The Gaggia TS was working well, and providing excellent espresso, probably due in part to its electrically heated group head.
However, today the boiler heated up to pressure once, and then cooled right down until it was back at zero pressure.
First check is to make sure there is water in the tank, and that the boiler has filled sufficiently. It may be (depending on the controller fitted) that the controller only allows the pump to run for a fixed period each time, so if the boiler has been drained, one run of the pump may not be sufficient to fill it. To check, switch the power off for 10 seconds, then turn on. The pump should not run if the boiler is full. The boiler has to be full (ie touching the sensor probe) to allow the element to be powered.
Having determined that the boiler had filled, looking at the circuit diagram, the control unit provides a live feed to the pressurstat, which then connects to the element, and then through an overheat thermostat back to neutral.
My problem was that there was no live feed to the pressurstat. This feed originates at the control unit, which switches its live feed (coming in on pin 1) out to the pressurstat (connected to pin 6) via relay K3. The most likely fault was a dirty contact of relay K3 so I began stripping down the control unit to see if I could repair or replace the relay.
First I labelled the connections to make reconnection easy.
The after removing the control unit, the cover can be unclipped to reveal the two PCBs.
One PCB has the mains wiring connections, an 18V transformer which is regulated down to 12Vdc and relays K1, K2, and K3 (labelled on the PCB as RL1, RL2, and RL3. The other PCB has circuitry to detect the level in the water tank and boiler, and driver circuitry for the three relays. The interconecting ribbon carries 0V, 12V, and the three relay feeds.
Here is the bridge rectifier, input capacitor, and regulator.
Here are the three relays. Each has a diode (1N4148) across the coil (bottom right of the picture). The beige one drives the heater.
On the second board, there are three transistors to drive the relays (they ground the relay coils down to 0V to switch the relay on). The remaining three pins of the ribbon cable are +12V, and two ground connections.
Here is the remainder of the measurement board.