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Hi there! This is my first thread on this forum so thank you all in advance.
I am troubleshooting a Marshall MG100HDFX solid state amp. This is a two channel amp. Each channel has two settings: clean/crunch & OD1/OD2.
My client says the amp than in second setting of both channels (crunch and OD2) the amp starts to squeal in a high pitch frequency. Nothing weir: the usual feedback of a high gain setting, but at low pot setting. In clean and OD1 position the amp behaves as expected.
Once in my shop I saw in the scope that switching from clean to crunch in channel one caused the gain to increase by 10 times at the output of IC5A (see attached schematic). For example, if the gain control is set on 3 you could see a normal signal on the clean mode but the gain goes full just switching to crunch mode, causing the amp to feedback like crazy.
The gain pot is on the negative feedback circuit of IC5A and every thing looks fine here. The switch bypasses two big resistors (2.2M) R87 and R86 to modify the input impedance in the loop and to connect the wave shopping LEDs respectively.
I guess a change in the input impedance in the negative feedback loop can make a big difference in te closed loop gain. And if I look into the schematic I see that the switch changes from 33K (R89) to 470R (R88). So if I were by the schematic I would expect such a gain increase!
So for me everything is like the schematic says, but it doesn't work fine.
Any suggestion before I surrender to the mystery?
Thank you all!
Marshall MG100HDFX.pdf
I am troubleshooting a Marshall MG100HDFX solid state amp. This is a two channel amp. Each channel has two settings: clean/crunch & OD1/OD2.
My client says the amp than in second setting of both channels (crunch and OD2) the amp starts to squeal in a high pitch frequency. Nothing weir: the usual feedback of a high gain setting, but at low pot setting. In clean and OD1 position the amp behaves as expected.
Once in my shop I saw in the scope that switching from clean to crunch in channel one caused the gain to increase by 10 times at the output of IC5A (see attached schematic). For example, if the gain control is set on 3 you could see a normal signal on the clean mode but the gain goes full just switching to crunch mode, causing the amp to feedback like crazy.
The gain pot is on the negative feedback circuit of IC5A and every thing looks fine here. The switch bypasses two big resistors (2.2M) R87 and R86 to modify the input impedance in the loop and to connect the wave shopping LEDs respectively.
I guess a change in the input impedance in the negative feedback loop can make a big difference in te closed loop gain. And if I look into the schematic I see that the switch changes from 33K (R89) to 470R (R88). So if I were by the schematic I would expect such a gain increase!
So for me everything is like the schematic says, but it doesn't work fine.
Any suggestion before I surrender to the mystery?
Thank you all!
Marshall MG100HDFX.pdf