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: