This is a spin-off from this thread, where both Vince and myself had similar issues with the pre-reg board on some Tek 2215 oscilloscopes
https://www.eevblog.com/forum/repair/tektronix-2215-scope-repair/50/I thought I'd start a new thread as I'm now really scratching my head and thought I'd throw it out to a few others for an opinion.
The unit has no trace, however there is a (VERY LOUD ... which is driving me crazy every time I power it!) high pitched whine. Flicking things like the trigger switch etc on the panel sees the LED flick on, and pushing the beam find loads the supply (whine changes). So it wants to work, but doesn't. I thought pre-reg, as that's what failed previously, but I couldn't fault that.
As suggested by Vince I am now powering the unit from a bench supply, that takes the pre-reg out of the equation. The unit is drawing approximately 1A @ 42.2V from the supply. The standard test output voltages are all present, but low: TP500 -5.6V (-8.6V); W975 5.6V (+8.6V); W985 4.0V (+5V); W965 20.7V (+30V); W966 71.0V (+100V), (brackets = nominal correct voltages as per manual). The manual provides a typical resistance for those points, and the 5V rail is reading 160 ohms vs book value of 330 ohms, but all other resistance values are close to suggested. I isolated most (if not all) those rails and it didn't seem to make much, if any difference.
Next is to check the Head Room voltage at TP952 and confirm/adjust it to +4.3V. Well here is a problem as it was reading +15.3V and could not be adjusted. A relatively easy fault I think so I check the power supply regulator and everything looks ok. Thinking maybe I've missed something I swap Q948 and Q954, but no change. Much head scratching. Everything I check looks ok, but something isn't working quite as it should. So I have written down the DC measurements to see if I'm missing something here. Attached is a photo of a copy of the circuit .
I'm also wondering if the circuit is actually fine and I'm chasing a red-herring here? Could it be that there is something (like the HT supply for example) that is loading the output and causing the symptoms I'm seeing?