Hi! I've been lurking around here forever and appreciating it, but this is my first post. I hope I get it right!
I just got a nice HP 3561A Dynamic Signal Analyzer "For Parts/Not Working" on the Bay. Looking at the chips, it seems to be about a 1991 model. Just as the seller stated, it boots up but the screen will only output for a short time. If I wait a few days, it will fire up, pass its POST, then go into the regular screen. It will last about 90 seconds and then fritz out. Subsequent power on tests result in that 90 seconds going to 60, 30, and so on until the screen barely comes on unless I wait another few days or something. During the time the screen is off, the thing seems to operate normally. No error codes on the screen or via the LEDs on the front. Wish I could hook up HPIB to look around, but I don't have that capability right now.
I've got a PDF of the manual, and I've gone through a good bit of the power supply testing. All the rails look in spec with the exception of the 24V one, which is about .3V under allowance. I think that rail runs the input board. Doing the troubleshooting procedure, removing the boards in sequence to see if they are causing the issue doesn't reveal anything. I've not yet looked for ripple.
Cosmetically, it's in beautiful shape inside and out. No goo or extra dust in there. Someone's been in it before me, because I did see the screws for one of the shields taped to the shield instead of screwed in. I can't tell what they did, though.
I took all the cards out, cleaned the contacts and reseated them. They didn't look dirty, though. I've not looked very close to the CRT, because I don't want anything to do with 8kV. I've fixed up several pieces of equipment, but stayed away from high voltages.
I've got a searchable manual on the way from QService, and I'll go farther into the troubleshooting in there. But I wanted to post here to see if anyone had an idea off the top of their head. I've Googled around quite a bit and haven't seen a lot. Here's a video of the startup so that you can see it happen:
https://youtu.be/WRjR0Wevgvc (It happens first briefly at 1:02 and then gets going finally at 1:18).
Thanks in advance!