Sony STR K502p Protect Fault

[A_Tom]

This is a home theater amplifier which immediately goes into the protect mode after power on. It does this even when disconnected from all other devices including speakers.It also does it even if I immediately turn the speakers off and with the volume turned down to minimum.

I have already purchased a replacement, but I thought I would take a stab at repairing this one since I have a second unit just like it which is in service. My only test equipment is a multimeter.

My first guess would be one of the ten output transistors, but I'm not sure how I would test them while connected to the unit.

Visual inspection does not show any obvious problems.

Thanks for any help.

 

[tweakjunkie]

Hi Tom, you are likely correct on your assumption. However, depending on the model, some of these Sony's used a driver IC, Part # usually starts with uPCxxxx. These have been known to fail as well. If your unit has a discrete driver stage (individual transistors), there is a 50/50 chance there is failure there too.

The big problem comes if you don't have a Variac to bring the line voltage up gradually. If you replace outputs and still have a problem in the circuit, when you power up you will likely see your hard earned cash go up in smoke, literally.
To complicate matters even further, you will have to somehow bypass the standby relay that puts mains to your main power transformer (the big one) or you won't be able to use the variac anyway.
As well, I can guarantee you there are likely some burnt resistors that won't show it. These are flameproof and need to be checked. Luckily they are almost always very low in value and can be checked in circuit.

If you want to start checking outputs, make sure the unit has been unplugged for a while and the caps are discharged. Put your meter on low ohms or preferably "diode" test and when looking straight at the transistors, test for continuity on the center and right most pins. The reading should be very high, if not infinity. This will get you started, but is not a complete test.

Hope this helps

 

[A_Tom]

Hi Tom, you are likely correct on your assumption. However, depending on the model, some of these Sony's used a driver IC, Part # usually starts with uPCxxxx. These have been known to fail as well. If your unit has a discrete driver stage (individual transistors), there is a 50/50 chance there is failure there too.

The big problem comes if you don't have a Variac to bring the line voltage up gradually. If you replace outputs and still have a problem in the circuit, when you power up you will likely see your hard earned cash go up in smoke, literally.
To complicate matters even further, you will have to somehow bypass the standby relay that puts mains to your main power transformer (the big one) or you won't be able to use the variac anyway.
As well, I can guarantee you there are likely some burnt resistors that won't show it. These are flameproof and need to be checked. Luckily they are almost always very low in value and can be checked in circuit.

If you want to start checking outputs, make sure the unit has been unplugged for a while and the caps are discharged. Put your meter on low ohms or preferably "diode" test and when looking straight at the transistors, test for continuity on the center and right most pins. The reading should be very high, if not infinity. This will get you started, but is not a complete test.

Hope this helps

Thanks for the comments. As I looked at how this thing is put together, I decided it's not very repairable, so I've just given up on the idea altogether.