I took apart the scope a bit, and I've had a long look at the schematic and the voltage levels I'm seeing in the horizontal circuit. I am seeing some strange voltages, some of which are definitely intermittent, which was a bit of a surprise, and makes it more difficult to solve of course.
Unfortunately, the schematic is not clear about most of the voltages I should expect to see, but it states two voltages, both of which are intermittently incorrect. Also, when one is correct, the other is incorrect, but I think this is incidental. The first is -4V at a voltage divider, which is sometimes -7.3V instead, and the other is 2.5V at the emitter of a transistor Q2124, which is sometimes 5.3V instead.
Voltages on power supplies are all good.
From what I can tell, the 5.3V on the 2.5V part are simply a consequence of the -7.3V on the -4V voltage divider.
That voltage divider is between ground and -8V to create -4V at the base (pin 2) of a transistor U2126, part of a five transistor 3086 DIP array. However, the voltage divider sometimes reads -7.3V instead of -4V, meaning additional current is being drawn from R2127, the one with one
lead to ground, causing the voltage to drop.
The base of the transistor is also decoupled to ground and to +8V. The collector of U2126 is connected directly to +8V as well, and the emitter is connected to the emitter of a second transistor on the 3086. Those emitters are connected by a 560 ohm resistor to -8V. Thus, the only path from the voltage divider other than the resistors is to +8V, ground, or through the base of the first transistor.
So the suspects have to be the resistors, the transistor, or some short. I poked and scraped at anything that might cause a short, but found nothing. I took the resistors out of circuit and measured them and they were fine, but they were old and crusty, so I replaced them while I was at it. So if the resistors are fine, the current has to be through the transistor. As far as I know, this means the transistor is drawing LOADS of current through its base, about 5 or 6mA by my calculations.
Obviously, that was an immediate red flag, but I took some voltage and diode drop measurements of the transistor in circuit, and they seemed okay. So, I desoldered it from the board and measured again and everything seemed nominal, as far as I could tell.
I don't have a component tester so I couldn't test the hFE or anything, but it still seems very bizarre to me that measurements of the transistor would be reasonable even though it is somehow passing 6mA through its base. I thought 6mA would destroy the base of a transistor. What would it take to destroy a BJT like that?
I assume that 6mA through the base will make it function improperly even if it reads okay out of circuit, so I will definitely replace the chip, but I would still like to know whether it could feasibly be okay for that to happen.
Am I missing something? Is that kind of current reasonable? Could the incorrect -7.3V at the -4V divider be because of something else affecting the emitter of the transistor?
In the meantime, I'll order that part and see if that fixes it, but I'd love to hear anyone's thoughts on this!