2. Improvements

The power switch in the back of the T962A is a double pole single throw (DPST) switch. However, the wiring is such that both poles are connected in parallel and thus only work on either the phase or neutral line. Another feature of the DPST switch is an internal indicator light. This indicator consists of a small neon bulb with series resistor and is connected between the two center pins. So, two failures in one: the oven may remain hooked up to the phase line and light from the indicator in the power switch will never shine. Dooh …

One of the clips holding the DPST switch in place was found to be broken and fixed by white silicon glue. I replaced the switch with a new one and changed the wiring. All masking tape has been replaced and the grounding of the case has been fixed. The two green LEDs in the front panel have been replaced by a red and blue one. The small system fan has been made controllable and the USB interface has been installed. Next, and best of all so far, the controller has been upgraded to a fresh compiled version of the firmware by Unified Engineering of Sweden AB.

The FTDI cable is detected in Linux:

usb 1-1: new full-speed USB device number 35 using xhci_hcd
usb 1-1: New USB device found, idVendor=0403, idProduct=6001
usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
usb 1-1: Product: TTL232R-3V3
usb 1-1: Manufacturer: FTDI
usb 1-1: SerialNumber: FTHBX1SL
usbcore: registered new interface driver ftdi_sio
usbserial: USB Serial support registered for FTDI USB Serial Device
ftdi_sio 1-1:1.0: FTDI USB Serial Device converter detected
usb 1-1: Detected FT232RL
usb 1-1: FTDI USB Serial Device converter now attached to ttyUSB0

Communication is verified by reading the part ID:

# /opt/lpc21isp_197/lpc21isp -detectonly build/T-962-controller.hex /dev/ttyUSB0 57600 11059
lpc21isp version 1.97
Synchronizing (ESC to abort). OK
Read bootcode version: 14
2
Read part ID: LPC2134, 128 kiB FLASH / 16 kiB SRAM (0x0002FF12)

The first two attempts to flash the firmware failed:

# /opt/lpc21isp_197/lpc21isp build/T-962-controller.hex /dev/ttyUSB0 57600 11059
lpc21isp version 1.97
File build/T-962-controller.hex:
loaded...
Start Address = 0x000000C4
converted to binary format...
image size : 52672
Image size : 52672
Synchronizing (ESC to abort).................................................................................................... no answer on '?'

The third attempt with data verification and logging to a file enabled worked out:

# /opt/lpc21isp_197/lpc21isp -verify -logfile build/T-962-controller.hex /dev/ttyUSB0 57600 11059
Verify after copy RAM to Flash.
lpc21isp version 1.97
File build/T-962-controller.hex:
loaded...
Start Address = 0x000000C4
converted to binary format...
image size : 52672
Image size : 52672
Synchronizing (ESC to abort). OK
Read bootcode version: 14
2
Read part ID: LPC2134, 128 kiB FLASH / 16 kiB SRAM (0x0002FF12)
Will start programming at Sector 1 if possible, and conclude with Sector 0 to ensure that checksum is written last.
Erasing sector 0 first, to invalidate checksum. OK
Sector 1: ...............................................................................................
Sector 2: ...............................................................................................
Sector 3: ...............................................................................................
Sector 4: ...............................................................................................
Sector 5: ...............................................................................................
Sector 6: ...............................................................................................
Sector 7: ...............................................................................................
Sector 8: ...............................................................................................|
.............................................................................................|
.............................................................................................|
.............................................................................................|
.................................................................................
Sector 0: ..............................................................................................
Download Finished and Verified correct... taking 17 seconds
Now launching the brand new code

Replaced the masking tape by Kapton tape:
T962A_kapton_mod
Addition of a Cliff CP30207NX USB feed through connector and proper case grounding:
T962A_rear_modT962A_rear2_mod
Marl 357 series LEDs (blue and red):
T962A_leds_mod
Fairchild 2N7000 MOSFET for system fan control:
T962A_fan_mod
Maxim DS18B20 1-wire cold-junction thermometer:
T962A_coldjunction_mod

A new mod enables accurate temperature measurements down to the cold-junction temperature. This requires a split power supply of the thermocouple preamplifier. First step was to lift the GND pin of the TLC27L2CD from the board and solder a wire to it. A small piece of shrink sleeve was placed over the pin to ensure isolation from the pad.

T962A_opamp_pinAn Adafruit SOT-23 breakout board was used to mount a Linear Technology LTC1983ES6 charge-pump inverter and convert the +5.0 V supply from the LM2575S to a -3.0 V. The rear side of the tiny breakout board has two SOT-23-3 pads and could be used to hide the input, output and charge pump flying capacitors.

T962A_inverter

The breakout board was glued in the corner of the thermocouple connector and adjustment pots. The +5 V source and GND connectors can be found on the other side (just in front) of the thermocouple connector. The output of the breakout board is the new -3.0 V supply to the thermocouple preamplifier.

T962A_inverter_installedNote that the minimum temperature that can be measured is now the cold-junction temperature reported by the DS18B20 temperature sensor. Measuring temperatures below the cold-junction temperature would require the A/D converter to allow negative input voltages. The cold-junction compensation could also be implemented differently to increase the measurement range. However, temperatures below the cold-junction temperature are not likely in a reflow oven.