If you haven’t been following my blog, you should go back to the first post of this series, in order to get caught up. At this point, I’ve disassembled and cleaned all the spilt coffee from the machine, repaired a trigger control, and replaced a bad electrolytic cap with a temporary substitute. However the machine still didn’t exhibit a horizontal trace. The -8 volt supply was showing it was shorted to ground. I thought that this was due to some problem in the horizontal timebase/sweep circuit.

At this point, I decided to try to further isolate the problem by disconnecting a number of components from the bottom (interface) board. This was quite easy to do, as I had already done this as part of the disassembly process of the trigger and timebase boards. I knew that the system wouldn’t work, this way, but I could check to see if the problem with -8 volts was on the bottom (interface) board or some other board. I unplugged whatever I could from the interface board and powered on the unit. At this point, -8 volts showed as correct, so I started plugging things back in, starting with the trigger control that I had fixed way back in part II of this series. Sure, enough -8 volts went back to ground and scope appeared to have horizontal sweep again.

Duh – my problem was with the repair I had made, and wasn’t due to something else that failed. I did notice all along, that the stop at one end of the rotation didn’t seem quite right. Before removing the switch I checked and sure enough, it seemed shorted to ground. I removed the switch and checked it out. After removing it, everything seemed fine and I could find no short in the switch and the glitch in the rotation disappeared. I reinstalled it and found no short and rotation continued to behave normally. I could only guess that the first time that I installed the switch, that I hadn’t put the index correctly in the slot. When tightened down, the shell had deformed and caused an internal short and also the problem with rotation.

At this point, I figured that this was likely to be the last time that I had the timebase and trigger boards disconnected. As I reconnected everything, I cleaned any soldering residue off the connections that I had to solder to remove or reattach. I did this with isopropyl alcohol applied with a small, stiff, paint brush. I scrubbed the residue until it dissolved and cleaned it up with a paper towel.

I reconnected everything, except the external trigger input, and powered up. The trigger input connected to a large area of copper that required a lot of heat, so I decided to do this later on. Besides, I didn’t quite remember how it was connected. Once I connected everything else, I powered up. This time, my treasured Tektronix 465 seemed to behave correctly. I connected scope probes to both inputs and to the calibration output and got good waveforms and the “A” trigger control worked like it was supposed to.

However, trigger “B” still would only trigger on the downslope and the horizontal position control had the same jerky operation as before. By now, I was getting more comfortable with working with this scope, so I decided that I would investigate those issues before reconnecting the external trigger input and putting it all back together.

to be continued…