Prototype Audio Controller Main Board

posted in: Audio Projects 0

Sometimes when you etch your own PCBs using resist ink, you can have issues such as over or under etching the board. But I hadn’t expected that my failure to clean and seal the board properly after soldering would result in oxidation eating the board away over the course of a year until my most complex hand-etched board would fail, taking my favorite and most used / enjoyed project with it.

This board was removed from the Prototype Audio Controller one year after being built. This photo was taken after the board was removed due to a ground plane problem, so some items have been removed from the board during testing. The unit had four stereo inputs, selectable by an LM1037N audio switcher chip, which is, of course, no longer in production. That chip is shown top-left, with eight capacitors and eight resistors mounted around it. It also had a tape monitor circuit, incorporated using two PC-mount 5VDC relays (removed from bottom-right) and two audio op-amps, shown top-right in the photo.

The second photo shows the functional blocks of the board. Top left (yellow) was the LM1037N switching circuits. The LM1037N used 8 resistors and 10 tantalum capacitors and was capable of switching 4 stereo inputs to the outputs. Top right (blue) is the output pre-amp, which is composed of two low-noise audio op-amps running at +/- 12 VDC. Bottom right (red) is the 555 timer and 74LS74 flip-flop which controlled the tape monitor circuit via input from a button on the front panel and switched via two micro relays (green blocks) through the output pre-amp.

You can also see in the first two photos the red wire and another wire which were my first fixes to the PCB from the oxidation. In the third photo you can see how bad the damage was when I removed the PCB from service. I no longer have the schematic for this old circuit and probably won’t redraw it since I will be revisiting this again in a new project at some point.

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