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Old 03-17-2017, 06:32 PM
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1970s Maytag Oven Igniter - Fixed!

Granddad's house is a bit of a dump. I'm in the middle of renovating the bathroom. The kitchen needs a little love too, including the wall oven. I don't have a picture of the oven, but to me, it's a very stylish and timeless design of black glass and polished and brushed aluminum. In a time of avacado and turd-brown appliances, it's quite the odd one out.

It has a rotating drum-type digital clock that hadn't worked for years, which I managed to fix with a little elbow grease, and some real grease.

Also out of commision for the past few years is the electronic ignition. I eliminated all the stuff other than the module itself. I opened it, and found immediately the 110vac intputs on the pcb had cracked solder joints. Silly me, I thought this would be an easy fix. Reflowed, didn't work. Took it home, took the pcb out, no visible problems. Hooked it all up with alligator clip wires, and made a crude sparky thingy with 2 nails in a piece of wood. Still didn't work. Took the capacitors off the pcb one at a time and tested for capacitance and shorts, found that C2 didn't want to test right, and it blew up after a second of testing. I thought for a minute I tested it on the wrong setting and fried it.

Replaced it, and the thing worked! It sparked regularly about every half second. Reassembled the module, tested again, and no dice. It did nothing. Took it apart, back on my little jig, still doesn't work. Looking at it while plugged in, L2 (a neon lamp O_o) was flashing regularly, whether the switch was on or off. I messed with it a bit more, and got a spark at the connection to the ignition coil INPUT, not output. Only once, at that, it wouldn't repeat. Nothing visibly fried though.

Le sigh. I took the liberty of drawing up a schematic. It should be right. Kool-Lite 6513

Ideas?
Attached Images
File Type: jpg igniter1.jpg (58.9 KB, 24 views)
File Type: jpg igniter2.jpg (94.6 KB, 31 views)
File Type: jpg schematic.jpg (42.7 KB, 19 views)

Last edited by MadMan; 03-20-2017 at 11:01 PM.
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Old 03-17-2017, 06:51 PM
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What's the black thing that looks like a Horizontal AFC diode?
Maybe just replace all the caps regardless how they test.
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Old 03-17-2017, 07:38 PM
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The only black things are 3 diodes and 1 thyristor. I might recap. I've honestly never heard of a thyristor until now. Apparently it's just a transistor with some minor technical differences.

The more I'm chewing over this, the more I feel some component must be at least partially shorted. Because of the flashing neon lamp, I'm thinking it's getting bursts of power leaking through something else. Idk.
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Old 03-17-2017, 07:53 PM
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I wonder if the flashing light is supposed to indicate it's working?

The only things likely to short would be the diodes and thyristor, I wouldn't even know how to check a thyristor.

I suppose the ignition coil itself could short out?

A Power outage killed the controller board for my Carrier HVAC unit, it's 38 years old but the board is a common part and easily available on the Web. I searched for your board but Oven parts are a lot less common.

I wonder if the ignitor from a modern oven could be adapted?
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Old 03-17-2017, 09:12 PM
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Actually, it is available on ebay for like $40. I'm just a cheapass. Plus I don't like being defeated by 1970s technology.

Also, I really don't think it's the coil itself - it worked so nicely - but I really don't know how to test it, I don't know what to input. Is it ac, dc, 110v, 50v, I just don't know. I'd assume it works like a car's igntion coil, where it has power applied, and disconnecting power makes it spark, but uh, yeah, don't know how to test it.

The diodes all test fine by ohm meter. I'd assume a thyristor is the same as a transistor, but you know what they same about assuming.

The interesting part of the design is that it has to be able to self-terminate. Otherwise it'd keep making sparky noise until you turned the oven off. I believe this is accomplished by detecting the flame at the 'spark plug.' I forget the exact theory, but apparently when two metal contacts are inside of a flame, they act like a diode. Kind of like how a tube works... or something.
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Old 03-19-2017, 07:35 PM
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One quick and easy solution...

Muahaha
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Old 03-19-2017, 09:56 PM
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Not sure what's weirder. Your suggestion, or the fact that it looks like your avatar is the one holding the lighter fluid.
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Old 03-20-2017, 11:30 AM
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A thyristor, also called a silicon controlled rectifier (SCR), is a 4-layer device that acts like a diode, that is, conducts only one direction; but, it doesn't conduct at all unless triggered during the forward half cycle. Hence, it is the device used in light dimmers, where the timing of the gating voltage controls how much of the half cycle it conducts.

More: once the forward half cycle ends, the reverse voltage across cathode and anode shuts it off automatically and it will not conduct on the next forward half cycle unless it gets another trigger pulse.
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Last edited by old_tv_nut; 03-20-2017 at 11:36 AM.
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Old 03-20-2017, 11:37 AM
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Also if they latch: Once they are conducting they keep conducting till the voltage on the anode/cathode hits ~0V.
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Old 03-20-2017, 12:15 PM
zeno zeno is offline
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Dont think its your problem but its common for crap to get
on the spark gap & kill it especially on top burners.

73 Zeno
LFOD !

BTW in New England its not cheap but FRUGAL !
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Old 03-20-2017, 12:49 PM
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Have you checked continuity of the primary and secondary of the coil?

jr
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Old 03-20-2017, 04:38 PM
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Thanks for the input, guys!

Quote:
Originally Posted by old_tv_nut View Post
More: once the forward half cycle ends, the reverse voltage across cathode and anode shuts it off automatically and it will not conduct on the next forward half cycle unless it gets another trigger pulse.
I was reading up on thyristors, apparently they latch on, but it hadn't occured to me that on AC, they'd unlatch on the reverse part of the cycle.

Quote:
Originally Posted by Electronic M View Post
Also if they latch: Once they are conducting they keep conducting till the voltage on the anode/cathode hits ~0V.
Which on ac, would be as the cycle... cycles. Yeah?

Quote:
Originally Posted by zeno View Post
Dont think its your problem but its common for crap to get on the spark gap & kill it especially on top burners.
Yeah dude, that was literally the first thing I considered, so with the unit still in the oven, I unplugged the sparky thingy and used 2 screw drivers to simulate it. It's definitely a problem with the module itself. :/

Quote:
Originally Posted by jr_tech View Post
Have you checked continuity of the primary and secondary of the coil?
Uh... no. Since it has 3 inputs (as opposed to 2 that I'm used to), idk which one is which. I just ohmed it now, the top and bottom leads are continuous at 1 ohm, all other combinations are open.

Anyway, I've literally recapped the entire thing, and no change. I'm assuming the neon lamps are circuit protection devices, so given that one is flashing, that would imply that one part of the circuit is being overloaded. I've even tested all the diodes, and I removed the thyristor entirely, and the lamp still flashes. So I think I've ruled out evertyhing but the resistors. I think I'm gonna take them all off and test each one.
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Old 03-20-2017, 05:11 PM
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Have you heard of the neon bulb oscillator circuit? Basically you put series RC circuit with a high value R across a DC power supply and a neon bulb in parallel with the cap. Neon will not conduct till it hits it's threshold voltage voltage then it will conduct heavily. Basically the cap will charge till threshold is hit then the neon nearly shorts the cap, the neon voltage drops below current sustaining level and the cycle restarts. RC determine the time constant. It was common in early intermittent wipers in cars I was told.

That circuit may or may not be in your igniter (did not check), but it could explain how it pulses the spark if not at a clean multiple of 60Hz.
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Old 03-20-2017, 05:28 PM
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O_o

Um. Idk lol? It very well might be. Honestly, I have no idea how this thing works. I drew up the schematic to help me understand it, but I still can't quite wrap my head around it. :S
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Old 03-20-2017, 06:17 PM
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"Uh... no. Since it has 3 inputs (as opposed to 2 that I'm used to), idk which one is which. I just ohmed it now, the top and bottom leads are continuous at 1 ohm, all other combinations are open."

So there is no continuity at all in the secondary winding that drives the spark igniter?

jr
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