Friday, June 28, 2019

SPX90 Repair

This repair would probably have taken someone who knows what they're doing about 5 minutes... it took me significantly longer than that. 

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.
Here is the board with all the components installed and back in the chassis. That long metal bar that connects the power switch to the front panel is a real pain, since it requires the removal of the front panel to get the PS board in and out. I assume it's set up this way to keep the mains voltage wiring segregated to the back corner of the chassis and away from the audio signals. I tested the supply voltages with the headers disconnected, and all the rails looked good!

It's alive! Now I can make all my cool lo-fi 80s gated reverb drum sounds that this thing has in its presets. Next time I have to mess with a switching power supply, I will hopefully not make so many mistakes. Overall, this probably cost me $25 or so in parts and shipping. Part of that is the sellers of these obsolete transistors make you buy more than 1.

When I got the SPX90, I also got  a lot of documentation. In addition to the manual, I also got these:
The one on the left has some pretty funny stuff in it. Here are my two faves:


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