By my standards, this is quite a long video. This 7854 oscilloscope has a power supply fault that is proving elusive to find. It’s also quite complex, so easy to add additional faults if clumsy. It is so well made I am reluctant to give up on it, despite many modern digital scope’s specifications surpassing this one these days.
Repairing Switch-Mode power supplies can be challenging and this one has some features not often seen in modern power supplies.
It's now back up and running but I'm not convinced I have completely solved the problem. There is also a logic fault still to find, within the vertical channel mode. This may be on the back-plane board which is difficult to access. Connectors are not often keyed, so it's possible to mess up when reassembling from memory. So far, I have not found any out of spec capacitors or dreaded Tantalums shorts. The scope is from the 1980s and also have many hours on it. It could be at the stage of going from one fault to the next.
Sorry for the out of focus schematic section. This was actually because I had to zoom in during editing so is a resolution issue.
CONTENT:
00:00 - Intro
01:10 - power supply issue
01:48 - PSU overview - block diagram
04:34 - Circuit schematic
06:45 - Switch-mode internals
08:40 - testing and fails
09:33 - repair and testing slowly
10:11 - Scoping switch mode
11:40 - Fault summary
11:53 - Soak tested and still running
12:25 - Warning on working on live equipment
26 авг 2023
