If you haven’t been following my blog, you should go back the to first post of this series, in order to get caught up. At this point I have the trigger and horizontal control board out of the scope, looking for a problem with the -8 volt power supply.

After examining horizontal control board, I’m kind of surprised to see that there isn’t a whole lot circuitry on it. I’m sure that was by design, because, for the most part, this board is unaccessible when installed. It mostly contains the horizontal sweep control, which is hidden under a metal shield. Curious about it, I unbolted the shield to see what was underneath. What I found is a masterpiece in electro-mechanical engineering. I’ve always wondered why I didn’t ever have trouble with the time base control on my 465 and now, I knew why. If the bandswitch control on my Kenwood 530S was built this way, I’m sure I would have never needed to fix it.

Horizontal Timebase Control

Now that I had this board out, I cleaned the switches that were so sticky from the spilt coffee. I also looked for leakage down into the circuit, but found no sign of any coffee beyond the buttons that extended out the front the machine. Since I had so many controls removed from the front panel, I cleaned it up as much as possible. The lack of components and signs of coffee made me think that the problem with -8 volts was not on this timing control board.

I kind of stepped back and took an overall look at the machine. It was at this point, that I noticed a bad capacitor on the bottom (interface) board. One lead had fallen off and in general this capacitor looked like it had seen better days.

Bad Cap

I looked this capacitor on the schematic and it didn’t seem to have a big role, but it was near to the -8 volt rail, so there was slight hope that this could be the root cause of my issue. It was a 1000uF electrolytic cap, which was rated for 10 volts.

Bad Cap on Schematic

I removed the cap and found a 470uF radial cap in my stash, which was the closest thing in value to the original that I had. I soldered it in place as a temporary patch to see if it would help. I put the cover back on the timebase switch, I plugged in the two boards I had pulled and reconnected most of the wires that I had disconnected to see if this replacement cap would solve the issue.

I powered up and the scope had no horizontal trace, exactly the same as prior to the capacitor replacement.

to be continued…