Gallien-Krueger 700RB problem

[davidturner]

Greetings!

I have a GK 700RB bass head that works great, except for the following issue:

With No signal running through the amp and only when the Woofer X-Over is NOT engaged, if I turn the boost and woofer up to about 5 or 6, The amp starts to act like there is a short and will run away with current. If a speaker is connected, you hear a thump and see the speaker bulge forward. This also happens with a resistor load connected instead of a speaker to rule out the cab.

I tested with the amp plugged into a dim bulb (current limiter) and an amp meter.

I also monitored the bias at P2. I set it at Idle for 5mV and I see it run away at the same time the Dim bulb lights and the amps start to climb.

If I engage the Woofer crossover, the problem goes away and the amp works perfect.

I am hoping a tech would be able to point me as to where I should look for a fault, and save me from countless hours of hunting and pecking!

Thanks!
David Turner

[nevetslab]

I haven't encountered this one on any 700RB's, 1001RB's or 2001RB's. Is the front panel on your amp labeled Woofer X-Over? I see the buttons on the 1001RB's in our inventory labeled as Woofer Hi Cut, which is probably the same, it using the same crossover circuit as that of the tweeter. So, without pushing in the X-Over/hiCut, the signal is full BW, and coming from IC U6B, fed from the Boost Control, which then feeds the JFET Q4, then thru IC U9A and thru the Woofer Pot to the power amp.

I've had issues with Q4 in the past....it doesn't have the headroom that the IC's do, and distorts easily, but, signal in both cases are passing thru it on the woofer channel, so doesn't sound like the problem is there. Might be something with U6B, which when using the Xover, you're now passing thru U6A, then thru the HiCut filter of the xover, and perhaps that removes whatever LF issues are occurring.

I've attached the schematics below:

700RB-II_1001RB-II_PREAMP_PN206-0251.pdf

1001RB-II_POWER_AMP_PN_206-0260.pdf

[g1]

Does the issue still occur when not on the dim bulb tester?
Just wondering how you came upon the problem, and if it is possibly just caused by the test set-up. Was there a fault you were investigating that caused you to run the amp on the dim bulb? You said the amp 'works great' so some back story would help.

[davidturner]

Nevetslab,

Thanks for your quick response.

my amp is the older 700RB, the button is labeled "WOOFER X-OVER" See link for screenshot on my google drive:

https://drive.google.com/file/d/1JC9...w?usp=drivesdk

The schematic is not the same as the RBii's. I found a service manual for the 1001RB that is closer. heres a link to it on my google drive:

https://drive.google.com/file/d/1J0R...w?usp=drivesdk

I suspect the problem is something in the pre amp that is filtered out by the Crossover. just a guess for now.

Thanks!

[davidturner]

Hi g1,

When I first bought the amp, it wasn't passing any signal. The problem was a broken pot for the WOOFER / MASTER.

I replaced the pot (2 times to rule it out as the problem) with a correct valued replacement, B2k.

once the pot was replaced I found the problem when testing the amp. I have the same results with or without the dim bulb (current limiter)

Thanks!

[Helmholtz]

Could be HF oscillation. Do you have a scope?

[davidturner]

Thanks Helmholtz,

yes, I was thinking some parasitic oscillation of some sort could be the issue.

I do have an old Hitachi v-522 Oscilloscope.

David

[davidturner]

I set up my oscilloscope and made a quick video demo of what I see so far. Heres the YouTube link:

https://youtu.be/6GZYVGDxfcM

[Helmholtz]

Try to capture a scope picture with the HF oscillation (on the bulb limiter). MIght be MHz frequency. I would expect the oscillation to be present at the power amp input as well.

[Jon Snell]

What is happening on TP15 and TP21 DC offset wise?

[davidturner]

Hi,

here's some measurements:

TP21 1.1mV (U1 pin7 to chassis ground)
TP15 6.3mV (U3 pin1 - to chassis ground)

Thx,
David

[davidturner]

I made another video today clearly showing the oscillation signal that is being created by the amplifier. this test is done with NO signal going into the amp. Im monitoring out the Woofer speaker connector.

my oscilloscope is set to:
5 uS Time/Div
5V Volts/Div
probe is set to X1

https://youtu.be/CpJyaXWDKJ4

[davidturner]

Here's what the pre-amp looks like on the scope...

https://youtu.be/cFAT7yvCrSs

[vintagekiki]

https://www.electronica-pt.com/esquema/audio/amplificadores-audio/gallien-krueger/gallien-krueger-700rb-13793/
Gallien Krueger 700RB

[nevetslab]

If you disconnect the 2-pin connector J4 from the Power amp PCB, leaving no signal connection from the Woofer Output of the Preamp, do you still have that oscillation on the power amp output?

Your finding -14.42VDC on U1 pin 7 (of the Line Output IC), do you find similar +14.4VDC or similar at U1 pin 1? Is there DC voltage on the Direct Level Pot R10, it being sourced from the input Preamp 'PRE' or "POST' ? Both of those sources are AC coupled, so I wouldn't expect to find DC voltage there.

The earlier schematics you included are still essentially the same, though clumsily drawn, trying to follow the signal path thru the Post-patch point from J9 (Return jack). If you don't find oscillation on the power amp with P4 unplugged, I'd look at the crossover circuit to see what happening with the LPF (Hi Cut) from U6 Pin 7 on out.

I usually have to remove the front panel preamp PCB assembly so I can power it separately from my lab supplies to troubleshoot.