aving completed the PC board assemblies and made up the connecting cable as described last month, it’s now time to test the tachometer. However, before applying power, check that all parts are in their correct locations and are correctly oriented. Check also for any solder shorts between the board tracks and IC pads.

Do not connect the display PC board to the main board yet – that step comes later, after the initial voltage checks. In addition, the PIC micro (IC3) should be left out of its socket.

Once you are satisfied that everything is correct, apply power (12V DC) to the main PC board and check that pins 4 & 14 on IC3’s socket are at +5V. That done, monitor the voltage at TP1 (with respect to 0V) and check that this voltage can be varied from about 2-5V using trimpot VR1.

If this checks out, switch off the power and install IC3 in its socket – see Fig.5 last month. Make sure that this IC is oriented correctly; ie, with its notched end towards the adjacent 100nF capacitor.

Next, connect the display board to the main board using the IDC cable, then set VR1 fully anticlockwise and VR6 fully clockwise. Trimpots VR2, VR3, VR4 & VR5 should all be set to mid-position.

Apply power and you should be greeted by a single "0" on the digital display (ie, on the righthand digit). In addition, LED1 in the bargraph should light. If this does not happen, switch off immediately and check for assembly errors (ie, parts placement, faulty or missed solder joints, solder shorts between IC pads, etc).