So, i bought the (faulty) electronics out of a Randall RG200G3 guitar combo a few years ago, and at the time, determined that one of the output transistors had failed shorted.
Here's the kicker: the power amp employs lateral MOSFETs (@SK1058 & 2SJ162)
You can find the schematics over here.
Finding replacements that 1) can be had, 2) aren't fakes, and 3) don't cost an arm and a d*ck was about as successful as finding hen's teeth, as you might imagine.
So recently i got a devious idea - retrofit a more conventional BJT output stage. I "mocked up" the circuit in LTspice, and got a seemingly working option easily enough. I then whipped up a board in Eagle, then drilled & scored the PCB in an afternoon.
I went with some free OnSemi samples i got ages ago - MJE15032/33 drivers, and MJW21195/6 output transistors. All was well, until i got to the biasing stage. As soon as i approach the "optimal bias" area (ie. around 2V across the Vbe multiplier), the thing breaks into oscillation
Preemptively, i had already added a 100pF drain-to-gate cap across the MOSFET source-follower that drives the power stage (since "even" the simulation broke into oscillation without that), but that doesn't seem to have helped much.
Attached you'll find the schematic of the power section (screenshot and LTspice file), as well as the (zipped) triode library.
I'd dearly appreciate some thoughts and ideas
Here's the kicker: the power amp employs lateral MOSFETs (@SK1058 & 2SJ162)
You can find the schematics over here.
Finding replacements that 1) can be had, 2) aren't fakes, and 3) don't cost an arm and a d*ck was about as successful as finding hen's teeth, as you might imagine.
So recently i got a devious idea - retrofit a more conventional BJT output stage. I "mocked up" the circuit in LTspice, and got a seemingly working option easily enough. I then whipped up a board in Eagle, then drilled & scored the PCB in an afternoon.
I went with some free OnSemi samples i got ages ago - MJE15032/33 drivers, and MJW21195/6 output transistors. All was well, until i got to the biasing stage. As soon as i approach the "optimal bias" area (ie. around 2V across the Vbe multiplier), the thing breaks into oscillation
Preemptively, i had already added a 100pF drain-to-gate cap across the MOSFET source-follower that drives the power stage (since "even" the simulation broke into oscillation without that), but that doesn't seem to have helped much.
Attached you'll find the schematic of the power section (screenshot and LTspice file), as well as the (zipped) triode library.
I'd dearly appreciate some thoughts and ideas
Attached Files
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