Transformer Problem In Espresso Machine
We have all been told about how to diagnose. The K.I.S. method – ‘Keep It Simple’ and also ‘Do Not Assume’. Well, in this repair, I managed to forget both!
A friend that I do some domestic electrical work for is a coffee fanatic. He has his own machine and grinds his own beans. His espresso machine had failed to work correctly and he was desperate for a decent coffee! He had a look at it himself, but was not keen on the electronic side of the machine. I was interested in repairing the machine to show my electronic skills that I had acquired as a hobbyist.
The Bezzera BZ99S Espresso Machine would operate for up to ½ hour and then the operating light and boiler light would disappear. The power light would remain lit. You would have to wait a few hours before it would work again. Clearly it was temperature related.
The system uses a combination of sensors and an electronic control unit.
As I had no schematic or voltage details, I had to check the basics. I decided to check the voltages at the control module before and after the unit failing to work. I was able to isolate the area where there was a voltage loss. After opening the unit, I checked for obvious signs of failure – burnt components, cold solder joints and damaged tracking. Everything seemed OK. Rather than doing further testing and thinking about how the system must work, I started to check various components. Mistake number 1!
I checked the relay points as I reasoned that if they were burnt, they may create high resistance and cause the unit to fail. I examined them under my USB Digital Microscope (these are an excellent tool for finding cold solder joints and other faults)
Although there was some decolourisation, there were no pits that I expected to find. I used 1400 grit sandpaper to clean the surfaces.
I did however find several capacitors that were far out of range after being checked with my Blue ESR meter. The capacitor shown is 63V 10uf and should have a reading of 2Ω, but shows a reading of 7.7Ω and was clearly faulty. I decided to replace all 3 electrolytic capacitors.
I tested the unit for 3 hours and felt confident that I had repaired the machine. My friend would be pleased that I had repaired his beloved espresso machine so quickly. I was feeling quite pleased with myself. The next day I reassembled the unit and decided to make myself a cappuccino, purely for testing purposes…….
The machine ran for ½ hour and then stopped. What had I missed? I pulled the unit down again and did what I should have done all along. Not assume that just because I found a few faulty components, that I had completed the repair. Also keep my diagnostic path simple. I had a close look at the control unit and tried to understand how it actually worked. Once I did that, the correct diagnosis became easier.
The transformer used has 240 Volts (here in Australia) on the primary side and 18 Volts on the secondary side. By running the unit open and carefully separated by cardboard, I was able to test these voltages while the unit was both operating and not operating. The fault became obvious. Now that I had kept the diagnostic path simple, understood how the system operated and not assume that it may only be one fault, I quickly arrived at the correct diagnosis. The transformer lost its 18 Volts on the secondary side and the unit would no longer operate.
But………I had learned my lesson. I would remove the transformer and do further testing before I cried ‘Eureka!’ Since the fault was temperature related, I used both freeze spray and a heat gun to replicate the fault. The results were conclusive. When cold, I had continuity and when heated, the transformer open-circuited.
Unfortunately, I was unable to locate a transformer exactly the same as the one that needed to replace. I found one that had a different pin configuration and by soldering on extra ‘legs’, was able to fit this to the circuit board. (Excuse the soldering!)
By thoroughly testing the machine (and several coffees later) I felt very confident that I had repaired the unit. It helped me to create new diagnostic paths, start with the basics, realise that there may be more than one fault and modify components to adapt to control boards. My friend was more than happy and was keen to get his precious espresso machine back in action.
This article was prepared for you by Mark Rabone from Australia.
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