1999 earth stove mp35/50 circuit

igobill Posted By igobill, Jan 6, 2019 at 12:34 AM

  1. igobill

    igobill
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    I have a 1999 earth stove mp35.50. The control board died and I would like to fix it, however, there is no circuit diagram that I can find. Would save a lot of time if someone had figured it even at a block diagram level. A $500 charge for a replacement is like robbery. I had a career as a field engineer fault isolating and repairing Broadcast equipment at the component level, so Im sure I could fix this circuit more quickly with a diagram. However, I will go ahead guessing at the most likely suspects in the circuit path. SYMPTOM: It stopped feeding pellets, motors and wiring are good. Any help appreciated.
     
  2. Stovensen

    Stovensen
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    Hello Igobill, and welcome to the forums,
    Finding the circuit diagram for your control board may never be possible. Trust me, I've tried for years finding one for my Whitfield, and with no luck.
    But as you put it, finding "the most likely suspects in the circuit path" should be possible without the diagram.
    The first area I would suspect and inspect, is the functionality around the safety switch, since the symptom is a non working pellet feed.
    Now, the safety switch is normally open at room temp. and should close at 110 ::F
    Until the stove is warm enough to make the safety switch close its contacts, the safety switch must be bypassed by a timer controlled relay on the control board. At least that's how the warm up procedure works on the Whitfields ( and many others ), but your stove is different here: At start up, pressing the start button on a MP35/50 will make the feed motors run continuously for app. 12 minutes.
    Question: Is the pellet feed inoperative at all times, i.e. both at start up and when stove has warmed up?
    If we take a look at the power flow diagram, we see that the safety switch has three wires connected to it. As I understand it, the purple wire from the start button circuitry is bypassing the safety switch for 12 minutes and thus delivering start up power to the feed motors in this period.
    If this bypass power is not delivered to the fuel feed, you have something faulty on the circuit board related to the start button circuitry. A worn out/burnt relay could be a likely culprit. Are there any relays on the board?
    Bypassing the safety switch temporarily with a jumper wire should deliver power to the feed motors and tell you if the 12 minutes start up power is missing. Jumping the safety switch is of course only temporarily, and remember to unplug stove first.

    Power flow diagram:

    Power flow MP 35_50.jpg

    I have attached both the owners and service manual below in case you don't have them. A lot of useful info can be found there

    Good luck and take care. Keep us posted
     

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  3. igobill

    igobill
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    Wow, Stovensen, thanks for the comprehensive reply. The MP 35/50 in question is from the 1990's and it is a Tank! I've replaced the blower motor and welded some cracks in the burn pot, otherwise it just ran. As you point out, I did check the associated heat activated switches in the circuit and all seems fine when heated and cooled. None needed reset. I'm miffed at the fact that schematics are not available, seems cheesy of the manufacturer. I had a career as a circuit and system designer so I can design a replacement, but lots of other projects happening. I was Anticipating a failure of a 20 something controller, so I bought a Raspberry Pie microprocessor intending to design and code a replacement control board, BUT, the stove got wise to me and broke a few days after. So I bought a uustove Ashley AP5710, mistake since it arrived already broke, no control of feed rate, runs hot all the time. Tech service is supposed to send new auger motor assy. It also has a small hopper, which I'll cure with an aux hopper attachment of my design. However, it is light enough to handle by myself and has it's big brother burn engine. It does keep my house warm here in N Ca, and has a nifty auto start, no more messing with starter chip, fluid, matches, etc. Thus my wife can easily run the stove if I needs be gone for a short time. Again, thanks for the documentation, I did have a very tattered old manual, so your gifts are very timely.
     
  4. Ssyko

    Ssyko
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  5. Mspath

    Mspath
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    May 13, 2019
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    Hello Stovensen:
    I just signed up to the forum and actually posted an issue similar to the one that Igobill posted. My Earth Stove MP35/50 was not feeding during the warm up phase and I suspected (and purchased) a new safety disk. I have not been back to camp to try it but I suspect that was not the issue.
    The main thing I noticed as I read your post was that my safety disk only had TWO wires (red & purple), not three. Jumping the disk started feeding so that is why I jumped at suspecting the disk. The control board's task of the bypass for 12 min. seems like that was the intent by the wiring diagram but I don't see it wired that way.
    Any ideas?
    Mike
     
  6. Stovensen

    Stovensen
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    Hello Mspath, and welcome to the forums. The power flow diagram above is a very simplified version of the wiring diagram in order to get a quick overview of how the system works.
    On the power flow diagram it certainly looks like the purple wire from the control board is connected in parallel directly on the output side of the safety switch. I understand that your observation is different - only two wires on the safety switch - This means that the connection point is made around one of the Molex plugs.
    Now, the wiring diagram is a bit more complex, but I've combined the diagrams below, and after some scrutiny, it should appear how things are wired.
    Hope this helps.
    Good luck and take care. Keeps us posted

    Power flow diagram and Wiring diagram combined.jpg
     
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  7. Mspath

    Mspath
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    Thank you for your quick reply!
    I did start looking at the wiring diagram but it is quite confusing. I suspect that there is an issue on the control board with a timer used to set the relay to switch power.
    Any chance there is a block diagram of the components on that board? I'm sure I can no longer order a new board but maybe I can shoot a timer chip or something like that.
    Thanks again!
    Mike
     
  8. Stovensen

    Stovensen
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    The purple wire from the start button circuitry is connected to the auger motors on the lower rightmost pin on the upper 6 pin Molex plug, (the male half?)

    Most likely, since the augers work, when you bypass the safety switch. Do the auger motors run ok, when the stove has warmed up and the safety switch has tripped?

    A new control board is available, but with a price tag of $432,- it certainly makes sense to try and repair the old one.
    https://woodheatstoves.com/products/earth-stove-pellet-stove-control-board_28m61
    I hope you have some experience with electronics repair, otherwise go for the new replacement. I'll be happy to help as far as I'm able to, remotely.
    A block diagram should not be necessary. With a lamp and a magnifying glass it should be possible to trace the components in the start circuitry. Is there a relay? Are there any cmos timer ic's of the 4000 series, or is the control board a newer type based on a microcontroller?
    Warning: If the control board is an older type with a capacitive voltage divider instead of a transformer, there is a risk of one or more capacitors carrying a rectified mains voltage, even with the board unplugged. Please check for this before disassembly.
     
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  9. Mspath

    Mspath
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  10. Stovensen

    Stovensen
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    This is good news. Now we know that most of the electronic parts on the board are in working order, except the 12 min bypass function at start up.

    Not bad news at all. Microcontrollers are to my best experience some of the last components to fail. The control board in my 1998 Whitfield pellet stove is also based on a strange labelled microcontroller, and it is still working perfectly. Never made any hiccups.
    Since we don't know the exact type of microcontroller that is used on your board, we don't know on which pins the data ports are located.
    We do know, however that data output ports on microcontrollers are not able to handle more powerful signals directly. Auxiliary circuits, relays etc. are needed.
    Therefore we will have to do our tracing from the outside and inwards...If you follow the copper trace on the board that connects to the purple wire, what major component does it lead to? A relay?

    Suggestions on what components to look for:
    On my control board a single bit on the data port actuates the start up bypass relay through a circuit quite similar to this:
    jMxj6.png

    The contact points in the relay have a limited number of actuations, before arcing will wear them out. And the flywheel diode has a hard time protecting the transistor from the rather high reverse voltage that is generated in the coil whenever the relay is deenergized.
    If the diode burns off, the transistor won't last for long. Result: No bypass power to the augers at start up.
    Does your control board look similar to this?
    28M61-1__35697.1528138718.jpg s-l1600'.jpg s-l1600''.jpg
     
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  11. Mspath

    Mspath
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    Thank you so much for your help! The control board is the one you have in the pictures you posted. No relays but three Triacs and 6 pin dip isolator Triacs. I will trace the circuit as you suggest to get an idea on how they are used to turn things on. I took the board with me to home so I do not have the stove connections to test, hopefully I can figure out how to emulate a startup so I can test the 12 min. bypass. I will insert pics of the board.
    Mike
     

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  12. Ssyko

    Ssyko
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    I use light bulbs for testing

     
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