I’m really stuck on this PS Audio and hoping someone can help. I cannot find any schematic information online, so i feel a bit blind. This amp had stopped outputting on the left channel, it just had a loud buzz. If left on long enough, a burning smell would gradually occur. I opened it up and checked the main transistors which are screwed onto the massive heatsink at the back of the unit. One of them - a A1360 was shorted on the left channel. I ordered a replacement online and fitted it. Then, I turned it on while connected to my dim bulb tester. Everything came on fine, and no smoking. I then tested both sides of the unit. When plugging the speaker into the right side - the dim bulb didn’t really light up at all and sound worked great. When plugging into the left side, the bulb lit up a bit and a loud hum was heard initially (2 seconds) and then it faded away and the sound worked perfectly. The following day I tested it again and this time (with the speaker cables plugged into the left side) as soon as i turned it on the dim bulb lit up and stayed lit, and I saw an area of the board light up and smoke came out of it. I quickly turned it off. I couldn’t see any burned components, but there was a burnt bit of the board around another A1360 in the left channel (top middle of the board). Unfortunately none of the components are marked with component numbers on the board. So - once again I took the board off and replaced this A1360 (it wasn’t shorted, but i replaced it anyways to be safe). Turned the unit on again, this time with no speakers connected and did what I should have done after the first A1360 replacement - I checked the speaker outputs for DC offset. turns out the left side has 1.7vdc on the outputs.
Since this discovery, I have checked all transistors on the left side for shorts or partial shorts, as well as the large capacitors for shorts and capacitance readouts. I don’t really know what else to do in trying to find the fault when i have no schematic information available. Can anyone help? I recognise it might be impossible. I’ve attached some photos of the board anyways. The red circled component is the first one which was shorted and replaced, the yellow circled component is the one i saw burn up a bit (but didn’t appear to be shorted) it was also replaced.
I don’t think we have a schematic anywhere for this older piece as it was produced after I had left the company and before I returned. The circuit is pretty standard output stage with drivers and outputs separated by a vbe multiplier. If the outputs were to die standard procedure would be to also replace the drivers feeding them and the emitter resistors of the dead output transistors.
The amp was designed and manufactured by Greg Shug at Monolithic Sound in Nipomo California. Perhaps you can reach out to Greg for some help. On occasion he has fixed such problems.
I really appreciate the help here, but I have been unsuccessful at finding the issue so far. Partly because there are no component markings on the board and some of the transistors are NOT what the schematic says they are.
I have replaced two A1360 transistors which were getting hot (but not shorted) - no help, and the new ones still get hot. I’ve replaced a 200ohm resistor which was only reading 180ohms, either R14 or R15. I’ve replaced the output transistors. Still, i have around 1.7-2.7vdc on the left outputs. I have tried doing the DC offset adjustment - if i turn it all the way in one direction, the voltage goes down to 0v and then quickly climbs back up to around 2v, if i turn it all the way in the other direction the voltage goes up to 6v and then quickly drops back down to around 2v again. In other words, something is pulling the voltage down or up to around this value, regardless of the potentiometer setting. If i put a 10ohm resistor across the outputs, two A1360s get hotter on the left side than the right - up to around 40c, and one output transistor gets a bit hotter too - again, already replaced it with an identical component. I’ve also replaced a capacitor on the board which was in value, but had leaked some crap on the board (and cleaned that up too, and replaced a resistor with corroded legs from it). I just don’t know what else to try and I’m finding it so hard to fault find due to the lark of pcb markings. Can anyone give me some suggestions or tips?
