Gallien Krueger 1000RB Oscillating

[rf7]

This amp has a nasty 247Khz oscillation coming off of the power amp. How do I figure out what's causing this? I dead patched the return, and have the master on zero, but it's still oscillating.

[Enzo]

At the output of the power amp is your zobel network, a stability circuit. R410,R411 are in parallel. They are each 10 ohm 2W resistors. Together, they are in series with C409 between the amp output and ground. Make sure those resistors are not open, and that the cap is intact. Cap may have broken off, but in my experience it is never the cap that has failed.

And somewhere nearby should be the output inductor L405 - a coil of wire. Make sure it is not broken off at one end. There is a 1 ohm R412 in parallel with it, so don;t let that fool you taking resistance readings.

[g1]

If an amp in the forest oscillates at 247khz, does any one hear it?
Or are we talking weird switcher "noise"?

[zero cool]

I have a GK 1001 that oscillates also. but it only does it when the Crossover is engaged. a signal injected directly into the amp board runs fine. the pre-amp disconnected from the amp board runs fine. but connect the two together and it turns into an AM radio transmitter.

The pots for these models are notoriously bad. even confirmed by a GK engineer. tap on the pre-amp board and it goes crazy. I have already replaced all of the pots in mine and its much more stable now. still oscillated but it is easier to troubleshoot now as i am not hunting phantom problems.

[Enzo]

Oh, so many things make amps oscillate. SImple things like mis-adjusted bias, or poor ground connection at the power amp input. But it can be darn esoteric at times as well.

[rf7]

The Zobel network looks OK, but the bias sits at 0.1 mV no matter where the pot is turned, so there's a problem there. Some parts look to have been replaced previously in that part of the circuit too, along with a burnt trace.

[zero cool]

Well I have made some progress with my GK 1001RB. there is a J113 Jfet just after the Boost controller that I think has the gate gone funky. I removed the fet and jumped the I to the O and the amp works great. no oscillations at all. much more stable. Not even sure WHY that fet is there. seems maybe just for flavor??? it's possible its providing some gain but there is an op-amp just before and after configured for unity gain that could be setup differently. so...Shrug?

I also noticed that there is no Gate resistor on this Jfet. I am used to seeing gate resistors on Mosfets. not sure if Jfets benefit from having a few ohms in series with the gate's to prevent oscillation?

I don't have the schematics in PDF form or i would post the page here.

[rf7]

Well I have made some progress with my GK 1001RB.

Congrats!

I've got -3Vdc on my output. All the semiconductors measure OK in circuit. More fun.

[rf7]

Got it fixed, R217 was open. Looked like it had been replaced once already.

[rf7]

AACK! It blew up after the guy took it home.

I ran it for 8 hours and played through it on and off all day. Even bench tested it (again) for the customer at full power into 4 ohms.

[rf7]

Well, half of the output devices blew and both bias strings blew up also. And a filter cap broke off a leg. Got it running on the bench and it blew half of the output devices again--such fun! It turns out that the owner installed a large muffin fan, running off of the AC, conveniently aimed only at the portion of the heat sink where the thermal sensors are. The thermal sensors never actually got hot enough to do anything (like turn on the heat-sink mounted fan or shut the unit down on over-temp conditions), and the rest of the heat sink got hot enough to destroy output devices.

[zero cool]

I have a newer backline 600 that has the same problem. i have figured out that the Jfet after the boost controller is indeed the problem! this jfet is there to add color to the sound by being overdriven asymmetrically. replacing the device does not help! I removed the device. wired from the wiper of the boost control directly to the input of the next stage opamp and the amp works 100% reliably and has full power. it just does not have the flavor control. I see looking over the various revisions that this circuit has changed several times. this seems to be a problem area with these amps.