I wonder, why is there a 4071 IC there in the circuit in the first place?

 

It can be hard to trace a pcb. The inputs probably go somewhere.
Anyway, the 4017 can't drive the transistor with enough current to fully turn it on, so maybe like #12 said a FET.
You don't need the 33 ohm for the FET, but if it is there you can leave it.

 

maybe like #12 said a FET.

Brevor gets the kudos for the 2N7000 suggestion.

 

Brevor gets the kudos for the 2N7000 suggestion.

+1 for using a 2n7000 .... remember though that its gate is a lot more sensitive to static discharge than the base of a bjt. Handle as you would any cmos device.

 

OK I have installed the 2N7000, it works well. I spent a lot of time trying to figure out why the second stage compressor won't come on. I assumed both would be on until it reaches temp, this however is not the case, stage 2 comes on at around -20degC. I left it running overnight to get it to -20 but the next day it was hardly any colder, looks like I'm on to checking for leaks and re-gassing.
Thanks everyone, I really appreciate it.

The more I learn about electronics the more painfully aware I become of my lack of knowledge- being an electrician I haven't had to touch a PCB or a transistor. Now I'm suddenly working as the sole electronics tech in a lab and I have had no training and noone to ask questions, except you guys and the allaboutcircuits textbooks.
Thanks again.