All,
I was recently doing some work on a HP 141T spectrum analyzer, and while I was debugging it the display started gradually flickering and jittering. (see my thread here about trying to fix a 8553 plugin:
https://www.eevblog.com/forum/repair/blown-input-on-hp-8553b-sa/)
Looking at the power supply, all of the rails are reading low. The 141T uses a series regulated PSU that uses a +100V rail as its master reference, so I suspect that something on the +100V rail is starting to fail.
I have already tried swapping out the Q2 power transistor-- no change. I also tried connecting another capacitor in parallel with the C3 filtering cap, no change. I tried swapping the plugins out for a set of known good plugins, no change. so I do not believe there is any fault with the plugins, and I am guessing that the fault is after the regulator transistor?
Normally, this wouldn't be the first time that I have debugged power supply issues on a 141T, but the issue here is that this is an unusually late revision 141T (SN 2233-, so mid-1982), but the only manuals I can find online only cover up to 1977. It seems that sometime in between these dates they revamped the power supply board so that it doesn't use a neon regulator as the voltage reference, and eliminates the 10V oscilloscope calibrator rail. It has a completely different board layout from any of the manuals I have been able to find. a lot of the resistor values look to be different as well.
Another quirk of these late 141Ts is that they have a protection relay mounted above the CRT that prevents the unit from powering up without any plugins in, which is actually a bit frustrating given that I would prefer not to risk the safety of my plugins while working on the power supply. I have seen one other 141T from 1985 and it also had this feature.
Does anyone know where I could find a manual that would cover this unit? Ideally, I would even like to find a manual to cover my 1985 141T (SN 2506-), but the power supply board looks to be largely unchanged from the 2233- unit.