Emerson MW8675W microwave oven teardown: salvaging the LED display

10 min readOct 13, 2016

My old microwave oven, an Emerson MW8675W:

It held up well over the years, but one day a power surge damaged the touchpad. All I could do was cook popcorn, every other button was unresponsive.

Found at Emerson Microwave Buttons: “If some of the buttons respond correctly while others don’t, replace the touchpad.”

The rear view, for reference:

Wow, from 1996? This unit lasted almost 20 years on the dot. About time for a replacement. Replaced it with a 700W Sunbeam from 2012, free on Craigslist and completely functional. Now we can disassemble the Emerson:

Disassembly

The sheet metal case slides off after removing three or four screws. The magnetron and high-voltage transfer are visible on the side:

A pleasant surprise is the label on the top containing a schematic:

Removing the Control Board

The touchpad connects to a printed circuit board with some circuitry:

RY1 is relay 1, connecting white and red from mains to the H.V. transformer. The board can be screwed off, cables unplugged, and gently removed:

A side view of the board:

Removing it completely reveals the entirety of the circuitry:

and the reverse/solder side, including the 4-digit 7-segment LED display:

The thin green ribbon cable connects to the touchpad, directly in the front panel. Replacing the touchpad seems to require replacing the front panel, I’ll pass on this repair (got a replacement oven anyway, can salvage this one).

A Closer Look at the Control Board

DAEWOO-MWO
KOR-611M/Q
HD404314B21S
6C1 JAPAN

is what the larger chip reads. Daewoo KOR-611 is another microwave model — the Emerson MW8675W may just be a rebadged Daewoo KOR-611. Searching finds service manuals, but behind paywalls. Though I did find the comparable Daewoo KOR-161 service manual, on daewoo.com.mx. This manual describes how to short the high-voltage capacitor, in a diagram reminiscent of the 611:

Exploded view of the control panel on page 15:

The mechanical safety interlock mechanism is pretty cool:

It would make sense for Daewoo to reuse components and design as much as possible between microwave models. The KOR-161 may not be identical to the KOR-611, but the board schematic on page 29 looks close enough:

What went wrong?

Back to the Emerson MW8675W I have. Close-up of the microcontroller IC1:

and the second IC (IC2), which reads T H SIA 9538U TD62004AP:

This may be a Toshiba 7-channel Darlington sink driver (datasheet). The TD62004 model has a 10.5 kΩ base resistor and is intended for 6~15 V PMOS, CMOS. Package type AP is 16-bin DIP, as we see here. Pinout:

Looking closer at this chip, the board appears to be burnt! IC2 is described as “high-current, high-voltage… Applications include relay, hammer, lamp and display (LED) drivers.”, perhaps the surge overpowered this chip?

The service manual also refers to the Dongbu Daewoo Electronics Corporation — Service Information Center website: http://svc.dwe.co.kr, but it requires ActiveX and an account to sign in. Passing on this, too.

But if I wanted to get this device working again, first thing I’d try is replacing IC2 (TD62004AP) and the components near it, it may be that the touchpad is functional but the control circuitry is damaged.

The touchpad on the front panel can be pealed off, revealing the sensors:

Segmented Display

Removing the 4-digit display:

The reverse reads “DYS-DDG 611”, likely a reference to KOR 611, this oven’s internal model, then U.L. No RUEW94VO, and a stamped lot number (barely visible in this photo) of 9607:2:

Onboard Transformer

The transformer on the control board (not the high-voltage transformer) reads “DMR-604P SEPU041362 96_F28 CEC”:

Searching finds an out-of-stock Emerson DMR-604P Microwave Display Control Board for $75. No plans what I’ll use this transformer for, but transformers are generally a good idea to salvage due to cost.

Relays

Scored two relays from the control board:

Both are from Omron. The first is an Omron G5B-1, 24VDC, IEC255 (IEC335–1) 3A250V 3A30VDC 1116Y5, SPST normally-open, the second larger relay with contacts on the top (for the mains input) is Omron G5J-1-TP-M 24VDC Contact: 16A20VAC 16A30VDC Coil: 24VDC, 0936YH.

Relays can be especially useful for home automation, as detailed in Home automation with Raspberry Pi + Homebridge, so I’m keeping these for sure. Unfortunately, their coils are 24V, not convenient for interfacing with lower-voltage systems which usually only provide 5V or 3.3V. Still good to have.

Curiously, the PCB silkscreen shows three relays, but one is unpopulated:

Other Components

A handful of other components, not too many interesting:

I took the electrolytic and ceramic capacitors since I am low in supply (lost my stock, still building it up after Salvaging a Samsung DVD-M101 Player and a salvaged Uninterruptible Power Supply), but they probably are not worth salvaging otherwise. A magnetic buzzer, crystal, and resistor array labeled 6A104J with 7 pins and 100 kΩ. I hoped to use the resistor array along with the 7-segment LED display, more compact than a bunch of individual resistors, but the resistance is too high.

Passed on the resistors, transistors, diodes, and other discretes.

Reverse-engineering the Segmented LED Display

The 4-digit LED display is most what I’m most interested in from this salvage. But how to find the pinout?

Searching for the part number fails to find any relevant datasheets. We’ll have to reverse-engineer it by hand. A handy tool for this purpose: a multimeter with diode mode. Probe each pair of pins and see what lights up!

Testing reveals of the 14 pins, the first 5 are for positive voltage (= anode) and select the group, the last 9 are for negative (= cathode). There is a 1.8424 V voltage drop against these LEDs. Labeling the pins as follows:

1: G1, 2: G2, 3: G3, 4: G4, 5: G5
6: i, 7: h, 8: g, 9: f, 10: e, 11: d, 12: c, 13: b, 14: a

Here’s out the LEDs light up:

The only inconsistency is group 1 for the center dots, all the other groups are the same, fairly straightforward. This display is quite similar to the “multiplexed 4-digit” seven-segment display pictured on Wikipedia:

but it has a center colon, and upper/lower cursor indicators instead of decimal points. When it was working, this display showed the current time of day or cooking time remaining.

Application: Clock

It would be nice to wire up this 4-digit 7-segment display to show the time or other useful information. However, my Raspberry Pi happens to be occupied with many various other functions, so I only have a few GPIO pins available. This could be solved with a I/O expander like the MCP23017 (adds 16 extra GPIO), or other circuitry; I may later use a different board altogether. 9 ports would be needed for the cathodes, and 5 for the anode groups (switching each group on one by one, rapidly to take advantage of human persistence of vision to show the full 4 digits), a total of 14 inputs.

For now, I’ll settle for blinking the “:” in the center of the display, once per second, as a homage to this device’s original purpose. gpdot.py:

and the /etc/init.d/gpdot startup script. Wire up the LEDs pin #1 (G1) through a 220 Ω resistor to the Pi’s GPIO physical pin #36 (BCM G16), and the LEDs #12 and #13 to ground, run the script and watch it blink each sec:

Update 2016/10/16: Taking Apart the Rest

The LED display has been salvaged from the electronic circuit board, but there’s more to disassemble in this microwave oven.

The cooler fan comes off easily, labeled Daewoo MW10XA-R01 120V 60 Hz 0.30A MAX Z.P. A Class:

A closeup of the dirty blades:

and the shiny exposed copper coil:

Lots of stuff runs off 120V mains voltage in this unit. Not sure I have much use for a 120V fan, perhaps could reuse the magnet wire for something.

Moving on, the food rotating motor also runs off AC, labeled Synchronous Motor ST-16 6722 KX63MQAD AC120V 60 Hz 6 RPM 2 W Korea Z. P. Jae Il Engineering Co Ltd:

It comes off easily after removing the screws:

The switches/transformer/capacitor wiring comes off together:

The interior is lit with an incandescent bulb (labeled on the bottom “K.E.I Korea 125V 25W K.J”):

Three tactile switches, part of the safety interlock mechanism. From Deco Korea. Some are normally-open, some normally closed. Rated for “15A-1/2HP-125 OR 250VAC, 0.6A-125VDC, 0.3A-250VDC, 15(3)A 250V~, µ 185 +++ (2783)”, stamped date code 96.07.12 VP533A-0F:

These are nice switches, produce a satisfying click when pressed, I have a couple other like it (maybe from another long forgotten teardown?), could be useful in the future for anywhere switches are needed, not necessarily high voltage. Speaking of high voltage, the high voltage transformer:

As briefly mentioned in Opening a Winsson 62–1225 Power Transformer, some people take to opening these transformers by grinding, since they are welded shut. Very serious transformer. Connects to a “Capacitor for Microwaveoven 2100 V.AC 0.70 µF±4%b TUV Rheinland 560–22 NONPCB 21H704SO66R JQF T-80” from Samwha Capacitor Co., Ltd.:

Even though it is only 0.70 µF, this capacitor is rated for 2100 volts, high voltage from the transformer for the magnetron. A peak from the inside:

This is the wave guide of the magnetron that cooks the food. What’s dangerous about magnetrons? When they are off, the insulation could pose health risks if abrated. This component was the most difficult to remove, the screws were extremely tight, fortunately they had hex heads, and yielded to a socket wrench.

The front of the magnetron reads “Daewoo 2M218H MFI 960724BN Korea Caution High Voltage Microwave Radiation”. Looking inside: