Well Murphy has not had a go by the looks of things.
Well done great work, pity Sunbeam did not think of this.
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EM6910 Control Board Testing
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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
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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.
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
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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.
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)
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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.
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
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After reading this comment I decided to check the thermal grease under the thermistor.Originally posted by Xanthine View PostGlad 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
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.
SteveLast edited by SteveDB; 2 August 2015, 11:41 PM.
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G'day Trev
I am even more impressed with your effort
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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
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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
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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
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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
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Hi Steve
It sounds like your problem could be with the steam side of things.
If you listen carefully during warmup you can hear the two relays (steam and water) clicking on & off independently.
I would suggest that most likely the steam thermoblock mains circuit is at fault rather than its thermistor.
If the steam thermoblock is not heating you would need to check the following components in its circuit -
Heating element - two terminals at the top of the thermoblock. 45R to 50R.
Thermal cutout - lower left of thermoblock. close to 0R.
Thermal fuse - lower right of thermoblock in a white silicone sleeve clamped under a small metal plate. 0R resistance.
Thermistor - upper left of thermoblock. resistance depends on temp.
The last possibility is the relay on the control board which could have burnt contacts or an o/c coil (or, least likely, the control board itself)
Good luck with your search and let us know how you get on.
Trev
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G'day all.
I'm new here, been looking over your shoulders for a while now.
The reason I'm posting is that my EM6910 has started to play up, and I have been looking through thesse posts for some clues.
My machine now goes into warming mode, with relays clicking and thermoblock heating, but it never gets to the ready stage. The power light just keeps flashing, and therefore no way to make coffee.
My first thought was the thermistor, so I measured its cold resistance, about 150kOhm, then fired it up and let it sit for about half an hour so it was nice and hot, then switched off and unplugged and measured again as quickly as I could. The resistance was now around the 10kOhm range (from memory). According to the spec I found (see ATC Semitec GT-2 NTC Thermistor) this would equate to the 80-90 DegC range.
So I think my problem is not the thermistor.
I did a system reset, hoping to maybe fix something, but no change.
So does anyone have any suggestions on where to look next?
Xanthine, I was told that the optimum temperature for the water into the coffee was 92DegC. The spec for the NTC above would equate this to about 7.2KOhm. So your 390 Ohm would equate to 200DegC. I would hope it thinks this is hot enough!Originally posted by Xanthine View PostRe the mods, the first problem to solve will be finding the optimum resistor value to parallel with the NTC thermistor which reads about 150k at room temp (it's only about 15deg here at the moment) so would be specified as a 100k thermistor (25deg C). The temp curve graphs for a 100k thermistor would probably be a good match and could be a good starting point for some parallel resistor calculations to save a lot of trial and error testing (does anyone know the normal operating temp of the steam thermoblock?).
The shunt resistor would need to be carefully chosen so that the controller cycles on & off as I think it may otherwise signal a fault and cause a shut-down - another function which I hope to look into with the 'simulator'.
I might try jumping a variable resistor across the NTC to see if i can get mine to settle.
Oh, just found another thread that says the steam block and its NTC also as to be operating. Hadn't checked that, as I just assumed that group heater was all thats need to start. I'll have a look tomorrow and see if that's my problem.
Steve
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Tamplt-
Glad you liked my letterbox. There's not too much of the original 6910 left as I didn't want to waste any of the 'good bits' on it.
The control panel buttons are ex computer keyboard.
I bent the wands from a straight length of chromed steel tubing from a lamp fitting and stuck them in the original holes with epoxy putty.
The water tank lid was sealed in place with silicone left over from the bathroom reno, as was the cup warmer tray on the top and the steam and hotwater knobs on the sides.
The group handle cost 40cents at the local op-shop.
I drilled drain holes in the waste tray at the bottom and the coffee cup so they wouldn't hold water and breed mozzies in summer.
The coffee cup is bolted and siliconed to the perforated cup stand.
The letter slot was cut with a thin abrasive disk in the angle grinder. This was also used to remove all of the internal mounting spigots under the lid so the mail could slide in.
Aluminium panels were fitted over the water thermoblock hole top and bottom and another half-height one behind the door at the back to stop the mail falling out when it is opened. I siliconed a magnet to this to hold the door shut as the original ones came out with the plastic water tank liner.
So, as you can see there was quite a bit of modding to do, but all good fun.
Now to get on with the steam thermoblock descaling experiments - will post more on this soon.
Trev
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Absolutely love it, well done!!!
I love the old "cheapie" p/f addition, just completes the art.
Hmm, I wonder if I can "enhance" the rust on my current letterbox surreptitiously? Plotting, plotting, plotting...
TampIt
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