The high striking voltage of the aged neon must be the problem here, as it lights briefly at power-up, maybe the voltage is just high enough before the PSU starts regulating, but I haven't checked this.
Yesterday I had a look at the switch & neon problem, first I noted for the single sweep mode the switch had to be pressed quite hard to get the neon on the circuit board to light.
As for neon in that switch, well it's part of the Schmitt trigger circuit on the main sweep circuit diagram.
As before I set the controls & input as listed opposite the circuit diagram page and went through checking the waveforms at the test points (I've added the missing text using a 1820A manual as a reference
), nothing seemed to be wrong with any of them.
Then I checked what was happening at the collector of Q203, again I was getting something that looked OK (no waveform shown in the manual), I pulled a couple of the resistors in the chain to check they had not drifted and they were all OK.
That only left the neon lamp in the switch, to check this I found one from another 1821A (that has been a parts donor for various repairs) and temporarily connected it across resistor R215. It was working fine and this proved the original was knackered.
The switch with neon isn't the easiest part to replace, but probably not the most difficult either. Once the dozen or so screws were removed, along with the side panel, it wasn't too hard to replace, note the wire colours have faded a bit in this 1821A.
The replacement switch seems to be in better condition too, as it requires less force to operate in single sweep mode.
That's two vintage
hp scopes here that work & trigger
, I currently only have this 180A & the 140A here (others are in storage) I had to bring back the other 1421A plug-in from storage to prove the operation of the reset neon, the 1421A I already had here failed last weekend, the sweep length gradually reduced and stopped working.
David