Hello every one, here is my current quest. I have an 2006 Philco PHT500. initially the power amplifier was failing and i managed to repair with no problems, but after a few months all channel gone mute and after some poking around, i isolated the problem being the volume control system.
Initially i thought that the PT2258 chip was bad, but after replacing it, the volume controls still didn't worked, i poked some more and discovered that the reason for the malfunction is that the chip is only receiving 2.3V on the VDD pin instead of 7.5V that is defined on the diagram.
Some more testing later i found out that if i plug the amplifier without the display/control unit connected the correct tension is present on the chip and for whatever reason, once i connect the display unit the tension on the PT2258 goes to 2.3V and stays like that even if i unplug the display until i power cycle the amp.
I checked the tension on the zenner diodes, resistors and didn't managed to find any shorted capacitor in the board, the control/display unit works just fine (and the 5V rails is normal with it plugged in), but something that the control unit activates is causing a voltage drop on the 12V rail, and i can't get my head around it.
I have attached the schematics for the board, so if anyone can point me on the right direction i would be more than glad.
Update:
Following jw0752 tip i started to to look for the reason of the voltage drop, and i found two incorrect behaviours:
1 the display/control unit is shorting the SCK and SDA lines to ground after the initial communication occurs. Because of that, i made some tests on it and came to the conclusion the chip is damaged.
2 For testing purposes i have replaced the control unit with an Arduino, and sending the initialization commands for the PT2258 does makes the audio work for a split second, but after the initial "pop" in the audio, it goes silent again and i get the 2.3V on it's VDD pin, and i don't seam to get an ACK from the chip.
In short, i have a damaged control unit, that i will replace by an Arduino and the PT2258 seams to be the reason behind the voltage drop.
I will investigate further, and depending on the result i might just replace the entire volume control sub-system by an analog one since the control interface is damaged.
Update 2:
Things got a little more complicated now, I removed ZD202 and T204, and then connected the VCC pin from the PT2258 to the 5V line. Not only i can now talk to the chip using the Arduino, but as wheel i get sound on the frontal right channel (connected to channel 1 on the PT2258), and no voltage drop appears to happen in the 5V line.
Right now i'm trying to figure out why i don't get audio in channels 2-6, I'm not sure if my code on the Arduino is wrong, or if it is something else like the tension on the 5V is not enough to the PT2258.
Update 3:
Solved!
Finally found the problem: U206 is damaged, apparently what was happening is that once the PT2258 was setup the lower impedance on the input 5 was enough to the U206 cause a false ground connection internally that was causing the voltage drop and turning off the PT2258.
Now i will just get a replacement for the U206 and make a new control interface for the AMP since the original is indeed is dead.
Thanks everyone for the help.
Also sorry, but i forgot to mark the discussions as question, therefore i can't mark this discussion as answered.
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