This guide is for anyone who has an SC140 that does not work.
TODO
Unless otherwise specified the following tests are performed with a low cost digital multimeter.
Results, below, are shown in a table which has a column for each of the following:
- Test – Description of the test
- My unit – Results on my SC140
- Min. – Minimum acceptable value
- Max. – Maximum acceptable value
Perform each check in the order described and, if possible, resolve any problem found before moving on to the next check.
Visual Inspection
From my experience I think the most likely problem will be an assembly issue. These can often be found with visual inspection, so take your time and carefully do the following:
- Check components are in the correct locations and are fitted the correct way around.
- Look for any IC leg that is bent under the chip and not making contact with the socket.
- With a magnifying glass (or similar) inspect all solder joints looking for any that may have a short to other components or where the solder has not flowed nicely onto the pad and component lead.
Never underestimate the importance of visual inspection.
Jumpers
The following tests assume a jumper shunt is fitted in the position shown below.
The write protection jumper (JP1) should have a shunt fitted in the ‘Write protect flash’ positions.
Power supply
With the board not connected to a power source and nothing else connected to the motherboard, use a multimeter to check there is not a short on the power lines. The meter reading may take a while to settle due to capacitance.
| Test | My unit | Min. | Max. |
| Power supply resistance measured at screw terminal (J2) | > 1 Mꭥ | 10 kꭥ | n/a |
A value below the minimum would likely indicate a short circuit, such as a solder splash. A less likely cause would be a faulty component, such as a decoupling capacitor.
The following tests are carried out with a 5 volt power source connected to either the screw terminals (J2), the barrel socket (J1), or one of the serial port headers (P2 or P6). When using the serial port power source, fit a jumper shunt to P4 (serial port A) or P8 (serial port B).
| Test | My unit | Min. | Max. |
| Power supply voltage measured at screw terminal (J2) | 4.93 V | 4.75 V | 5.25 V |
If you have a means of measuring the power supply current, do so now.
| Test | My unit | Min. | Max. |
| Power supply current running RomWBW | 40 mA | 30 mA | 60 mA |
A value below the minimum suggested value might indicate a broken track such that power is not reaching all of the components. A value around the minimum suggested value could be caused by the oscillator not running so that there is very little activity on the board, and thus low current consumption.
A value above the maximum might be caused by a faulty component, a short circuit somewhere, or perhaps an IC inserted the wrong way around.
Reset
The processor won’t run if the reset signal is low.
| Test | My unit | Min. | Max. |
| Voltage from U6 pin 7 (GND) to U6 pin 1 (/RESET) Reset button not pressed | 4.9 V | 4.0 V | 5.25 V |
| Voltage from U6 pin 7 (GND) to U6 pin 1 (/RESET) Reset button pressed | 0.0 V | 0.0 V | 0.5 V |
Check this voltage drops to less than 0.5 volts when the reset button is pressed. When the reset button is released, the reset signal should go high (> 4.0 V) within about 1 second.
If the reset signal stays low, there could be a problem around U7 (the voltage supervisor and reset chip), C10, or the reset button (SW1).
Power LED does not light up
Skip this section if the status LED (LED1) lights when power is applied, then blinks off for about half a second, then turns on and stays on.
If the LED lights but does not then blink off and on as described above, it is likely that there is a fault around IC U6 or the processor is not running the firmware correctly when it comes out of reset. If you can’t see a problem around U6 then skip the rest of this section.
Press and hold the reset button. If the LED lights but did not light at power up, then go back and check the reset signal again. If the LED does no light then continue with these tests.
| Test | My unit | Min. | Max. |
| Power supply voltage at IC U6, pins 7 and 14 | 4.93 V | 4.75 V | 5.25 V |
If the voltage is missing at IC U6 then check bad solder joints on the power tracks.
| Test | My unit | Min. | Max. |
| Voltage from screw terminal (J2) GND to resistor R13 (near power LED) Test end of resistor furthest away from LEDs | 0.17 V | 0.0 V | 0.5 V |
If this voltage is above the maximum, then U6 pin 5 is not pulling the signal low. At reset the two D-type flip-flops in U6 should be cleared. A cleared output on pin 5 should be low. As we have already checked the reset signal it is possible that IC U6 is faulty.
| Test | My unit | Min. | Max. |
| Voltage from screw terminal (J2) GND to resistor R13 (near power LED) Test end of resistor nearest the LEDs | 2.99 V | 2.5 V | 3.5 V |
If the voltage is not within the specified range, it is likely the LED is fitted the wrong way around or is faulty. Also, check solder joints and tracks around R13 and LED1.
Clock
The processor won’t run without a clock signal.
| Test | My unit | Min. | Max. |
| Voltage from any ground point to X1 pin 5 (CLKX) | 2.48 V | 2.0 V | 3.0 V |
If the voltage is not within the specified range, it is likely the oscillator is faulty, power is not reaching the oscillator, there is a bad joint around X1 or there is a short on the CLKX line.
Self-test LED sequence
Following a reset, the status LED should light for a fraction of a second, then blink off for about half a second, then light again. This indicates the self test has passed.
RomWBW uses an output port driving 8 LEDs to display the results of self-test diagnostics following a reset. A suitable port is SC129 set to address 0x0D.
RomWBW lights the LEDs in sequence, such that a successful test results in all 8 LEDs being turned on. This is described in detail here.
More power tests
Check the 5 volt supply reaches the following places by measuring the voltage at the points indicated.
| Test | My unit | Min. | Max. |
| IC U1 between pins 16 and 32 | 4.94 V | 4.75 V | 5.25 V |
| IC U2 between pins 16 and 32 | 4.94 V | 4.75 V | 5.25 V |
| IC U3 between pins 8 and 16 | 4.94 V | 4.75V | 5.25 V |
| IC U4 between pins 36 and 34 This can be tricky so skip this measurement if you are concerned | 4.94 V | 4.75 V | 5.25 V |
| IC U5 between pins 10 and 20 | 4.94 V | 4.75 V | 5.25 V |
| IC U6 between pins 7 and 14 | 4.94 V | 4.75 V | 5.25 V |
Bus signals
Check the voltage on the bus signals indicated in the tables below.
The voltages will depend on what the processor is doing, so on a faulty system they may not follow the pattern indicated by my test results. Also, a different version of the firmware will result in a different pattern of voltages.
If any of these signals are say less than 0.1 bolts or within 0.1 volts of the power supply voltage, it is likely there is a short between the signal and ground or the signal and 5 volts.
If any of these signals is outside the suggested range but not within 0.1 volts of ground or the power supply, then there could be a short between this signal and another signal, or it could just be the processor is running a tight loop and keeping the signal in one state nearly all the time.
If you have an oscilloscope, you will get a better idea of what is going on by looking at the signals at these locations, looking for floating signals or poorly shaped signals.
| Test | My unit | Min. | Max. |
| IC U2 between pins 16 and 13 (D0) | 2.7 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 14 (D1) | 2.4 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 15 (D2) | 2.6 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 17 (D3) | 2.4 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 18 (D4) | 1.9 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 19 (D5) | 2.4 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 20 (D6) | 2.3 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 21 (D7) | 2.5 V | 1.2 V | 3.5 V |
| Test | My unit | Min. | Max. |
| IC U2 between pins 16 and 12 (A0) | 2.5 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 11 (A1) | 2.4 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 10 (A2) | 2.5 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 9 (A3) | 2.8 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 8 (A4) | 3.2 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 7 (A5) | 3.2 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 6 (A6) | 2.5 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 5 (A7) | 2.0 V | 1.2 V | 3.5 V |
| Test | My unit | Min. | Max. |
| IC U2 between pins 16 and 27 (A8) | 2.1 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 26 (A9) | 3.6 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 23 (A10) | 3.3 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 25 (A11) | 3.4 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 4 (A12) | 1.5 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 28 (A13) | 1.4 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 3 (A14) | 1.5 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 31 (A15) | 4.9 V | 1.2 V | 3.5 V |
| Test | My unit | Min. | Max. |
| IC U2 between pins 16 and 24 (RD) | 2.3 V | 1.2 V | 3.5 V |
| IC U2 between pins 16 and 29 (/WR) | 4.6 V | 4.0 V | 5.25 V |
| IC U2 between pins 16 and 22 (CE) | 1.9 V | 1.2 V | 3.5 V |
If any of these signals looks wrong, try measuring the voltage when you press and release the reset button.
If the voltage levels do not appear to change during that first second, it is likely the processor is not running the firmware code in the expected way.
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