Fusion PP-AM20001

[spooney]

When I received this amp most of the output and power supply fets were shorted . I replaced all output (IRF640N) and power supply (FQP50N06) fets along with their gate resistors. I also replaced the MPSA56 transistors that are driving the power supply fets and two more MPSA56's that I think are driving the outputs. After replacing those parts the amp powers on normally without drawing excessive current. The speaker relay engages a few seconds after power up as it should and the rail voltage is sitting at approximately +/- 67 volts. The amp will not produce any output though. The only audio that the amp is producing is a hum on the output that changes volume with the turn of the gain control. It does it with or without the rca's plugged in but it seems to get worse when the rcas are plugged in. There is approximately 250 mV of dc on the output terminals as well. Not sure where to start looking from here. I have a service manual for this amp but there isn't really a ton of useful info in it. Just schematics and a component list. There isn't a list of voltages to look for.

[Perry Babin]

Do you have audio on pin 7 of IC8?

[spooney]

There is audio on pin 7 but it is very fuzzy looking. Also noticed while I was testing for audio that the dc on the outputs drops considerably when the rcas are not plugged in. It gets down to approximately 30 mV which probably isn't anything to be concerned about.

[Perry Babin]

How much DC is there with no RCAs plugged in?

What do you have on the center leg of Q42? Scope set to 20v/div. timebase set to 5uS.

[spooney]

I think I found part if not all of the issue. I am using two power supplies for my testing. One runs my test head unit and a small amp I use for my shop stereo and the other is strictly for the amplifier I am working on. I was running signal into the amp again and I wasn't getting anything that looked like clean audio even on the center pin of the rca cables so I knew something was up. My oscilloscope probe was grounded on the negative terminal of the amplifier which was using one power supply and the head unit is run on a completely different supply. These two supplies are not grounded together. I ran a small ground wire from the negative of one supply to the other and I finally had a clean audio signal on the rcas. When plugged into the amp I was happy to see that it now played audio cleanly and that hum was completely gone. Dc offset measured at the speaker terminals was only 30 millivolts. Now I am wondering if there is a defective ground shield on the rcas of the amplifier. I haven't had an issue like this before now and today was the first time I grounded the supplies together. It makes me think its either just dumb luck or there is some small issue in the amp. It seems like the differing ground sources would have caused other issues before now.

[Perry Babin]

Do the power supplies have 3-prong power plugs?

[spooney]

Yes and they are both plugged into the same outlet.

[Perry Babin]

You probably burned a trace in one of the power supplied. The grounds were probably connected together through the ground pins in the power plugs before.

With no RCAs plugged in, measure the resistance between the RCA shield on the amp and the secondary center tap on the power supply. You should read ~0 ohms.

[spooney]

In a car the amplifier should work correctly because the amp would share a ground with the head unit and everything else in the car. I just wanna make sure I am not missing something here. Is there something else in this amp that the rca shields should have continuity with that I can check for?

[spooney]

You beat me to it. I will check that out Perry