Safely Testing EM6910 Control Board
For anyone wanting to work on the control board out of an EM6910 the biggest dis-incentive is the very real hazards of the 240V mains power required to power it up.
After studying the circuit diagrams in the service manual I concluded that it should be possible to get it working with a small 6V - 7V AC plugpack after a few simple mods.
This was OK in theory but what I didn't allow for was that the circuit in the manual was for the EM6900 board and that it seems to have been modified for the EM6910. My board is marked as EM6900 V1.3.
Still, after much trial and error and tracing out the circuit I was able to get a beating heart working out of the body and, in the process, was able to learn a lot about how it functions. I was also able to track down a shorted triac in the steam pump driver.
First of all, I would like to thank TommyV for his donation of the patient in the "Pay It Forward" section of the CS Forum. The clapped out machine he was ready to toss was just what I wanted. Since it was pretty-well beyond repair I was able to do a full teardown with a clear conscience and learn a lot in the process. Unfortunately he was at work when I picked it up so - "A big thanks Tom!"
The main differences between the 6900 circuit diagram in the manual and the board from the 6910 are-
Relays instead of Triacs on a sub-board to control the heaters in the two thermoblocks.
Extra optocouplers to monitor the heater circuits and mains power.
Basically the board has two sections, the microcontroller with its associated inputs and outputs running on a 5V regulated suppy from a small on-board 240V to 7VAC transformer and the driver circuits controlled by the micro and switching the mains supply to the pumps, heaters and solenoid valve.
The first thing I did once I had the board out of the machine was to remove the transformer. This came off fairly easily once I had sucked up the solder. (solderwick would probably work just as well as a good solder sucker). I then bridged the 2 pairs of pads with 2 insulated wire links - soldered from each prim to sec pad.
I rather optimistically figured that at this stage it should work so I attached a little 6VAC (230mA) plugpack to the "Line" & "Neutral" terminals and all of the other peripherals to their respective white sockets down the side of the board - 2 NTC sensors, 2 microswitches, the flowmeter, control panel, piezo buzzer and lastly a little magnet on the reed switch under the white silicon. I did not bother with the low water LED.
To simulate the 240V water pumps and solenoid valve I used 3 little 12V torch globes wired between each respective terminal and the "Line" terminal.
OK - time to power it up. All good - no smoke, nothing getting hot but the globe attached to the steam pump terminal was lit (Tom had told me that the steam pump ran as soon as the machine was plugged in so the triac was probably shot). A quick check with the multimeter showed there was 5VDC from the voltage regulator.
A press of the "Power" button caused its LED to light - but only for a second then it went out and nothing! The only way to get a repeat of this was to power the whole lot down briefly (I wonder where I learned this trick!). Time to start looking more closely at the circuit for something I had missed. But first I tested the driver circuits by jumpering between pin 2 of each opto to ground with a 390 ohm resistor causing the 12v globes to light - so the the triacs seemed quite happy to work with 7V AC!.
A couple of hours poking around the board and sketching circuits revealed that this later version had some extra optos to monitor the mains voltages through the two thermoblock heaters and also that the mains is present. Two of these optos are on the relay board and another just below it on the main board.
These had high value (100k/120k) 1w dropping resistors on their inputs. Shunting them with resistors around 4k provided enough current to trigger the sensors at the lower supply voltage. It was also necessary to simulate the heater circuits which normally connect to the relay board. A jumper between the neutrals on the two boards and a couple of 220 ohm load resistors resistors to the "Line" terminal finished the job. Now a press of the "Power" button got the board into warmup mode and jumpering the thermistors with 390 ohm resistors made it think it was up to temperature and light up the relevant LEDs.
A press of one of the brew buttons had the water pump globe flash a couple of times with pre-infusion then come on solid. Sucking air through the flowmeter hose to spin its impeller caused it to go out after a few seconds. Pressing the steam microswitch caused the steam pump globe to emit a string of double flashes - all good!
My next project, now that I have a working test rig, is to see if I can find the most suitable value resistor to use in parallel with the steam thermistor to get the steam thermoblock to hold a steady temperature below boiling point. I figure that if I can switch this in and out of the circuit I will have a convenient way to pump descaler through the steam circuit without the problems of it boiling. This would make regular descaling of this section simple and safe as it would not be necessary to take the covers off the machine.
As soon as time permits I will try to post some photos.
Trev
For anyone wanting to work on the control board out of an EM6910 the biggest dis-incentive is the very real hazards of the 240V mains power required to power it up.
After studying the circuit diagrams in the service manual I concluded that it should be possible to get it working with a small 6V - 7V AC plugpack after a few simple mods.
This was OK in theory but what I didn't allow for was that the circuit in the manual was for the EM6900 board and that it seems to have been modified for the EM6910. My board is marked as EM6900 V1.3.
Still, after much trial and error and tracing out the circuit I was able to get a beating heart working out of the body and, in the process, was able to learn a lot about how it functions. I was also able to track down a shorted triac in the steam pump driver.
First of all, I would like to thank TommyV for his donation of the patient in the "Pay It Forward" section of the CS Forum. The clapped out machine he was ready to toss was just what I wanted. Since it was pretty-well beyond repair I was able to do a full teardown with a clear conscience and learn a lot in the process. Unfortunately he was at work when I picked it up so - "A big thanks Tom!"
The main differences between the 6900 circuit diagram in the manual and the board from the 6910 are-
Relays instead of Triacs on a sub-board to control the heaters in the two thermoblocks.
Extra optocouplers to monitor the heater circuits and mains power.
Basically the board has two sections, the microcontroller with its associated inputs and outputs running on a 5V regulated suppy from a small on-board 240V to 7VAC transformer and the driver circuits controlled by the micro and switching the mains supply to the pumps, heaters and solenoid valve.
The first thing I did once I had the board out of the machine was to remove the transformer. This came off fairly easily once I had sucked up the solder. (solderwick would probably work just as well as a good solder sucker). I then bridged the 2 pairs of pads with 2 insulated wire links - soldered from each prim to sec pad.
I rather optimistically figured that at this stage it should work so I attached a little 6VAC (230mA) plugpack to the "Line" & "Neutral" terminals and all of the other peripherals to their respective white sockets down the side of the board - 2 NTC sensors, 2 microswitches, the flowmeter, control panel, piezo buzzer and lastly a little magnet on the reed switch under the white silicon. I did not bother with the low water LED.
To simulate the 240V water pumps and solenoid valve I used 3 little 12V torch globes wired between each respective terminal and the "Line" terminal.
OK - time to power it up. All good - no smoke, nothing getting hot but the globe attached to the steam pump terminal was lit (Tom had told me that the steam pump ran as soon as the machine was plugged in so the triac was probably shot). A quick check with the multimeter showed there was 5VDC from the voltage regulator.
A press of the "Power" button caused its LED to light - but only for a second then it went out and nothing! The only way to get a repeat of this was to power the whole lot down briefly (I wonder where I learned this trick!). Time to start looking more closely at the circuit for something I had missed. But first I tested the driver circuits by jumpering between pin 2 of each opto to ground with a 390 ohm resistor causing the 12v globes to light - so the the triacs seemed quite happy to work with 7V AC!.
A couple of hours poking around the board and sketching circuits revealed that this later version had some extra optos to monitor the mains voltages through the two thermoblock heaters and also that the mains is present. Two of these optos are on the relay board and another just below it on the main board.
These had high value (100k/120k) 1w dropping resistors on their inputs. Shunting them with resistors around 4k provided enough current to trigger the sensors at the lower supply voltage. It was also necessary to simulate the heater circuits which normally connect to the relay board. A jumper between the neutrals on the two boards and a couple of 220 ohm load resistors resistors to the "Line" terminal finished the job. Now a press of the "Power" button got the board into warmup mode and jumpering the thermistors with 390 ohm resistors made it think it was up to temperature and light up the relevant LEDs.
A press of one of the brew buttons had the water pump globe flash a couple of times with pre-infusion then come on solid. Sucking air through the flowmeter hose to spin its impeller caused it to go out after a few seconds. Pressing the steam microswitch caused the steam pump globe to emit a string of double flashes - all good!
My next project, now that I have a working test rig, is to see if I can find the most suitable value resistor to use in parallel with the steam thermistor to get the steam thermoblock to hold a steady temperature below boiling point. I figure that if I can switch this in and out of the circuit I will have a convenient way to pump descaler through the steam circuit without the problems of it boiling. This would make regular descaling of this section simple and safe as it would not be necessary to take the covers off the machine.
As soon as time permits I will try to post some photos.
Trev

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