Fridge Freezer Thermostat - Adjustment Screws

I have a 20 year old Hotpoint RF60 2-door upright fridge freezer. The bottom half is the freezer and the top half is the fridge. There's only one compessor, and a single electro-mechanical thermostat in the fridge is the only temp control for the whole unit; it's sensor tube is attached to the rear of the heat exchanger in the fridge compartment.
Last week the thermostat packed up. I called a repair firm and they changed the thermostat. However, even on its warmest setting, the freezer is at -24C (at least - end of thermometer scale) and the fridge is colder than I would like - naturally, the compressor is running for much of the time but the unit does go through a cycle i.e there are times when the compressor is off and the heat exchanger in the fridge defrosts itself. However, something's not right. Yesterday, the firm replaced the new thermostat but the unit is still running too cold.
To try and understand how the thermostat works, I partly stripped the old defective one. The outer 2 contacts appear to be controlled by the bellows at the end of the sensor tube. However, there is a central contact which seems to be connected to one of the edge contacts at all times except when the thermostat is turned to Off. I don't understand what this central contact is for.
There are 2 factory preset screws which control spring tensions and I presume these screws control the temp at which the compressor contacts make and break. One's accessible on the top of the thermostat and the other's visible through a hole adjacent to the first screw. I'd be grateful for advice on how best to go about adjusting these so that with the user control knob in the middle of its scale, I have a freezer temp of around -18C and a correspondingly cool fridge.
I've tried phoning Hotpoint for help but there is no technical department. Any guidance and clarification would be much appreciated.
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Those thermostats are generally not user adjustable (exept for the obvious knob).
They may have used a universal aftermarket part instead of a factory original. They may also have positioned the temp probe (wire like lead) inside the insulation (or otherwise in the wrong spot) causing it to run too cool.
Turn the other knob to "freezer colder" to close the vent and at least the fridge will warm up a bit even if the freezer is still too cold

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Many thanks.
I'm sure you're right: they've used a universal thermostat instead of the original part - assuming it's available. Now, the compressor has been running for hours and the fridge is below zero. I watched him reconnect the sensor tube and I'm sure that was positioned correctly. The fellow thought it possible he'd reconnected the contacts wrongly, but as the fridge has gone though on-off cycles, I imagine he reconnected correctly, but he was unsure where the central contact leads to i.e. the contact that breaks when the thermostat is switched off. He thought it might be the light!
I've run some simple tests using the force required to make and break the contacts seeing what effect both the user control and the factory preset screws have. It appears that the user knob controls the point at which the contacts break i.e. when the compressor is switched off.
The screw accessible on the surface of the thermostat appears to affect the make and break force: clockwise increases both the make and the break forces. The screw inside the body appears to affect only the break force - clockwise lowers the force.
I need to sleep on this, and tomorrow I will have a go at adjusting the screw(s) and will report back in due course. This fridge freezer has no vent - there are heat exchangers in both the freezer and the fridge. There are no fans in either compartment and no visible physical connections between the 2 compartments.
In future, I don't buy any appliance unless I can get hold of the service manual. And the Hotpoint Indesit range is definitely out: they refuse to sell the manuals and have no facility for technical help - unlesss you call their own service personnel out.
I'll report the outcome for future reference.
Thanks again.

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