I'm looking for a reasonably low cost first oscilloscope for power supply repair, audio and hobbyist microcontroller work. If it could decode serial signals that would be a fantastic bonus (RS232, 485, I2c ...)
General principle is that a scope is an analogue domain tool used to view analogue waveforms. Sometimes those analogue waveforms are
interpreted by the receiver as being digital signals. If the analogue signals are imperfect (voltage, timing, noise, transitions) then they may be misinterpreted by the receiver. Hence use a scope to ensure signal integrity, i.e. that the analogue waveforms will be correctly interpreted. After that, flip into the digital domain and use digital tools (logic analyser, protocol analyser) to debug digital signals and the messages contained in the digital signals.
What you want is a tool that lies idle until a message to "address 59" or an "overload occurred" or whatever message occurs.
The analogue waveforms contain a lot of information that is completely irrelevant in the digital domain, and the digital signals are far more information than is easily captured in the digital domain. Hence the first benefit of flipping to the digital domain is that the digital domain tools allow you to concentrate on what is important, by triggering on the
message information and filtering out the "boring stuff". The second benefit is that for i2c, spi and similar, there are
very inexpensive logic/protocol analysers available - and some allow you to transmit signals as well as simply analyse them.
You may find that some scopes claim to decode digital signals, but the decoding is very limited - e.g. only the trace on the screen is decoded, and the rest of the stored trace is ignored.
Careful use of a (cheap) LA/PA/printf() statements may mean that you only need simpler and cheaper scopes - and that you won't be limited by the features (and lack of features) in any single tool.
Apart from that, if you want to "repair power supplies", then be aware that incorrect use of scopes and scope probes might damage the power supply, the scope, and even you. Make sure you use the
right class of probe. You may need to leave budget available for them. FFI see the references given in
https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/