I had a similar problem with the controller for a double oven. When the manufaturer's repair guy came out he told us the control unit was no longer produced. We would have to reaplce the over. This was a $2K+ proposition. Oh, and by the way, the new units were slightly larger, so the hole in the cabinetry would have to be enlarged, if possible. Sounds like a way to make work for the contractors. I took the unit out and confirmed the problem. I isolated the problem to some power transistors. Rather than fixing it myself from that point I found that there were several companies that would refurbish the units. One was a short drive from home. So, I took it there and they fixed it. It has worked ever since.
This controller sounds very alike the technology that I saw in the Luigjes Hottop solar boiler which I installed in my home (in the last century ;-). Including the single sides PCB and dual comparators. Only this one controlled the pump via a relay contact, so there was no high current through the electronics.
Its operation was extremely simple and I doubt that even today you'd find a digital controller because the analog controller was so simple. Maybe just for calibration purpose it would be made digital (and to allow the boiler temp to be displayed so the user would have a nice display to look at instead of the two LEDs on the original controller.
The function of the entire controller was executed by the two comparators:
1. Is the temp of the solar panel a set amount of degrees higher than the water in the boiler? If yes, switch the pump on.
2. Has the temp of the solar panel reached close to boiling? If yes then switch the pump off.
The first comparison guarantees that only energy (hot water) will flow into the boiler, no hot water will cool down in the panel without the sun shining. The second will avoid the water getting pressurized from steam production and the boiler be damaged. The solar panel may get (very) how without water in it (NOTE that there was a quantity of air in the system that normally filled the solar panel when the pump was off - that protected the system so it could not freeze or boil). When the pump engaged, the panel was filled with water and the air moved to either the return-flow pipe or the designated container in systems with large height difference between boiler and panel, like in my system.
The calibration required for the controller were two potmeters to set the temp difference between boiler water and panel before the pump turned on and the max temp of the panel before the pump was turned off again.
The system was very simple and reliable and I enjoyed it for many years until I sold the house. I still plan to go back and ask if it is still working - I left a spare controller just in case.
The older style controller that used analog components was indeed repairable, which is the main reason that units made now are digital. Harder to troubleshoot and six months later you can't purchase the controller chip if you wanted to. Also, they would be programmed and the only source of the programmed controllers is the manufacturer. And by the way, they don't sell just the controller chip.
We had a similar failure on a Sears microwave oven, in that the failure was an opto-isolator that drove the power switch, but the only recourse was to replace the whole circuit board at a cost equal to the price of a cheaper replacement oven. Not as many features, but would you pay $250 to repair a microwave oven?
I've lost track of all the non-user-serviceable products and components I've fixed.
There was the PC motherboard which wouldn't drive the printer. Had to replace a surface-mount IC on it.
The 1984 washer that was incredibly robust but stopped working in 2008. It would never progress from the washing cycle to drain, fill, and rinse. The controller board would have cost $350, except you couldn't even get them. The triac which fixed it cost 37 cents from Mouser.
Two Roombas, each of which had failures in the battery charger. Each took two surface mount transistors (didn't bother to figure out which one had failed).
The loss of innocence of my son, at about age 12: "Dad, this stereo receiver is dead. It won't go on. Nothing at all." "Son, get a #2 Phillips screwdriver from the workbench." "Dad, the label says 'No user serviceable parts inside.'" "Yes, son. no user serviceable parts inside." as screws are removed from the cover. "Son, no user serviceable parts inside," as the cover is lifted off, revealing a blown glass cartridge fuse in clips, in plain sight at the top of the unit.
My son is now 37. I'm still using the receiver, haven't had to change the fuse a second time yet.
The $1.50 picofuse that put the $200 laser printer back into service in 2005 or so. Still using it.
There are dozen more instances, but they all follow the same pattern. If you simply think a little, you can replace inexpensive components instead of expensive assemblies or complete products.
We'd love to use some of your stories at Sherlock Ohms postings. Could you send one or two of them along? They need to be at least 350 words, and we also need a shor bio to go along with it -- about two to four sentences would be find.
I forgot to mention the Mitsubishi ECM (car engine control module "computer") which needed four inexpensive capacitors and some jumper wired where PCB traces had been eaten through by poorly-selected capacitor adhesive. That baby would have cost $700, had I not been able to install about $4.75 worth of capacitors.
And the dishwasher control which was over $12, but not available, fixed with a couple of "bullet" connectors costing pennies.
And I already promised to write you about the GM high-mounted brakelights. They will have to be a destructive analysis. I tried to open them to effect a repair but they were bonded (ultrasonically welded? epoxied?) so well that I will have to break them apart to find out what failed.
Terribly busy right now--besides my day job I'm leading the volunteer recovery effort from the tornados which struck here on April 16, 2011. This consumes all my nights, but it's nearly done. I should be able to send you something in May or June.
I'm watching my favorite program and there is a knock on the door. My neighour needs leads for his multimeter. I lend him the whole Harbour Freight special with clamp on ammeter with a breif explaination.
Then I think, he knows enought to to pull the plug, right. He is trouble shooting his electric dryer. I had to check. Yes, he knows. We looked a lot of the part. The schematic used non standard symbols but we figure out what was what.
I mensioned that what works is as important was what does not work. It does not do any thing. Oh, yes the door light comes on and the clock works runs. I showed him how to jumper around components. I explained the schematic some what. With a half hour we found the thermal limit component that we jumped allowing the other phase to power the motor.
He went on line and found the component. He phone a friend in the business who had the part. He replace the part and had the dryer running about an hour and half after I started.
He thinks I should go into the business. I think it could never pay enought fixing things rather than Oh its broke and cant be repaired I'll sell you a new one. Just too honest. An not worth my time. But it was fun.
Reminds me when I had a VW Diesel Rabbit. The car ran well, but one time a water leak got in the glow plug time delay relay. It failed and I had to lift the hood and apply a jumper between the battery and the glow plug bus for 30 seconds or so to get the car started. I checked the VW dealer and a new plug in relay was $35 a lot in 1985. So I took the old relay apart, and replaced the comparator ICs, which cost less than $1 each, but with shipping ended up costing $4. It was good for the rest of the car's life.
I had a simular experiance with my solar pool heater. The controler stopped working, I removed the controller from the circuit and found the power relay tha operated the water valve was defective. I went on line to find the part. The venders had a $50 mimium order so it was prohibitively expensive so I bought a new control;ler for $100. Problem solved!
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