I want to re-build this simple circuit board

Thread Starter

mangotango

Joined Dec 8, 2023
6
Hi all. I'm a novice who tinkers with electronics to try to get broken things to work again. I don't like throwing away an otherwise great device because a few $.05 components burned out. I have no formal (or informal, for that matter) training in electrical engineering, but I have a long track record of figuring out where problems are and fixing them. I've successfully re-capped hi-fi speaker crossovers, re-wired entire rooms, replaced computer components, etc.

So, I recently acquired a vintage Excalibur food dehydrator. The previous owner said it needed a new thermal fuse. I went to the company's website and confirmed that those were a thing and that I could still find replacements, so I went ahead and bought it from him for $20.

I purchased the appliance and ordered a replacement part, and when it arrived, I replaced it. However, the heating coil still didn't heat up. I have a multimeter, and I'm handy enough with it that I can usually figure out how to identify a short or failed circuit.

After taking apart the unit, I looked for obvious signs of problems. I immediately noticed a discolored resistor and assumed that was it. I checked it with my multimeter and could not get a reading. I desoldered it and looked it over; it was very brown and cracked. I assumed this was the problem.

I live in a rural area and don't have access to replacement electronics (no Radio Shack for hundreds of miles), so I identified (I think) the resistor I needed from the four color bands. It's either 10k Ohms 5% or 10k Ohms 0.5%. (I can't tell for sure from the fourth color, which I can't tell if it's green or gold). Either way, I ordered an assortment of resistors from Amazon. (should arrive next week).

Additionally, a pin diode doesn't appear to be working. I don't know how to test it (I did put the multimeter in diode mode on both the anode and cathode and got no readings from either side), nor do I know how to identify a replacement if, in fact, it needs replacing.

I'm attaching pictures of the circuit board and the parts and hoping someone here can help me through this. I'm happy to replace all the components on the circuit board if necessary, however, I don't know which replacement parts I need.

Thanks for any and all help and guidance!
 

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MrChips

Joined Oct 2, 2009
30,415
Welcome to AAC, mango!

We don’t like to throw away anything that still has some life. That looks like a simple enough board to fix.

So what is in a food dehydrator? Basically a heater and a fan.
First things first. When you replaced the thermal fuse, did you check for continuity?

Next, a 5% or 0.5% tolerance resistor, what difference does it make to you or the board? I can tell you with certainty that it is not 0.5%. What is more important is the wattage. If it is burnt out, you certainly want to replace it with a 5W or 10W resistor.

As far as I can tell looking at that board, you only have a few more components that can be bad, the IC, transistor, capacitor, and the pot.

BTW, those are really nice sharp photos.

Edit: Tell us which coloured wire goes to what device, e.g. the heater or fan. What is the part number on the 3-legged device? It is probably a triac.
 

SamR

Joined Mar 19, 2019
5,019
Also, if the resistor is burnt, why is it burnt? What else in the circuit may have caused too much current across the resistor? Are there any old electrolytic capacitors that may be failing and leaking current? Is the heating element burnt out? Check it for continuity. Good Luck and Welcome to AAC!
 

Thread Starter

mangotango

Joined Dec 8, 2023
6
Next, a 5% or 0.5% tolerance resistor, what difference does it make to you or the board? I can tell you with certainty that it is not 0.5%. What is more important is the wattage. If it is burnt out, you certainly want to replace it with a 5W or 10W resistor.
I'm not sure what size resistor that is. It's difficult to read the fourth color band. I'll take your word for whatever resistor you think I should replace it with! Can you point me to a good source (other than amazon) to purchase these components?

Tell us which coloured wire goes to what device, e.g. the heater or fan.
White wire, "B" on the front of the circuit board, goes to one end of the heating coil. From that same nut, another white wire leads to a wire nut connecting to a yellow wire, a black wire, and the cold side of the power cord. From there:

- The Yellow wire returns to the circuit board in position "C" on the circuit board.
- The Black wire runs to the fan motor

From the fan motor, another black wire leads to a wire nut connecting to another black wire, leading to a crimp connector. A red wire leading from the "on" position on the power switch connects to that same crimp connector, which then attaches to the circuit board at point "A".

The other red wire leading from the power switch, from the "off" position, connects to the hot wire on the power cable.

What is the part number on the 3-legged device?
I assume you mean part inside the metal fin (assuming that's the thermostat)?

"MAC15A4 M8213"

Yes, it appears to be a Triac Thyristor.

It is interesting to note that on further inspection, the middle terminal on the pot has what appears to be heat discoloration.

I hope this information is useful in helping to diagnose the problem!
 

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Thread Starter

mangotango

Joined Dec 8, 2023
6
Also, if the resistor is burnt, why is it burnt? What else in the circuit may have caused too much current across the resistor? Are there any old electrolytic capacitors that may be failing and leaking current? Is the heating element burnt out? Check it for continuity. Good Luck and Welcome to AAC!
I checked the continuity on the heating element and it reads 20 ohm and beeps and indicates "short" when I set the multimeter to the alarm setting.

There is a 16v 100uf capacitor on the board. I don't know how to test it and would probably rather just replace it anyway given its age.

Thank you!
 

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MrChips

Joined Oct 2, 2009
30,415
First thing to replace is the 10kΩ resistor.
Next item on the suspect list is the MAC15A4 triac, 15A 400V, followed by the CA3079 IC.
 

MrChips

Joined Oct 2, 2009
30,415
Look for 10kΩ 2W resistor. While you're at it you should be looking for CA3079 and MAC15A4 from the same supplier to save on shipping charges.
 
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