QSC PLX1802 problems

[sparky94]

I have this amplifier that is having a multitude of problems. When I first opened it up of course there were non own solder joints in it. I thought “oh great someone has already been fiddling around in here. Plugging in the amp resulted in a quick show of leds on the front panel. This would flash on for a split second then wait a few seconds and repeat. After narrowing down to blown power transistors on the A channel. Upon removal of these transistors I am able to get the amp to power on. After this I checked all the power supply rails and they are all within spec. The current problem is that both clip leds will come on and stay on, normally they would do this for the first couple of seconds then extinguish. I have tested all components on channel B and cannot find any problems on this side. Inputting an audio signal results in a very dim flickering -10db LED and a faint brightening of the clip LED. I want to see if the amp will make sound at all from the working channel but have been unsuccessful. Does this amp have to have both channels to get past the initial power up. My goal is to see if the amp can make sound out of the B channel before I sink a bunch of money into it.

[CaptDon]

Is it quasi comp-sym output or class D switching output?? Most high power amps, especially comp-sym tend to slam the output hard against the upper or lower supply rail when ANY part is missing or off value, then the D.C. fault detection circuit kicks in and shuts down one or both channels. I am a bit surprised the channel with missing components didn't start to send smoke signals.

[sparky94]

That part I’m not sure of. Please forgive me as analog was certainly my weak spot in the electronics world. I have attached the schematic below. I’ve removed all the output transistors and tested them two tested with complete B to C to E shorts the other two tested completely open. R86 was burned wide open and R81 and 82 were visibly burnt. It must have been a pretty hard failure. I then removed the driver transistors (q26 and 27) and the pre drivers (q19, 20, 21, and 22). I removed the respective transistors on the b channels as well. Upon power up I am was able to test the power rails. All of them tested perfectly.

Anyway my problem seems to stem mainly on the negative side of the amp. Please I place all the drivers back in on either channel, on the positive side the amp will still power up and stay on. The second I install the last transistor (doesn’t matter which one in the chain) in the negative side the amp returns to power cycling every three seconds. If I disable U15 ( the optocoupler responsible for DC shutdown) I can see the full negative rail voltage at the output. Mind you this is still without the power transistors installed. It seems like to me what ever is controlling the base of q66 (probably something around U7) is telling they negative side to turn on full bore causing full rail negative voltage to appear on the speaker output. So at least the DC detection is working (and probably saving my a&& from blowing anything else.) this happens on either channel.

[Audiorepair]

The negative feedback in the amp is not obviously shown on the schematic.

This is actually designated SPKR_BUS_A.     or B.

It connects to R31 then U2.2.   or R153 then U7.2.


You cannot disregard the feedback when troubleshooting, as all sorts of red herrings will appear.

[sparky94]

Still no luck with this so far. The amp still cycles on the off with the negative drivers installed. I notice that after some time of cycling that transistor q22 begins getting hot. It appears that q22 and 21 are turning on from the -5 volt rail. I’ve checked every transistor, diode and resistor in the vicinity of this circuit and I can’t find a single component that is bad. Even the transistor that is heating up still tests fine on the meters diode check. Is it possible for a transistor to test good but still be bad? Unless there is something wrong with U2. I don’t think there is as I have four of these ICs that I have salvaged from various projects and the all result in the same problem.

[Audiorepair]

I have been in a similar situation where a transistor tested good on a component tester but replacing it cured the fault.
Presumably it leaked under higher voltages than the tester tests at.

Maybe you could just bite the bullet and replace all 12 of the small transistors.
Worst case scenario then if still faulty, is at least you know it is not a transistor and can logically use them to test everything else.

Not a bad situation to be in compared with what you have now.

[sparky94]

That is what I was thinking as well. With all the stories of amp failures that I have read where the outputs went full on short across the entire thing. They ended up with blown driver transistors as well. Given that four output transistors tested with major failure, I was amazed that the respective drivers still even tested fine on the meter.