My old Samsung HT-C460 home theater system has suddenly started going into protection mode – it displays “PROT” on the screen and then shuts down after a few seconds. I haven’t used its DVD player function for years, only using it to transmit TV audio, but I want to try to diagnose and repair it to extend its lifespan.
I carefully opened up the device and methodically checked for any burnt components, such as commonly seen bloated capacitors, but didn’t find anything out of the ordinary at first glance. I then checked the power supply area, verifying that the output voltages were correct, which are usually printed on the PCB. As a result, I discovered that the -5V rail was only outputting around -0.5V, which seemed very suspicious.
A bit of reversing engineering and probing around the power supply’s -5V rail resulted with this schematic and voltage readings:
I later searched for the part number of the PSU board “AH41-01311A” and found the full schematic anyway,
A 6V drop across two 10R resistors indicates a current consumption of 300mA. This also means that each resistor is dissipating 900mW of power, which is too much for its small 0805 package. It’s clear that something is consuming much more current than it should be.
Since all capacitors seem to be okay, I removed the 7905 -5V voltage regulator and tested it to see if it had an internal short circuit, but it turned out to be working normally. After removing the regulator, I connected a 5V bench PSU to the -5V rail, limited to 300mA (black wire to -5V, red wire to GND, reversed connection to produce negative voltage, as long as you use an isolated power source). However, even with the power board disconnected from the main board, something is still drawing a lot of current. Well, the only thing left is the 470u output capacitor.
After removing the capacitor, everything started working normally again! Using a detection tool to check the capacitor, I found that it had an internal short circuit, even though it looked physically fine, with no bulging or leakage, which are usually typical signs of electrolytic capacitor failure.
I ordered a batch of new 470u 25V capacitors and replaced the faulty one, as well as some other 470u and 330u capacitors, just in case. I also replaced the DVD disc tray motor, which had burned out a long time ago. After these repairs and replacements, my home theater system is finally working normally again! I expect it to last for a few more years.
