Microwave timer/clock 4 digit 7-segmnt display is out. Timer works fine.

Angelo Campanella · 2011-01-09T05:10:53+00:00

My ancient Amana Microwave oven numerical display is out. Has been acting up for years, but I now use it daily. (Batching it. Dear Wifey now in heaven.)I can punch in the time I want, and the oven will run and turn off accordingly. But I can't see the countdown any more because the display digits are out.Well they really are not out... just that the lower left segment on all four are lit continuously, not other segments ever avtivate. Once and a while #1 "4" and occasionally #2 "2" light on cue during the countdown. I've opened it up and measured power supply voltages and they are correct and normal at the power supply.I suspect it to be a power supply problem somewhere in the + busses, or possibly a common ground not making ground. But I have no clue as to where to start probing. Each digit is red gas discharge unit, unpluggable from their socket... really ancient... 1976 vintage. Any suggestions? Ange

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Spehro Pefhany 15 years ago

That's just a regular old 0.3" seven-segment common-anode LED display, like the old Monsanto MANxxx displays.

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Extremely low efficiency by modern standards, but good quality for the day.

We've got an antique Amana Radarrange with similar red LED displays.

Here's a more modern equivalent that is pin-compatible:

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(but more than 10x as bright at the same current)

But it's not likely the displays that are causing the problem- probably the drivers or some connections or circuitry further back.

Best regards, Spehro Pefhany

"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com

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Spehro Pefhany 15 years ago

(well, pin compatible on the segments, but it has a right-hand decimal point rather than the rare left-hand on the microwave oven, so not really 100% compatible...)

Best regards, Spehro Pefhany

"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com

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Angelo Campanella 15 years ago

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All well and good. But I need a typical driver circuit diagram to probe = voltages. It could be something as simple as a dropping resistor that's = opened up, or a local filter capacitor that is shorted... I need a pin = voltage map to find which voltage is missing.=20

=20 The brightness is plenty good for kitchens,=20

The decimal point is not used for microwave heating.

Ange

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William Sommerwerck 15 years ago

Akkk! If you keep running it empty, you might damage the magnetron.

I'm reminded of the opening of "The Outer Tsimmes"...

"There is nothing wrong with your television. But if we keep f***ing with it like this every week, pretty soon there will be."

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Geo 15 years ago

voltages. It could be something as simple as a dropping resistor that's opened up, or a local filter capacitor that is shorted... I need a pin voltage map to find which voltage is missing.

If you look at this page:-

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There are two gifs from an HP appnote. They shows suitable multiplexing circuits. It is possible that your circuit has stopped multiplexing so only single segments permanently on. May be missing a clock signal.

Geo

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N_Cook 15 years ago

solid

a

Can you view the board enough to count the pins on each of the non-display electronic chips?

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Angelo Campanella 15 years ago

Have to unscrew the bezel allen screws again. in a few days..

Ange

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Angelo Campanella 15 years ago

Seems to be near what I have.

I'm an old discrete component and vacuum tube guy.

I presume that "strobe" means hat there is only one driver, the SN7447 which is the only segment feeder, that the digit switched on at the moment by the strobe signal will take segment current and light up.

The current comes from the 7447, whatever of the 7 lines is to run to make the number of the instant.

In my uwave, only E segment is lit and the E of all 4 digits are on all the time. No other segments of all 4 digits ever light. Well, actually the

4 of digit 4 is lighting every time its turn comes in the countdown. Perhaps that digit is more sensitive.

One thing for me to look at is the health of the seven 47 ohm resistors.

Another is to check the + voltage supply. Do I expect 5 volts or some higher value? The two raw dc supplies are 10 or 20 volts (the filter capacitors gave away their location.)

Ange

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Microwave timer/clock 4 digit 7-segmnt display is out. Timer works fine.

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