Security panel board repair

Several of these boards have failing power supply capacitors affecting reliability.

These boards have internal power planes with little or no thermal relief. This means that removing, clearing the holes of solder and replacing the capacitors is tricky without damaging the board.

It requires a very powerful soldering iron and good timing. Too long and the board is damaged, too little heat and the through-hole plating can be damaged.

Fire alarm system board repair

This board just stopped working.

Powered by 12V or 24V DC, for the 24V option it has a linear pre-regulator. At some stage it had been repaired and the pre-reg transistor upgraded to a high-current type but with only a low gain. This stressed the zener voltage reference which failed.

The upgrade was probably due to the original transistor failing and/or overheating. The board pattern suggests a TO-92 package, which would not cope well with the 0.6 watts or more of heat it has to radiate. The larger TO-220 package can cope with a watt or slightly more without heat-sinking.

The solution was to replace the low gain transistor with a high gain darlington and adjust the zener reference to get back to about 12 volts.

Fluke 77 multimeter repair

This was reported as having a intermittent voltage range.

The first suspect was the range switch, often the problem. But after cleaning the switch and dusting out the meter, it was intermittent when the circuit board was touched, stressed while out of the case. This suggested a fractured solder joint or broken track.

A bit of poking, prodding and looking at it through the magnifier revealed two failed solder joints on the main chip. The repair involved re-soldering the 2 pins.

The pins were in position but the solder joints had broken. I bent them off-centre as I checked all the pins – and it made a better photo.