I was excited to take a look at the scope. On initial inspection I could tell that it had seen hard knocks. The vernier knobs for the A and B channels were replaced with knobs that did not fit the shafts and the A vernier shaft as well as V/div shaft were bent and binding.
But, it did produce not only traces but nice tracesHowever given it's age I wanted to open it up to see what else might need work other than the front panel knobs. In any event I needed to open it up to get at the channel A vernier and V/div shafts to straighten them. So in I went.
That plastic bar mates with the slot in the vernier pot shaft. So I had to hack something together to fix that at the very least. (As well as trying my best to straighten both the vernier shaft and the V/div shaft). So I cut a slot into the yolk
and used some cyanoacrylate to fix a new bar into the yolk.
The inside of the scope was meticulous so I did not need to do any cleaning.
All of the voltages tested fine so I put her back together. (I did savage one of the external sync input resistors so I had to solder in a new 15 ohm resistor on the delay external sync line, but no biggie.)
And this is where the problem started. Ugh. It is a bit of a puzzle to put back together but if you disconnect the delay side chassis bar from the rear panel it gives you the wiggle room you need to replace the amplifier board and it's associated daughter boards. When I tried to power it back up and I checked the traces with the calibration output channel B was acting weird and it was NOT acting weird before I took it apart. Sadness. There was only a noisy line if I set channel B to AC coupling. GND produced a noisy line
and there was a 10x attenuation on the DC coupling as well as the 50 ohm input ( did I mention that it has a low input impedance mode? No? Well it does! And it has a .005V/div sensitivity. I want this scope to work.)
So I go into troubleshooting mode. Looking at the theory of operation and schematics were key here. ( But dear God., Keysight can you please, please sort that page in some sort of order rather than date you added the document!)
1) horizontal is fine. (from the service manual both channels use the same circuit)
2) triggering is fine. (as above, same circuit)
3) There is a signal that is getting past the preamp/attenuation circuit so at least part of that is working.
4) Channel B was working before I took it apart. (Did I mess it up during re-assembly? Static?)
Anyway, apart it comes again. But what do I look for? It has to be with the preamp/attenuation board. So that is where I start. I pull that out and look for .... well ... something out of the ordinary and I ask myself, could I have got a solder ball from the external input resistor soldering caught under the channel B switch fingers? Or could I have dropped a washer and it slid under the switch fingers?
Not much harm to pull that off to see what might be going on. And lo and behold. The B element contact is broken. From left to right starting with A the contacts as they were when I first pulled off the switch...
And the middle contact of the first three is basically flapping in the breeze and not springing back like the others. Well, there is your problem right there!
So I start off on fabricobbiling a replacement.
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