Friday, 3 January 2020

ISOMEC Millenium Repair - No Heat after warming up

It was the most alarming development. Warm up the machine early in the morning, make a "cuppa" and then come back an hour later for another, and COLD is the only word to describe the machine. It got worse. Specifically, the symptom is that neither the READY or HEATING lights are lit. It took me weeks to shy up to diving in, mostly out of sheer laziness from the Christmas break. Yesterday I got to the bottom of it. Part of the issue is language. The Isomac Millenium circuit diagram is really a wiring guide and all the twists and turns on the diagram make it hard (for me) to figure out what is going on with the logic.  I re-drew it into a more classical schematic format as it is the "language" of electronics and wiring I am most comfortable with. You will notice I stopped once the problem was solved. Perhaps this will help someone else with this problem.

Solving the issue was not cut and dried, and it took a bit of creative hardware management to remove the water reservoir tray and boiler cover, placing the water inlet filter into a measuring cup full of water.

Here follow the wiring diagram and my own schematic:

The reader will notice that this schematic is missing the "water level" sense wiring.  Only a portion of the GIEMME controller is diagramed, that being the two relays.  Note that the common portion of both relays is connected to the Line level voltage through the Main Power Switch.  The heating element relay is solid-state and I noticed that there was no heat transfer paste on the underside of it.  The bottom of the relay and the base of the coffee maker were cleaned of corrosion from a leaking gasket on the boiler, the heat transfer paste applied and relay affixed.  The control voltage was noted to be 120VAC and the relay was actuated by pulling A2 to ground through the NC contacts of the Pressure-stat.  The Green Ready light is connected to the NO contact on the Pressure-stat, closing when the boiler is at operating pressure.  When the machine failed, both lights were extinguished and the boiler pressure gradually reduced to zero.  A voltage check showed that Pin 2 of the Giemme had mains voltage on it, but Pin 5 did not.  It was determined the problem was within the Giemme controller.

At $192 USD replacement cost, I decided to remove and inspect.  I could not make matters worse if I broke it further.   I spent several hours setting up a test jig for the Giemme and powering and watching it fail.  The symptom was repeatable in minutes now and it appeared my removing it from the machine had exacerbated the failure issue.  I powered the Giemme with a 1 ampere fused autotransformer to facilitate slowly raising the input voltage.  Looking back over the repair sequence, it appears that without the sensor probe and other loads connected, the Giemme was "timing out" and releasing the relays.  I was interpreting this as the fault manifesting.  I changed the bridge rectifier, then suspected a short in the transformer.  A suitable replacement was found in my junk drawers, scavenged from an old "wall wart" power supply.  As a general principle with old electronics, I reflowed (nice term for re-soldered) all connections on the two boards that comprise the Giemme.  The bench test was OKAY.  It had stopped "timing out" after two minutes or so.  I had somewhat inadvertently repaired the device!!  After removal, the secondary winding of the transformer was reading open.

Post repair, I found a circuit diagram for the Giemme controller while searching for the correct replacement for the transformer:

I also found a web post of others with the same problem:

If nothing else, this supported my findings.

and lastly, some close-ups of the errant transformer:

Continuing the search, I found a supplier of the transformer:

I have enquired if the transformer is available though the thought of disassembly and re-assembly just for the sake of a so-called proper transformer do not excite me.  I will post further information when and if I have it.  73  de VE2GYA

PAL 500 Power Amplifier