Many years ago, I got this Yamaha SPX90 as a throw-in when I bought my Tascam 246 Tape machine. It hasn't gotten that much use over the years - occasionally I would use it on the effects send of my headphone mixer to give singers a little reverb in their headphones, but that was about it. It always worked when I needed it to, until recently when I plugged it in and got nothing, no lights on the front panel at all.
I opened it up, put a voltmeter on the power supply rails and could see that it was not outputting any of the required voltages. The internal fuse was fine, though. This has a switch-mode power supply which is not a design I've ever worked on or troubleshot before. So, I did a little research first (this article by Scott Dorsey, in particular, was helpful) and then started hunting down sources of the issue.
The first thing I looked at was Q1, which is the main high current switching transistor that feeds the main transformer. I pulled it out and sure enough, it was shorted. This is a 2SC2555, which are no longer made but I found some NOS ones on ebay. Once those showed up, I put one in, and still nothing. At this point, I knew I should really get systematic and check everything in the circuit. I started with the other transistors, and discovered that Q2 was also kaput. This is a 2SD1207, also obsolete. I checked all the diodes, there are quite a few, and all those checked OK. Then I started looking at passive components. I discovered that R2, which directly follows the bridge rectifier and feeds the entire circuit, was failed open. So that would explain a significant reason that the supply wasn't working.
I decided that if I was going to be replacing a bunch of parts I should do the large filter capacitor, so I put together a Mouser order with the power resistor and the capacitor, and a substitute for Q2 that I thought was reasonable - not a pinout match but similar specs, or so I thought.
When all that stuff arrived, I loaded them into the board and applied power. The switching circuit fired up, but before I could even verify voltages, most of the electrolytic caps on the low voltage side of the supply let out their magic smoke. I guess my slick substitute transistor was not a great idea. I was really careful to modify it for the correct B-C-E arrangement, but I think there was some other specification that I overlooked that was causing trouble. Not something I am experienced enough to diagnose.
Once all the low voltage caps were removed, I made yet another order to get the replacements (even ones that hadn't popped). I sprang for low ESR Panasonics, since I read this is a good idea.
Here is the PS board waiting for its new capacitors and the replacement Q2 that I got off ebay - correct part this time.
When I got the SPX90, I also got a lot of documentation. In addition to the manual, I also got these:
