genlabs
New Member
You know, I realized I left this thread sort of hanging with no conclusion.
So here is the condensed version.
I really wanted to understand what was going on with this amp, not just fix it.
So I painstakingly reverse traced the entire amplifier assembly and created a working virtual model of it in SpiceLT. This included both the input board and amplifier boards.
I never used circuit simulation software before so there was a learning curve there. I got the main gist of it learned over a weekend.
As a reminder there are no known A202 specific schematics available. There are schematics for the larger A502 which does biasing temp compensation differently.
Now I have the entire schematic drawn by my own hands. With that in hand I could compare virtual voltages to the real voltages on the physical unit.
The root cause of the problem was a resistor that had wandered way out of spec. I had to pull that resistor out of the circuit to discover that.
I also upgraded the Toshiba output transitors to Sanken equivalents. The Sankens are slightly larger and have higher current capability but match the rest of specs.
I learned quite a bit from this amp and I have a better grasp of what is going on, at least with this design.
So here is the condensed version.
I really wanted to understand what was going on with this amp, not just fix it.
So I painstakingly reverse traced the entire amplifier assembly and created a working virtual model of it in SpiceLT. This included both the input board and amplifier boards.
I never used circuit simulation software before so there was a learning curve there. I got the main gist of it learned over a weekend.
As a reminder there are no known A202 specific schematics available. There are schematics for the larger A502 which does biasing temp compensation differently.
Now I have the entire schematic drawn by my own hands. With that in hand I could compare virtual voltages to the real voltages on the physical unit.
The root cause of the problem was a resistor that had wandered way out of spec. I had to pull that resistor out of the circuit to discover that.
I also upgraded the Toshiba output transitors to Sanken equivalents. The Sankens are slightly larger and have higher current capability but match the rest of specs.
I learned quite a bit from this amp and I have a better grasp of what is going on, at least with this design.