Upon documenting the switch and microcontroller pin connections in a table, the pattern emerged. That eliminates a slew of possible issues and points the finger of blame somewhere inside the ancient chip. But that part will have to wait for a future article. I bought a 100MHz Hewlett Packard 1740A dual channel scope to stand in.

The table below shows the matrix lines K0-K3 and P0-P3 connected to each button. As you can see, every button attached to the P0 and P3 lines are faulty in some fashion. If you found this useful and think others might too, could you take a second and share it?

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After messing with the malfunctioning scope haphazardly, scratching my head, and being overwhelmed by the circuit diagram (these are steps -2, -1, and 0, respectively), my "first" step was to systematically take detailed notes on the oscilloscope's symptoms and behavior, what was working, and what wasn't working. It's like I press one key but the scope thinks I've pressed an entirely different one. If you want the detail, take a look at the pdf I linked to above.

Basically you use the front panel controls to determine with what major block the problem lies. This is just a rough outline to give you the flavor of it.

When the sweep reaches the maximum (the beam reaches the far right), a blanking circuit turns off the beam until the sweep waveform returns to zero.

The output of the sweep generator is fed into the horizontal amplifier to generate a high enough voltage to deflect the electron beam horizontally.

Half of the board is used to pass the switch and LED signals to/from other modules.

With a better sense of all the boards involved, I started tracing individual switch pins on the schematic. It was finally time to open the case and perform further diagnosis.But I almost always use the A timebase, so it's a pain to have it in the other horizontal display modes.Eliminating the other blocks I'm left with one, unique to newer analog scopes like this one: the microcontroller.A malfunction in the vertical pre-amp or amplifier could manifest as non-linear or completely non-existent vertical deflection, or a beam that is deflected too high or low, despite adjustment.I had a Protek P3502 with this deflection problem due to a cooked power resistor in the vertical amp. A problem with the horizontal pre-amp or amplifier could result in a non-existent sweep with the beam stuck (a dot or vertical line on the screen), as was the case on my Hitachi V1050F, or a non-linear sweep as recently started happening with my Heathkit IO-12.In truth, I kind of had a feeling there was something logic-y about this problem. So I had to look at a service manual for more information.