QSC mx2000a

[e_c_mccollum]

Need to find out why amp has no voltage on the op-amp. It is acting like it is in protection mode even though prot. light not lite. Only affecting ch. one.

Messed it up in bridge mode. Replaced shorted output and driver. Replaced both drivers and their resistors, zener and non zener diodes in bias network for peace of mind. I have schematic available.

Planning on doing more testing when I finish the mackie 24.8. Any ideas will help. The last tech pulled the fuse for ch. one and cut off ch. one output and Y-ed the ch.2 output so it will output on ch.1 banana post. ?????????

[e_c_mccollum]

Got some voltage readings. Rail voltages are +58v and -110v.

Remove the resistor connecting to the gate of the negative jfet, no drop in -v. Remove negative jfet, no change. I removed the capacitors for the -110 rail, tested at 10000uf. Should be close to 12000.

Since one cap. is wired in series with the -110v rail, could it be the problem? Step driver board components test okay but without the jfet, the -110v should be gone.

[Jazz P Bass]

Opamp +- 15V

Did you check the 15 volt zener diodes?
+ 15 volt zener D923 & C928 (220uf/50v) come off the +110v rail. Through paralled R927 & R929 (8.2K/ 2W).
- 15 volt zener D924 & C929 come off of -110v rail. Through R949 & R947.
Good Luck.

[e_c_mccollum]

Remember, QSC amps can shut down if there is a problem. The Qsc RMX service manual expains how they do this. Once I get the -110v to go away, it should come back on.

The step driver circuit signals the gate on the jfet to increase the rails to 110v when more power is needed. Pretty neat but a pain when messed up. The step driver circuit monitors the condition of the amp. In my schematic it shows this.

[Enzo]

Have you removed the little controller card? If the commutating FET is removed and voltage still gets from point A to point B, then ther must be a path for it.

WHat rail levels are on the good channel? I am asking really if the problem here is one of the high voltage commutator is stuck on and the rails OUGHT to be +/-58 instead of 58/110. OR should it be +/-85 to start with and the whole thing is skewed over to one side. Very different problems to solve depending on which scenario.

[cjlectronics]

This may not have anything to do with your unit, but I recently repaired 6 QSC RMX series amps. Everyone of them have bad volume pots. I pulled each pot and totally dismantled them down to the resistive element. Cleaned with a very mild cleaner, lubed with a very light lube and put them back together. The amps are still working after 8 months.

[e_c_mccollum]

It's finding a path all right. I will try it without the cards to see if any difference. Out of curiosity, I checked the jfet gate voltage which came out to +58/-110. Probably a component leaking voltage in the cards.

There is really no good channel. Ch 2 works but heats up. Found it too has high voltage on one side. Kind of a mess, isn't it.

Anyway, I run this by QSC forum and see what they say. Probably rebuilding the cards will take care of it.

[e_c_mccollum]

Success! The op-amp was bad along with leaking (visually) capacitors that is used for the +/- 15v pwr supply. Did replace zener diodes for luck. Rail voltages are now +/- 58vdc.

[Phostenix]

Success! The op-amp was bad along with leaking (visually) capacitors that is used for the +/- 15v pwr supply. Did replace zener diodes for luck. Rail voltages are now +/- 58vdc.

Which opamp?

[e_c_mccollum]

U903, #5532