each, I ordered a couple to use a source of spare parts.

From what I can tell on the schematic I will need to replace R48, R43, C42 & C46; and if that does not solve the issue then I would I believe I would be forced to assume that the circuitry in the processor go fried. The thermistors connect side by side to pins 83 & 84 on the processor, which might explain why both are dead?

If anyone has thoughts I'd appreciate hearing them.