So I made it home and took off the cover. Most of the center (where the CRT is) is covered by the safety shield, which I didn't touch. I took a look at the parts I could see (sides and bottom) while the manual was printing, and the only thing I saw that was immediately obvious was corrosion on some of the IC legs. Looking at the manual, this is the Vertical Channel Switching section of the vertical preamp board, and two of those are Quad NAND-gates and the other is a dual Flip-flop. No idea what they do, though. Other than that, the coating on a ceramic cap had a tiny crack, but it's right at the leg.
That corrosion looks unpleasant - but contained to the IC itself. It doesn't
appear to have spread to the PCB or even the socket. If that's the case, at worst the ICs would need to be replaced, but I don't know whether that is necessary. Cleaning them with IPA might reveal the actual extent of the corrosion. Be careful not to allow any corrosion products to fall into any of the switches!
An interestion observation is that those ICs are in the ch1/ch2 trigger select circuit. As you will know from reading the manual (hint hint
), the trigger is the thing that kicks off a sweep. If you look at the manual, you will see that the ext trig input avoids those ICs and kicks off the sweep directly. So, try connecting a signal to the "ext trig" input, selecting "ext trig", and fiddling with the slope/level to see if you can kick off a sweep and/or get the "triggered" light to illuminate.
Replacing a socketed IC is trivial, provided you have a replacement. Now 74xx devices are rare, but, without checking the timing, I would try using a 74LSxx or 74xCTxx device. Be aware that 74xx devices have a 5V power supply - but in this case the +ve (pin 14 (or 16)) is
not at +5V and the -ve (pin 7 (or
) is
not at 0V. So do not connect a scopes probe's shield to the -ve ! Either use a multimeter, or do some mental arithmetic!
But before jumping to the conclusion that the ICs are faulty, it would be worth isolating the fault to that part of the circuit.
I followed tggzzz's advice and put the scope in X-Y mode, and lo and behold, with the right settings, the trace rotated 90 degrees and was horizontal(pic attached). With a little improvisation (I only have one probe!) I set the calibrator signal to both pins at once and got an actual 2-D shape, as well.
If the deflection is roughly equal in both directions, then the horizontal amp is OK and the problem is in the sweep circuits.
There are a lot of switches and ribbon cable connectors around the sweep circuit, and they might or might not be the cause of the problem. Some of the switches are easily accessible and others are not. Most have exposed contacts and can have grit or oxidation problems. If you really think the switches are the cause of the problem, then it is best to carefully ascertain what is still working so that you can avoid fiddling with working components.
Having said that, your audible "pop"
doesn't indicate a switch problem; a capacitor is still a likely cause. Look for tantalum or electrolytic caps which are damaged.