MK 138 no longer working

Kits Mini Kits Range
1

Amateur here. Have modified MK 138 with lower hysteresis and increased max temp for egg incubator following instructions elsewhere in this forum. Worked well during initial testing but having increased the wattage of the heater used, (more 12v bulbs) the unit now fails to activate relay on cooling. Have separate power supply for bulbs with plenty of amps and they work wired through NC with relay not activated.

Not sure where to start trouble shooting. Relay not stuck as works with power applied directly to base. Does not appear to be sensor as have closed the connectors with jump wire with no result. Have reverted to bulb used in testing and problem persists. Have I overloaded the board/relay? could it be a diode/other that has blown?

thanks

George

2

Hard to tell what went wrong. Most likely mechanical problem.
Return to original condition to test functionality.

3

It was a blown diode d2 and having replaced this I now have the relay switching at low temp as expected - however it does not deactivate once temp rises. Adjustment of the trimpot does not alter this.

Any idea why this should be?

replacement resistors are as follows:

Resistor 4 - 4k7
Resistor 5 - 3k9
Resistor 6 - 2k7
Resistor 7 - 10M
replacement 1K Potentiometer

4

Check voltage on (-) and (+) input of opamp.
if (-) is higher than (+) and output of opamp is still high, then op-amp is defective.