Posted Tue Apr 1, 2014, 12:04pm Subject: Gaggia Coffee mods resulting in temperature drop < 2 deg C
I bought neglected Gaggia Coffee for $30 and implemented some mods to improve its temperature stability. After mods, maximum temperature drop is between 1.4 and 1.6 deg C at the end of double shot. The same mods can be applied to Classic as well. Modifications include:
Internal RTD probe in place of brew thermoswitch.
REX-C100 PID modified from relay to voltage outputs (with alarm function for steaming); with RTD resolution is 0.1 deg C
SSR activated by PID output, PID alarm or preheat circuit
Circuit to add between 20 and 40% power to heaters when pump is activated (adjustable with pot)
I didn't have to adjust the safety valve - I've measured 12.5 bar on the pump discharge and around 8.9 to 9.0 bar after group valve (with flow throttled down to 75ml in 30 sec). My PID settings with 0 to 200 deg C input range are: P=12.8 I=40 D=2. These are different from autotune values for better control during the shot.
Coil + 6mm RTD (PT100) probe photo. Compression fitting installed in place of brew thermoswitch (drilled and taped to 1/8" BSP). End of probe is midway between boiler internal wall and outlet/raiser tube.
Posted Tue Apr 1, 2014, 12:11pm Subject: Re: Gaggia Coffee mods resulting in temperature drop < 2 deg C
Priming valve plugged off by soldering. Leaking steam valve repaired by filing off rolled edge, polishing the stem end and o-ring change. I've cut M12x1 external thread and installed stopper nut - valve can be repaired now any time.
Posted Tue Apr 1, 2014, 12:23pm Subject: Re: Gaggia Coffee mods resulting in temperature drop < 2 deg C
PID modification details. OUT and ALM1 relays removed, 4 jumper wires and 2 x signal diodes installed (1N4148 or similar). Take care not to bend wires joining the boards too much/many times - they are not too flexible and will break.
Posted Tue Apr 1, 2014, 12:29pm Subject: Re: Gaggia Coffee mods resulting in temperature drop < 2 deg C
Above mods will convert outputs to voltage type as marked on the photo. I've used 2nd Alarm terminal to connect low voltage ground signal (white wire on previous photo). I'm using it for preheating circuit (will post the details later after finishing wiring diagram).
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