Sony STR-D460Z Stereo Repair
For a while, my Sony STR-D460Z stereo was randomly (maybe once a week) going into protect mode (where it shuts itself off and flashes "PROTECT" on the display), but turning the stereo off and back on would always fix the problem.
However, one day it went into protect mode and wouldn't reset when I turned it off and back on. I spent a lot of time googling, and found that many people appeared to have similar symptoms, but I couldn't find many useful suggestions for fixing it.
So, having now fixed my stereo, here's my suggestions:
The first thing you should do is disconnect your speakers to make sure you don't have a short on a speaker wire or within a speaker. In my case, this didn't help.
Next, you should check the solder joints in the stereo. If you flip the stereo over, there is an access panel on the bottom which gives you access to the main board, where bad solder joints which could cause this problem are most likely to be.
On my stereo, nearly all of the solder joints near the voltage regulators, preamps, and main amps on the main board were cracked (probably from thermal stress over many years, as these areas of the board can get VERY hot under normal usage).
Cracks in solder joints can be very difficult to see, but generally look like a tiny circular ridge around the pin. Simply re-melting the solder joint is probably enough to fix it, but I always add a little extra solder to the joint just incase.
If fixing the solder joints doesn't help (as in my case), it's time to start testing circuitry.
I eventually found out that my center channel pre-amp had shorted internally, and this part is apparently well known for going bad, so this is probably the first thing you should check as far as circuitry goes.
The center channel pre-amp is a large black chip labeled 'STK350-230'. When the stereo is turned on (even if it is in protect mode), you can check the voltages of each pin on this chip (relative to ground) with a volt meter. You should get something close to the following voltages (starting from pin 0): 0, 0, 0, 0, -1, 1.1, -47.7, 0, and 47.7
If pins 5 and 6 are nowhere near -1 and 1.1 (mine were -45 and +45), then this chip is probably bad.
The stereo will work normally (minus the center channel) without this chip, so if you suspect this chip is bad, try desoldering and removing it.
If the stereo works fine without the center pre-amp, you're in luck, as numerous places sell identical replacement pre-amps, and they're fairly cheap (less than $10). I got mine from electronix.com. Or, of course, if you don't use the center channel, you can always just leave that chip out.
If the center channel pre-amp isn't your problem, you will probably have to trace out the protect signal to find what is causing it.
I managed to hunt down a copy of the Service Manual, which is infinitely helpful, as it provides complete circuit diagrams as well as normal steady-state no-signal/load voltages for most of the traces on the board.
Start at the connector from the display board to the main board - the protect pin is labeled on the main board. This pin will be at 0V normally, and 5V in protect mode.
From here, just follow the traces on the board, checking voltages against those in the Service Manual, and following any abnormal voltages until you find the problem. The abnormal voltages will likely branch many deceiving directions, so you may need to desolder and remove some components to determine which direction is producing the voltage.
If you disassemble the stereo to work on it, you can power up the main board for testing with only the power supply and the display board connected to it, but with two caveats:
First, all of the grounding contacts on the main board (the screw holes that hold the board to the chassis) must be grounded. (The stereo won't power up at all if they aren't grounded.)
Second, the two contacts on the bottom of the main board labeled "For Service" must be connected with a short piece of wire (don't forget to remove the wire when you're done). (The stereo uses a floating signal ground, and when the volume board is disconnected from the main board, the signal ground will float to +50V, causing all of the amps to open, and triggering numerous protection signals which will prevent you from finding the real source of the problem. Connecting these two contacts shorts the signal ground to chassis ground, which fixes this problem.)