Announcement

Collapse
No announcement yet.

EM6910 Control Board Testing

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #16
    Update on my no-start problem.

    As Trev suggested I checked the steam thermoblock power.

    Heating element - two terminals at the top of the thermoblock. 45R to 50R. - measures 48R - check!
    Thermal cutout - lower left of thermoblock. close to 0R. - measures 0R - check!
    Thermistor - upper left of thermoblock. resistance depends on temp. - measures 100k at 20C - check!
    Thermal fuse - lower right of thermoblock in a white silicone sleeve clamped under a small metal plate. 0R resistance. - measures O/C - found it!

    OK what temperature is this fuse set for? All the writing on the fuse is very faded/rubbed, so difficult to read.
    It has a Brown tip on it, and my memory says that should indicate its temperature. Look up Prof. Google and all I get is confused.
    Back with the magnifing glass and I think I can just make out 220C. Hope thats right 8-(

    Quick trip to Jaycar and they have 216C for $3.50 each. So Grab three, just in case, plug one in, and hey presto up an ready in two minutes, just like old times! Happy coffee maker again.

    Thanks for your help Trev.

    Now two questions to ask.
    1. Are these really 220C? Anyone?
    2. Why did it pop in the first place? Dodgy fuse? Blockage in the steam system causing less water to pump and therefore less cooling? Anyone?

    Steve

    Comment


    • #17
      Glad you found the problem Steve.
      I had a look at the thermal fuse on the spare thermoblock I have but could not read the last digit either - could have been a '6' or '8' but definitely in the 220deg C range.
      It could have just failed through thermal fatigue or through over temp. The thermal cutout is 200deg C from memory and the thermal fuse is a backup should the control circuit and thermal cutout both fail.
      It's always a concern when these go without knowing the actual cause.
      I know the values for these were mentioned elsewhere on the forum but not easy to find.
      One thing I did notice on the machine I scrapped was the way the thermal grease under the thermistor had dried out - I guess this could cause problems with accurate temperature control.
      Hopefully someone will have an answer as to the thermal fuse value or maybe its listed in the service manual which is available for download.
      Trev

      Comment


      • #18
        I just had a look at the circuit diagram in the service manual and it shows the steam thermal fuse as 240deg C and the thermostat as 210deg C - the only problem here is that the manual is supposedly for the 6910 and yet the circuit diagrams in it are labelled as 6900 - doesn't instil much confidence in the reliability of the info as it has possibly changed with the model upgrade.

        Trev

        Comment


        • #19
          G'day Trev

          I am even more impressed with your effort.

          Steam thermoblock descaling: I posted a long general post on this very subject tonight for Rawill on http://coffeesnobs.com.au/brewing-eq...tml#post560938.

          Hopefully it will save you some grief.

          TampIt

          Comment


          • #20
            Originally posted by Xanthine View Post
            Glad you found the problem Steve.
            ....
            One thing I did notice on the machine I scrapped was the way the thermal grease under the thermistor had dried out - I guess this could cause problems with accurate temperature control.
            ....
            Trev
            After reading this comment I decided to check the thermal grease under the thermistor.
            Just like you found, it was dry and hard. Could be the cause if it overheated and blew the thermal fuse.
            A quick clean and re-grease should see it right for some time.

            Thanks again Trev.

            Steve
            Last edited by SteveDB; 2 August 2015, 11:41 PM.

            Comment


            • #21
              Back to the 6910 de-scaling-

              Time to test my ideas on a 'real' machine.

              First I made up variable resistor box to simulate the thermistor using a 100k pot wired as a variable resistor in series with a 1k fixed resistor to limit the minimum total resistance. I fitted this with a lead and 2pin socket so it could be easily connected to the 2 matching header pins I soldered to the steam thermistor PCB pads on the back of the control board.

              Click image for larger version

Name:	Test box to board.jpg
Views:	1
Size:	77.0 KB
ID:	740912

              Once the box was plugged in I fitted the back cover to the machine, plugged it in and switched on with the pot set to minimum (1k) resistance.
              The green 'Power' LED started flashing as in a normal warm-up while the water thermoblock came up to temperature and then, after a couple of minutes, it went into 'ready' mode.
              Opening the steam tap produced a pulsing stream of water with a little steam so I figured that the 1k minimum resistance was still a little high.
              I then changed the pot in the test box to 1k and the series resistor to 82R.
              When this was connected to the machine it produced a pulsing stream of cold water from the steam wand.
              I played around with various resistance settings and finally decided on a value of 470R.
              Now to make it switch-able and permanently wire it into the machine for some more testing.

              Just another word of warning - whilst the control section of the board operates at 5V (the light green area in the photo) all of the upper section is potentially at mains voltage and is extremely dangerous.
              I like to lock up the mains plug on the machine before removing any of the covers. I have a screw top plastic container with a slot cut down the side which lets me put the plug in with the cord coming out and then screw the cap on. It doesn't get let out until the covers are back on. Sorry to sound like a nanny but mains appliances scare me!

              Trev

              Comment


              • #22
                Originally, I planned to tap into the thermistor leads using insulation displacement "T" connectors but found that they did not seem to make a reliable connection through the tough high temperature insulation so I decided to stick with the 2 pin connector soldered to the pads on the circuit board.
                I considered several locations for the switch before deciding on the lip in the bottom alloy casting just below the circuit board. This had the advantage of keeping the leads short, well clear of any mains voltages, away from heat sources and moving parts and not attached to removable panels.

                Click image for larger version

Name:	switch lead with resistor.jpg
Views:	1
Size:	40.3 KB
ID:	740913Click image for larger version

Name:	Heatshrink over resistor.jpg
Views:	1
Size:	44.5 KB
ID:	740914Click image for larger version

Name:	Switch & resistor assembly.jpg
Views:	1
Size:	48.5 KB
ID:	740915Click image for larger version

Name:	Ready to close.jpg
Views:	1
Size:	72.0 KB
ID:	740916Click image for larger version

Name:	Earth leakage test.jpg
Views:	1
Size:	79.0 KB
ID:	740917

                The main disadvantages of this spot are the difficulty in fitting the small switch in place and it's vulnerability to damage. The small aluminium shield I bent up to protect it made fitting even more difficult but is probably worthwhile considering its flimsiness.
                Mounting the switch only required drilling a 6mm hole through the soft alloy but had to be accurately located so that the nut could be fitted to secure it and the shield.
                Unfortunately the only suitable switch I had in the junkbox was a 'centre off' with a slightly dodgy mechanism but it will do until I can get a decent one to replace it. If the whole idea proves workable I will re-do the whole lot using high temp insulated wire and a cable tie around the board support mounting leg just to tidy things up.
                The 470R resistor was soldered into one of the leads and sleeved with 2 layers of heatshrink.
                Once the cover was back in place I did an earthing test with the Low Ohms meter and an Earth Leakage test before seeing what effect the mods would have on the steam operation.

                (more next post)

                Comment


                • #23
                  From a cold start with the shunt resistor switched in the ready lights came on after a couple of minutes.
                  With no steam nozzle fitted the steam wand emitted pulses of cold water which slowly became hotter until it reached boiling point and I closed the steam tap.

                  Click image for larger version

Name:	Water from steam wand.jpg
Views:	1
Size:	72.0 KB
ID:	740918 Click image for larger version

Name:	Back to steam.jpg
Views:	1
Size:	69.9 KB
ID:	740919

                  Although I have not tested it yet with vinegar it is giving every indication that it could be an effective way of descaling and flushing the steam thermoblock without having to open the machine up and dis-assemble anything - time will tell!
                  The procedure would need to be repeated several times, allowing cool down & soak times between each cycle, and then followed by several (many?) flushing cycles to remove any vinegar taint.

                  Please Note : This testing was carried out on an old but still working machine which only cost $50. I can not guarantee that it will not damage the machine . Anyone duplicating this mod. does so at their own risk. It would certainly void any warranty.

                  Trev

                  Comment


                  • #24
                    Just an update on the steam thermoblock de-scaling mods -

                    Since the spare red machine seemed to survive the mods and is still working, although not used very often, I decided to do the same to my 'good' machine which is in everyday use.
                    The switch and resistor were mounted in the same manner as previously, although this time the two wires were soldered directly to the thermistor pads on the pc board instead of using header pins and a plug. I also used wires with high temp insulation and cable tied them to one of the control board mounting pillars.

                    While I had the back off I measured the normal steam thermoblock temperature using a newly acquired IR thermometer and got a reading of 130degC.
                    Admittedly, this may not be terribly accurate due to the thermoblock having a shiny metallic surface and hence a lower emissivity than the 0.95 that the thermometer is calibrated for, but it should be somewhere in the ballpark.

                    I will post a further report down the track when I have done a few de-scales with this system. In the meantime any constructive criticism would be appreciated as I am always expecting 'Murphy' to step in and throw a spanner in the works!

                    Trev

                    Comment


                    • #25
                      Well Murphy has not had a go by the looks of things.
                      Well done great work, pity Sunbeam did not think of this.

                      Comment

                      Working...
                      X