What is the correct replacement?

Question

The thermal fuse on our old Magic Aire II dehydrator failed. I ordered and installed what appeared to be a correct replacement, and did some soldering to reinforce and repair some connections on the heater board. All original connections were rivets, so I don't know if solder will survive but am trying to save the device.

Everything seems to work except there's still no heat. The one bad component yet seems to be a switch of some sort built into the heater circuit. It is rectangular black, riveted to the board (asbestos(?) ring) with heat bands contacting the rivets, with brass connection arms in between, and a copper tongue inside that may have points on it, it's too hard to see very well inside. Another dark blade is loose inside and I cannot tell how it fits, nor maneuver it very easily. The switch is one and a quarter inch long overall and a quarter inch wide.

Is it a thermal switch of some sort, to keep the heater from getting too hot? Perhaps it's failure caused the thermal fuse to fail? After the time and effort so far, I'd like to see if I can get a replacement switch and probably a couple new rivets for it, since I'll try to drill out the old ones without destroying the fragile asbestos(?) board. Or has my resurrection effort all been a waste of time? There's a lot of extra trays and other accessories we've bought over the years for it... not crazy about tossing it all if I don't have to. A new one is $120 and just the base unit is $90.

Thanks for any advice.

jw0752 · Accepted Answer

Hi Joel,

ECG/NTE used to sell the proper crimps with the thermal fuses. If you do use a crimp make sure that it is a bare one so you don't get melted plastic. I like your small brass screw approach. Good Luck.

John

hafcanadian · Answer

Note where I had to notch out the end so it straddled the rivet hole and screw, and add a wide washer for the lower screw to effectively clamp the cutout. This particular device was 2mm shorter than the original and the other possible replacement (Klixon TH21), so notching was a way to make it reach and make good contact.

Strange... after I’d cut off the old cutout I happened to look closely at it, and now it’s contact point is sitting in a normally closed position. Don’t know what changed just from me removing it, but if it was installed now it would work as designed. Weird. Slight lift with a fingernail and the contact disengages the bottom brass, then springs right back like it probably did during an overheat/cool cycle.

Also on the underside of the old switch was stamped “PEPI M. 42”. I think that is Portage Electric Products Inc.

jw0752 · Answer

Hi Joel, I could not get your pictures to load. Try replying to your own post and use the picture icon on the tool bar to insert your pictures into the text. In my experience the thermal fuses blow for two reasons. First of all if there is an over heat situation and the second is if their leads do not have a good mechanical connection (Rivets) A bad connection produces heat and this travels up the wires and fuses the thermal link. Typically there are three ways to break the circuit. #1 is the thermostat which controls the temperature. The back up for this is usually a self resetting thermal cut out and the final protection is a thermal fuse. If the part you describe is the thermal cut out its failure could have caused the unit to over heat and take out the thermal fuse. Soldering will not usually work as the heat will eventually cause the solder to crystallize and make a bad connection. It is probably too late but the best way is to cut the original thermal fuse leads and use metal crimp splices to hook the new thermal fuse to the old leads. I look forward to seeing your pictures. While I have never worked on a dehydrator it sounds very similar to some of the dental lab heaters and sterilizers that I used to work on. John

jw0752 · Answer

Hi Joel, The pictured switch is the Auto Setting thermal cut out. It will open up if the element gets too hot due to lack of air flow and automatically close again when it cools down.. It should test with low ohms when it is cool all the way up to cut off temperature. Unless you know what you are doing do not try to test it with power applied. I would suspect clogged filters or fan problems if this were cutting out during use. The thermostat sensor is probably located in the air passage somewhere. You have already located the thermal fuse which may be close to the thermal cut out or at least somewhere near the heater. The material that you are dealing with in the picture is Mica as opposed to asbestos. John

hafcanadian · Answer

Not sure how a magnet would factor into this... it is not a motor coil. In the original post’s photo you can see the entire heat ring made of mica, not asbestos (thanks John). To the cutout’s right along the ring is the power source connections, to its left and up around the ring is the thermal fuse (which I carefully soldered in using multiple heat sinks so the fuse is still contiguous), and directly across the ring from the cutout is a simple copper? wire jumper bridge. Attached below find (hopefully) a closeup of the cutout. A probe is seen holding the copper strip up so the contact points disk can be seen attached to it underneath. In my experience points come in pairs, but no twin disk is seen on the brass bottom section. My take now is that a point set here is normally closed allowing continuity through the heater unit. The dark metal strip buried deep inside may be a heat spring material designed to change when overheated and pry the points apart until things cool to an acceptable temp. There is some sort of short and narrow raised bar transversing the bottom brass behind where a disk would be if it wasn’t missing. The bar appears made of cast material like a disk is. Perhaps one end of the spring strip is supposed to ride on that, supporting the copper strip, I don’t know. Regardless, the spring strip (if that’s what it is) seems to be laying loose on the “floor” of the cutout behind the bar. My reasoning is that the unit won’t heat because the cutout has a missing contact disk and the dark metal strip is mispositioned somehow. That is why I was asking if a replacement cutout could be found. But it hinges on my interpretation of the device’s construction being in the ball park. Not being familiar with such things, I have to guess at how they are supposed to look vs. what I’m seeing now. Perhaps there’s not supposed to be a second disk, and the bar serves to support the copper strip behind. Maybe my trying to probe in there just deformed the copper strip so it no longer lays down where it should. i think the closest modern device might be a TH21? GVZ Components - Klixon TH11, TH21

hafcanadian · Answer

Update.

A big thank you to John for the link. I received the “thermostat” and installed it tonight, and what to my wondering eyes should happen but... it worked! At least for the several minutes I had the unit running. I’m not sure it’ll hold up in the long run and didn’t have the unit totally reassembled, but after an afternoon spent at the hardware store getting the correct screws, nuts, and washers to mount the device with, I was elated to see at least the thing actually working about as it used to. I’m not sure that it heats up as much, but then it’s not reassembled and enclosed yet, and the “thermostat” didn’t break contact and kept current flowing through the coils.

In this case I simply used a Dremel to cut off the old thermostat/switch/cutout, leaving the rivets intact. By using washers to make good contact with the remaining rivet heads and brass ends of the old cutout, I planned to simply use small machine screws, washers, and nuts to clamp down the new cutout.

The Klixon TH21 I had my eye on had the same dimensions as the original and would’ve fit, whereas this one turned out to be just a little short (ironically, it’s stamped on the back as a Klixon product). I nipped off the very extreme of one end so it at least straddled the screw hole, inserted a larger washer to span the compromised area, and another washer atop of it appropriate to the screw size; the “miniature” class screws and such were size #1 (~1/16”) round head a quarter inch long. I was hoping for brass, but they only had stainless. Hey, whatever works at this point. I’m not sure I trust the unit to leave it dehydrating overnight, thermal fuse or not, but at least it can be used while monitored during the day and isn’t a bunch of money in the garbage can.

If anyone would like to see a photo of the final fix, I’ll be happy to supply it here.

Much obliged!

jw0752 · Answer

Hi Joel,

I found this switch on ebay that cuts out at 105C. I do not know if this is going to work but it is the closest replacement that I have seen. If you have to replace rivets do it with screws, washers and nuts as this will make a good connection that is not susceptible to heat failure like solder is.

https://www.ebay.com/itm/Bimetalic-Switch-Thermostat-Over-Temperature-Switch-221-Deg-F-in-USA/172517131295?hash=item282ad2841f:g:bkIAAOSwnHZYl5Px

John

jw0752 · Answer

Hi Joel, You definitely need to replace the cut out. Unless there is another sensor for a thermostat it is possible that it was being used as a thermostat. Are there any temperatures marked on it? John

jw0752 · Answer

Hi Joel, I know there are white LEDs that will operate at or below 1.25 V as they are used in the sidewalk lights that work off of a single 1.25 V NiCad or Nimh battery. Perhaps you can get white ones and coat them the color that you want? The other option is to use a boost DC to DC converter to get the voltage above the junction energy level for the regular LEDs. John

What is the correct replacement?

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