08 November 2009

ASM-2 (Part 9)

This evening after dinner I started to test the ASM-2 main PCB. I started with measuring the power. There is a +15/-15 Volt power rail and that seemed to work quite well. Besides there are some +10/-10 Volt reference points on the PCB to use for calibrating purposes. They all looked fine to me. The power regulators got a bit warm, but not hot, so I guess the PSU is holding fine. I hooked up a connector to the common power rail to get a ground for my oscilloscope to be able to look at some signals.

I started by testing the oscillator outputs. On VCO-2 everything looked very nice. I just had to calibrate some of the wave shapers to get symmetric waveforms, but on VCO-1 I found something strange. The sawtooth looked fine and also the square wave, but the sine wave and triangle were completely screwed up. In the picture on the right you see the waveform I got from the triangle wave. This looks more like a distorted saw actually. It clips on +15 and -15 Volt as well and it was supposed to be 10 Volts peak to peak. So time for some trouble shooting.

I started to study the schematics of the oscillator and found quickly that the base waveform is the saw and the other waves are generated from that one. The sine wave is generated from the triangle, so the problem had to be in the wave shaper circuit for the triangle wave. It is based on two diodes and a TL084. After a measuring and comparing to the other VCO I found that on one of the outputs of the TL084 I got no output signal. At first I thought the TL084 was broken, but when I wanted to swap it I felt it was warm. I looked under the PCB and found the problem. A tiny drop of solder was between the output of this IC and a through hole on the PCB. It was so tiny that I missed it on the two earlier inspections I did. But I could measure it.

After I removed it this was the result. On the picture on the right you see a very nice triangle wave. And after this also the sine wave was fine. When I tell it like this it sounds like this was an hour work, but actually it took me the whole evening to find this. But afterwards it is always a gratifying feeling when you solved a problem like this. The next thing is to calibrate the VCO's and test the rest of the modules. I hope that goes a bit smoother. At the moment I'm checking how to hook up everything, but I think I need some more crocodile clamped wires.

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