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Reading a few of the persistent drizzle of posters saying "uh, my (insert amp here) just died. What could be wrong?" made me think back to debugging, and then the solid state devil that lives on my shoulder whispered into my ear the following:
"Most of the time, you can debug an amp by just looking at what voltages are on the tubes. Why not just read all those... but automatically?"
The idea is of course that modern microcontrollers use so little power that they can run semi-forever from a micro-battery, the almost all include an A-D converter, and they can easily enough read voltages and make conclusions from them. The amount of conclusions to be made is really quite small. Great precision is not needed.
I suspect that with a remarkably small set of chips - a microcontroller and a few analog MUX selectors, mostly - you could have the thing read all the tube voltages and just tell you what's wrong with an amp in 90+ percent of cases.
Voltages are scaled to 0-5V by resistor dividers; slowed down by a small capacitor, and clamped to the logic power supply by diodes to prevent killling the logic.
Bias can be read on a positive-only voltage scale by having the bias voltage pull down on a pull-up resistor to +5 through a largish resistor with a clamp.
You can even sense oscillation by using ultrasonic/RF pass highpass filters that feed a rectifier and sense setup. Output is through a 2x16 LCD.
The thing is powered by a few small NIMH batteries kept charged by the heater supply, so it works when the AC plug is pulled out of the wall, even, or if the main power fuse is blown.
With a little work, you could have the thing tell you "AC power in, main power switch is on, but no power to transformer: Fuse is blown." as well as other stuff.
The uCs all have EEPROM these days, so you could have it read what your amp does when it's running correctly, store it, and tell you when the operating conditions vary more than X %.
The thing could normally do nothing, but come into action when you pressed the "diagnose" button, using the previously charged NiMH batteries to run even if the amp is stone cold dead.
Sorry for dumping this here. I had to write it down while I thought of it.
"Most of the time, you can debug an amp by just looking at what voltages are on the tubes. Why not just read all those... but automatically?"
The idea is of course that modern microcontrollers use so little power that they can run semi-forever from a micro-battery, the almost all include an A-D converter, and they can easily enough read voltages and make conclusions from them. The amount of conclusions to be made is really quite small. Great precision is not needed.
I suspect that with a remarkably small set of chips - a microcontroller and a few analog MUX selectors, mostly - you could have the thing read all the tube voltages and just tell you what's wrong with an amp in 90+ percent of cases.
Voltages are scaled to 0-5V by resistor dividers; slowed down by a small capacitor, and clamped to the logic power supply by diodes to prevent killling the logic.
Bias can be read on a positive-only voltage scale by having the bias voltage pull down on a pull-up resistor to +5 through a largish resistor with a clamp.
You can even sense oscillation by using ultrasonic/RF pass highpass filters that feed a rectifier and sense setup. Output is through a 2x16 LCD.
The thing is powered by a few small NIMH batteries kept charged by the heater supply, so it works when the AC plug is pulled out of the wall, even, or if the main power fuse is blown.
With a little work, you could have the thing tell you "AC power in, main power switch is on, but no power to transformer: Fuse is blown." as well as other stuff.
The uCs all have EEPROM these days, so you could have it read what your amp does when it's running correctly, store it, and tell you when the operating conditions vary more than X %.
The thing could normally do nothing, but come into action when you pressed the "diagnose" button, using the previously charged NiMH batteries to run even if the amp is stone cold dead.
Sorry for dumping this here. I had to write it down while I thought of it.
) was to integrate a small PLC within the amp's architecture. Such things are very cheap nowadays (in the 100USD "retail price" range), they can deal with both analog and digital signals, they're easy to wire and program ( ladder logic ) so you don't have to be a C++ Software Engineer to write a fully functioning program ( including software multiplexing of the inputs, if needed ) and most of them also sport an "user configurable" text display for state messages/alarms and the like.
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