need help with gallien krueger 800rb

[Paulmongan]

Hi Guys,

I have a Galluen-Krueger 400RB-III. I have the same problem as described above only my preamp board is the problem. I have sound when I plug into the return jack. I have no sound when I patch the send to return. When I turn up the master volume it pulls a lot of current. I can see this on my variac and lamp limiter. So I’m careful not to turn up the master volume.

Testing the TL072 opp Amps I test between pins 4 and 7 with volt metre. I only have voltage on U5. The other 4 opp Amos have no voltage on those pins.

Be great if somebody could help me to track down the problem?
Link to schematic
http://schems.com/bmampscom/gallien/Gallien-Krueger_400RB-III_SM.PDF

Many thanks
Paul

 

[PRR]

> Testing the TL072 opp Amps I test between pins 4 and 7 with volt metre. I only have voltage on U5. The other 4 opp Amos have no voltage on those pins.

Pin 7 is an output. Pin 8 is the other power pin.

The Master Volume should not have any effect on power current (unless it becomes VERY LOUD). Something odd there.

 

[Paulmongan]

Yes there is something odd. Alright.

It was late last night when I was working at that amp. I measured the wrong pins for voltage. I should have measured 4 and 8. (Was looking at the 5 pin example on the data sheet. Sorry ) There is around 15v present on all Opp amps. That was one of the first things I tested and yes voltage is there ok.

I fed a 200hz 100mv signal into the input. I can trace it to C44 which is not far in after the input. . Then I loose it completely.
I can’t see it anywhere after that. The signal should be available at the other side of c44 right?

I’ve checked the electrolytic capacitors and they test ok. Can’t find any shorts on the diodes. Resistors all test ok. I’m not sure about testing the FETS yet. I Will look at that later on this evening.

Paul

 

[Paulmongan]

Hi everyone,
I have the amp plugged into a variac @120v.

I’ve noticed that there is -15v missing from the Source of Q3A and Q3B (R69 voltage point indicated on schematic of the preamp board).

I’ve tested R69, R68 and C43 and they all test good.
Voltage on the Drain of Q3A and Q3B is +28v. On the schematic it states +35
Voltage on the Gate is -28v and at voltage point on schematic which s -35.
I’ve tested the JFETS with a multimeter and they seem to test ok.

Wondering now Would this be the reason why the preamp isn’t working.
The amp is working ok when I play through the return jack. But nothing when I test through the input jack.
I have 13.9v on all of the opp amp ICs.
Hope someone can give me a hand here.

Many thanks

Paul

 

[PRR]

> Voltage on the Gate is -28v

Seems unlikely.

35V versus 28V *could* just be general low voltage.

I've marked-up the expected voltages. Which ones are different on your victim? (If possible, crayon your voltages on the image.)

 

[Paulmongan]

I have taken some readings and entered them into the schematic attached.

Ive a funny feeling the -15v is not being sent to the preamp board from the main board. Might possibly be that cap c9 cap or D11 or D5?

thanks for the help.

 

[Paulmongan]

So after disconnecting the preamp board from the main board I noticed the -15v is not there at P2-4. I tested All the components around Q16 and found R30 Open Circuit. I replaced this component and straight away all voltages where present at P2 1 - 5

The amp is back working again and seems to be performing ok. I will run it for a few hours and hopefully it will be ok.

Thanks to PRR who give me a hand.