Harman deep technical dive

[Freedmeister]

I've replaced the control board on my older P-68 and I'm sure my ESP probe is good (600kohms at room temp, resistance decreases with increasing temp). I have the feed rate all the way up, just before the "test" setting. WIth my stove temp set to 7 (all the way up) and the stove mode set to "Stove temp" (all the way to high). my stove just doesn't get as hot as it should. (doesn't matter whether auto or manual - circulator function is correct).It stays just above idle warm enough that the circulator runs. I'm sure the DIP switches are correct (I've tried both feed auger rpm settings). I can hold my hand on the exhaust pipe right at the back of the stove and it used to be too hot to do that, before this problem. I'm wondering if the replacement control boards may not have the trimmer potentiometers set correctly for all stoves. What does each of those trimmers (on the back of the board) affect, and what are the test points and values I should get?

I've also seen folks wonder what's inside an ESP probe. Here is a tear-down of a "black" one. The failure was a disconnect of one of the wires to the thermister. Note the Kapton tape is charred. I think the failure mode for these is that the Kapton gradually breaks down from heat, and then the unit shorts to to the stainless sheath, or the connection to the thermister leads fails. Based on the construction seen here, cleaning the outside of the probe with alcohol is a waste of effort, the amount of impact on thermal conductivity or response rate that a layer of ash or creosote would have on this would not have a measurable effect on the response of the probe. If the Kapton tape is deteriorated, handling the ESP probe is liable to cause it to short to the sheath.

 

[Lake Girl]

Thanks for the visual on the ESP and the failure point description. There is another thread with a new board issue ... should compare notes
https://www.hearth.com/talk/threads/harman-p38-major-malfunction.151395/page-2

 

[heat seeker]

Nice photo - I was wondering just what's in there.

 

[frankie]

I just put a new board in my p61a. Board came with instructions for the dip switch settings for the board. That board is for several Harmans so here is what instructions say for the p68. There are 2 pics that show the dip settings and here they are. You may have to take the board out and change them as I had to set my settings on mine.

 

[Cold in W.I.]

I've replaced the control board on my older P-68 and I'm sure my ESP probe is good (600kohms at room temp, resistance decreases with increasing temp). I have the feed rate all the way up, just before the "test" setting. WIth my stove temp set to 7 (all the way up) and the stove mode set to "Stove temp" (all the way to high). my stove just doesn't get as hot as it should. (doesn't matter whether auto or manual - circulator function is correct).It stays just above idle warm enough that the circulator runs. I'm sure the DIP switches are correct (I've tried both feed auger rpm settings). I can hold my hand on the exhaust pipe right at the back of the stove and it used to be too hot to do that, before this problem. I'm wondering if the replacement control boards may not have the trimmer potentiometers set correctly for all stoves. What does each of those trimmers (on the back of the board) affect, and what are the test points and values I should get?

I've also seen folks wonder what's inside an ESP probe. Here is a tear-down of a "black" one. The failure was a disconnect of one of the wires to the thermister. Note the Kapton tape is charred. I think the failure mode for these is that the Kapton gradually breaks down from heat, and then the unit shorts to to the stainless sheath, or the connection to the thermister leads fails. Based on the construction seen here, cleaning the outside of the probe with alcohol is a waste of effort, the amount of impact on thermal conductivity or response rate that a layer of ash or creosote would have on this would not have a measurable effect on the response of the probe. If the Kapton tape is deteriorated, handling the ESP probe is liable to cause it to short to the sheath.

My P38+ is currently down as well. I've recently replaced the circuit board due to a failure of one of my pot switches shorting out. My new problem is the unit now turns it's self on while in the off position when I plug it in. Wiring checks out and my 38 also seems to be in a perpetual low setting preventing me from shutting the unit off.