Gallien-Krueger 700RB problem

[davidturner]

Nevetslab,

Thanks again.

Yes, It looks like you are still referring to the schematic for the 700/1001RBII. My amp is the 700RB. I posted a link to the right schematic above. They are quite different.

Ill try to answer your questions below:

"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?"

J4 is J1 on my amp. it connects to p1(woofer amp) on the power board. The oscillation isnt noticible with the pre amp disconnected. I have to have the pre amp connected to the woofer amp to see the problem. There are 2 ways I have found to manifiest the oscillation and thats by either cranking the boost and woofer to about 6 or 7 as described previously, or I also found that if I disconnect the pre amp from the tweeter amp and crank the Woofer I can see the oscillation with the boost and all other controls set to 0.

"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?"

Disregard those numbers I posted earlier about U1. They are wrong and I can't edit the post.

U1 is U8 on my amp (the TLO72 dual op amp behind the Direct Output). I checked all the TLO72s on the pre amp board and found -14.51V on pin 4 and 14.37v on pin 8 on all of them.

"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."

My direct level pot is R91
theres no voltage there, either pre or post.

"The earlier schematics you included are still essentially the same, though clumsily drawn,"

Very similar amps but not the same, also all the components have different labels.

"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 looked at what the crossover does on the scope. I can see it cutting HF. its working properly. Again, with the crossover engaged, the amp works perfect.

Quick fix would be to just jumper the Woofer Crossover switch so that the crossover is always in circuit regardless of switch position, but i really want to figure this out, obviously something is not right.

Thanks!
David

[davidturner]

Today's progress:

Disregard previous video #4. I think the pre amp is fine. I was just seeing my scope wrong.

I found another way to experience the oscillations. If I disconnect the pre amp from the tweeter amp and crank the woofer, it goes into oscillation with the Boost and all other controls at 0.

I replaced the caps that were missing. No change in regards to the problems.

I made another video of what the scope looks like when I run a signal through the amp and bring it up to see the oscillation. You can clearly see that one side of the wave is effected. This leads me to guess that there is a failing transistor.

Here's the video:
https://youtu.be/KTMaW98cct0

I think I will need to pull the heatsink next and investigate. I have also noticed the heat sink gets hot pretty quick even at idle for 5 or 10 minutes, yet I never seen the fan go on.... I'll address this later.

I started pulling the heatsink the other day, but decided to wait. I was having a hard time seperating the chips from the heatsink and didnt want to destroy the thin insulator sheet. Any tips to successfully pull the heat sink without destroying things would be greatly appreciated.

Thanks!
David

[Helmholtz]

Can you see the oscillation without any load connected to the output?

If yes, leave the amp's output open for further testing.

To see if only the power amp has a problem, disconnect the power amp from the preamp and feed your signal directly to the positive end of C13.

[davidturner]

Helmholtz

I tested the amp bypassing the pre-amp by running my signal straight to P1-1.

You must be looking at the schematc for an RBII. C13, the first cap at the power amp input is C8 on my 700RB. C8 is conenected to the input block so I conected to the pin instead of the cap.

Anyway, the power amp looks perfect on the scope and no sign of oscillation. I hooked up a speaker and it sounded correct.

I think this rules out the power amp for now. Now Im back to focusing on the pre-amp!

Heres the schematic Im using:
https://drive.google.com/file/d/1J0R...w?usp=drivesdk

Greatly appreciated,
Thx!
David

[Helmholtz]

Helmholtz

I tested the amp bypassing the pre-amp by running my signal straight to P1-1.

the first cap at the power amp input is C8 on my 700RB. C8 is conenected to the input block so I conected to the pin instead of the cap.

Anyway, the power amp looks perfect on the scope and no sign of oscillation. I hooked up a speaker and it sounded correct.

I think this rules out the power amp for now. Now Im back to focusing on the pre-amp!

Ok, then I meant the positive end of C8.

Could you achieve the same critical output level as before, which starts the oscillation?

Please answer my question about output loading.

If the oscillation is produced in the preamp, you should be able to see it at C8.

[davidturner]

Helmholtz

My results are the same with or without a load on the speaker output. Im not using a load for testing anymore, but it feels wrong coming from a background of tube amps, lol.

I was able to run full signal and turn the boost and woofer to 10 without issue when connected to C8, bypassing the pre amp.

The oscillations produced in the preamp are to small for me to see on my scope. I can only see them after they hit the power amp.

here's an update:

well, I think im getting close, divide and conquer.

I pulled C4, (to cut input through eq section) and tested with signal through the RETURN with no change. I left that cap out.

I pulled C16 (off Q1)



NOW I get good clean signal from the RETURN to the speaker output. I can crank both boost and gain with no feedback. I can crank the amp with the xover switch in or out. I see a change in the signal when I engage the crossover so it looks like the crossover is still in circuit.


I replaced Q1 (j113) just because I had some a few days ago. no change at the time.

So it looks like the problem is somehere near that boost circuit or the crossover.

I need to take a few minutes to understand what I did exactly when I pulled C16 to understand where to look next.

I put C4 back in circuit to enable the front of the pre amp and tested with a bass through the input. The oscillation is gone, but the amp doesnt seem quite right. low volume etc.

C16 is the only cap that I left out for now.

Thanks!
David

[Helmholtz]

My results are the same with or without a load on the speaker output. Im not using a load for testing anymore

That's good because it relieves the power amp from the load current.

I was able to run full signal and turn the boost and woofer to 10 without issue when connected to C8, bypassing the pre amp.

When you bypass the preamp, the preamps controls have no effect on the signal.

I need to take a few minutes to understand what I did exactly when I pulled C16 to understand where to look next.

Pulling C16 just lowers the gain of Q1. That might be enough to stop the oscillation but doesn't cure the root cause.


Oscillation can have many reasons, most likely ones being a bad (high ESR) filter cap or a bad ground connection.
I don't suspect a transistor.

[nevetslab]

I've had to rebias Q1 in the past, as that stage doesn't have the same headroom as the op amp stages, resulting in asymmetrical clipping. I don't recall it oscillating, but you were getting that someplace, and it was asymmetrical at that.

[davidturner]

nevetslab

Well, I put the amp back together, I put a new 47uF cap back at C16. The schematic shows it should be 33uf, there was a 100uF in there originally.

Anyway the amp still functions the same, it sounds great as long as the xover is enabled.

I did notice the problem getting worse, now, just a little woofer with no boost (with the crossover disengaged ) I can see, and now hear the oscillations through a speaker.

I did start looking at the components around Q1, but I didnt check the bias yet. I gotta read the technical data for j113.

I was looking at R31, The schematic shows 12K, it measures 24K in circuit, and I just couldnt make sense of the color codes.

Heres a pic of R31:


best I can make of it is:
Brn, Red, Blue, Violet, Gold = 1260M 5% tolerance.

so thats 2 of 2 components that didnt match the schematic so far around Q1....

I need to analyse that part of circuit and come up with my next move. I certainly want to make sure Q1 is properly biased. In the meantime, Im gonna play some bass, this amp rocks!

Thanks,
David

[Helmholtz]

nevetslab
I was looking at R31, The schematic shows 12K, it measures 24K in circuit, and I just couldnt make sense of the color codes.
best I can make of it is:
Brn, Red, Blue, Violet, Gold = 1260M 5% tolerance.

Measuring resistors in-circuit often gives too low (never too high) readings - especially with SS equipment. So lift one end and re-measure.

From the picture it could read as Yellow, Violet, Green, Red, Brown = 47.5K/1%.

R31 is the source resistor of Q1 and sets its bias.
Relevant Jfet parameters can vary up to 500%. That's why they often need to be selected for the specific application. And a reason to not unneccesarily replace them.

A different source resistor may have been necessary to set suitable bias, but high source resistance lowers headroom.