Gallien-Krueger Backline 210 Distorting

[Leooooh]

Hi there,

Got a Gallien-Krueger to fix here but I can not find the problem and need some help.

The amp is distorting in the clean and crunchy channel. I already tested the Pre Amp section and it is ok. No issues. Plugged the bass directly in the return to check the power amp section, and the issue is there.

I have emailed GK for the schematics and they sent it to me. I am attaching it to the thread, both schematics. The pre amp board and the power amp board.

One thing I need to mention before I start, is that I don't have a scope and neither signal generator. I am using only the multimeter. So, one of the first things I started to check was the bias. There is a test point for the bias measurement. In the schematics, they say to set the bias to 10mV. So, to my surprise I got nothing. 0 volts. So I took out the heat sink and tried to measure again the bias directly in the board. Same result. Time to check the other test points.

Measured the rails of 60Vdc and got +65Vdc and -65Vdc. The 15Vdc I got +14,8Vdc and -14,9Vdc. In the 35Vdc rail, something weird happened. Looking at the schematics, I should measure +35Vdc in one side of R36 and on the other side +60Vdc. The same goes to R38. In one side I should measure -35Vdc and in the other -60Vdc. But in the points I should measure +-60Vdc, I got a reading of +- 65Vdc. So, my initial thought was that R36 and R38 were shorted. But they aren't. They are ok. Measured both.

Do you guys have any tip about this?

Thank you a lot for the help.

[nickb]

The schematics didn't make it. The one I have of the power amp doesn't have 65V so we really need to see what you have.

[Leooooh]

Yeah...Sorry...Already updated the thread with the schematics. Thanks for the advise!

[nickb]

Yeah...Sorry...Already updated the thread with the schematics. Thanks for the advise!

Nah.. Didn't work. Try zipping and reposting. Or just change the filename a little.

[Enzo]

You have it. His is the series II version, same as the 400RB-IV
http://bmamps.com/Schematics/Gallien..._Schematic.pdf

Hi, welcome.

First off, resistors do not short. I suppose it is possible in some odd circumstance with certain type resistors, but in over 60 years of electronics, I have never seen one. Not unless it is a steaming pile of charcoal left behind.

Are the 35v rails even used in this model? The reason there would be the same voltage on both ends of a resistor is that no current was flowing through it - that is Ohm's Law. If there is no circuit on the other end of wherever those 35v rails go, then no voltage drops.

Back to your circuit. The schematic has DC voltages on it here and there. How do your match up to the drawing? I mean in the power amp circuit, not the power supply.

[Leooooh]

You have it. His is the series II version, same as the 400RB-IV
http://bmamps.com/Schematics/Gallien..._Schematic.pdf

Hi, welcome.

First off, resistors do not short. I suppose it is possible in some odd circumstance with certain type resistors, but in over 60 years of electronics, I have never seen one. Not unless it is a steaming pile of charcoal left behind.

Are the 35v rails even used in this model? The reason there would be the same voltage on both ends of a resistor is that no current was flowing through it - that is Ohm's Law. If there is no circuit on the other end of wherever those 35v rails go, then no voltage drops.

Back to your circuit. The schematic has DC voltages on it here and there. How do your match up to the drawing? I mean in the power amp circuit, not the power supply.

Hi Enzo... Thanks for the fast reply.

That is the same file I am using. I tried to repost the file, but I can't understand why it wont go.

I didn't understand the last part of your answer.

[Enzo]

Which part?

On the schematic are small voltages indicated, like the bias reading you already found. On the power amp part, look upper left, see these: 3v across R17, 3v across R15, 2.4v across R27, and so on. Then 1.0v on the base of Q18 and the collector of Q17. 0.4v on the emitter of Q18. And similar readings in other spots.

or did you mean the 35v remarks? I don't see the 35v supplies used anywhere in the circuits, power amp or preamp. If I am simply overlooking them, please point out their location. Wouldn't be the first time I missed something. This board is used in several amp models, and so some features of the circuit may not be used in every one. If the 35v rails are not connected to anything, then no current flows, and Ohm's Law - the fundamental law of electronics - tells us that if current is zero, there will be no voltage across a resistor. SO 60v in and 60v out if no current.

I calculate there needs to be roughly 10ma of current through R36 or R38 to make the 25v difference end to end. SO if those 35v sources are not being used, it would be normal for you to find 60v there instead of 35.

Alternatively, if there IS some sort of circuit in the amp that uses 35v, then it may not be functioning, and that explains the zero current.

[Leooooh]

Which part?

On the schematic are small voltages indicated, like the bias reading you already found. On the power amp part, look upper left, see these: 3v across R17, 3v across R15, 2.4v across R27, and so on. Then 1.0v on the base of Q18 and the collector of Q17. 0.4v on the emitter of Q18. And similar readings in other spots.

or did you mean the 35v remarks? I don't see the 35v supplies used anywhere in the circuits, power amp or preamp. If I am simply overlooking them, please point out their location. Wouldn't be the first time I missed something. This board is used in several amp models, and so some features of the circuit may not be used in every one. If the 35v rails are not connected to anything, then no current flows, and Ohm's Law - the fundamental law of electronics - tells us that if current is zero, there will be no voltage across a resistor. SO 60v in and 60v out if no current.

I calculate there needs to be roughly 10ma of current through R36 or R38 to make the 25v difference end to end. SO if those 35v sources are not being used, it would be normal for you to find 60v there instead of 35.

Alternatively, if there IS some sort of circuit in the amp that uses 35v, then it may not be functioning, and that explains the zero current.

Just checked the amp and indeed, those points were we should have +-35Vdc, they aren't connect to anything. My bad. Reaaaaaally bad!

[Enzo]

We have all done that, but it points out the lesson of always putting readings in their proper context.

[nickb]

That is the same file I am using. I tried to repost the file, but I can't understand why it wont go.

It's a bug, that is why I said "Try zipping and reposting. Or just change the filename a little". It's a workaround.

[g1]

Changing the filename won't help, the file size (bytes) must be different.

[Leooooh]

Hey....Some news. I began to measure the fuse resistors and found two open. R27 (1k ohm) and R31 (47,5 ohms). Replaced both and reasembled the amp but the issue remains. Then I started to check the test points marked on the scheatics. All of them are matching with the schematic, except for the test points in Q14. In Q18 (Q14 pair) I have +0,76Vdc in the base and +0,4Vdc in the emitter. Just like the schematics. In Q14 I have +0.46V and in the emitter +0,38V. As you can see, the values are quite different. I will take out Q14 and measure it out of the circuit to check it. Might as well replace it.

Another thing that is not matching with the schematics is the BIAS voltage. I am getting 0V in the J4 test point. That is really weird.

[Enzo]

You won't be able to get the bias right until the amp is working right. it is like setting the idle speed n your car, you cannot do it until you have the engine running.

[Leooooh]

Hey....Finally got the amp fixed.

Since the bad readings were around Q14, I decided to look around it. I found that R46 was open and that Q8 was shorted. Replaced both of them and finally could read something in the BIAS test point. Set the amp to 10mV, just like the schematics tell. Amp is clean again.

Thanks again for the help!!!