Snagged a us amps US25A circa 92-93. Little sucker wont power on. Visual inspection shows nothing out of the ordinary on the power supply side. Inspecting the output side I see two burned resistors that upon measuring are essentially open. These two resistors are in series with all 4 LARGE resistors on the outputs of the TIP35c/TIP36 bipolar transistors.
Any idea what value resistors are the burned ones?
Any idea what value resistors are the burned ones?
Are they in series with the emitter resistors or are they in series with a film capacitor and going to ground?
Just did some closer studying.
One end of the burned resistor connects to rca shield ground through that small rectangular resistor. The other end ties into the trace the emitters dump into that also has the positive speaker lead.
One end of the burned resistor connects to rca shield ground through that small rectangular resistor. The other end ties into the trace the emitters dump into that also has the positive speaker lead.
Those are Zobel resistors and won't stop the amp from operating unless the amp is badly oscillating when it starts to power up.
The rectangular resistors are film capacitors.
The rectangular resistors are film capacitors.
Gotcha. Any idea what value resistors would be appropriate to replace those Zobels?
Awesome thank you!!
Now as far as it not powering up, Im a bit at a loss. There is no tl494 or any sort of IC on the power supply side.
The amp in its entirety. (I removed TIP35 and TIPS36 to trst and both check out. Checked for drive at the base pin and i get a sudden glitch of power then nothing.)
An externally hosted image should be here but it was not working when we last tested it.
Power supply side close up:
An externally hosted image should be here but it was not working when we last tested it.
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There isn't a constant drive in a self-oscillating power supply. You have to have the PS transistors in the circuit for it to power up.
It didn't power up with them in place before i removed them. But that's good to know!! 🙂
So im guessing one or several of those transistors is bad in the power supply picture? (None show any shorts across the legs.)
Did you check the transistors for open junctions and leakage?
The amp could be going into protection from something in the audio circuit. Try removing the rectifiers to see if it will power up.
The amp could be going into protection from something in the audio circuit. Try removing the rectifiers to see if it will power up.
Pardon my ignorance, but how do i check to see if they are leaking?
Ill remove those rectifier diodes and see what it does when i get off of work tonight
I replaced the power supply transistors (both are TIP35C's) and removed the 4 rectifier diodes and the power supply fired up! Now here is the REALLY curious part....
Power supply base pin had a very small square drive wave that remains self oscillating even with REM+ removed. Collector pins had a 26v square wave, but the emitter pin which feeds the diodes, only had about a 5v square wave.
This cant be normal??
I was mistaken.
Base pin has a square wave driving it. Collector pin has a 25v square wave whose range is roughly ground to +25v. No negative voltage. Emittor pin has nothing on it and is tied to ground.
Output of the power supply to the rectifiers is a 11.2v peak to peak square wave that is equally spread across the Xaxis. That is +5.6 and -5.6 volts. Is that correct? Ive never know voltage rails to be THAT low?
Base pin has a square wave driving it. Collector pin has a 25v square wave whose range is roughly ground to +25v. No negative voltage. Emittor pin has nothing on it and is tied to ground.
Output of the power supply to the rectifiers is a 11.2v peak to peak square wave that is equally spread across the Xaxis. That is +5.6 and -5.6 volts. Is that correct? Ive never know voltage rails to be THAT low?
You were correct!! Secondary is isolated, 44v peak to peak!!
Found a dead output transistor. Pulled it (and the rest of the output transistors) the dead zobel resistors, replaced the rectifier diodes, and the amp fires up and i have rail voltage on all output collector pins!!
Thak you again for all the help!
Found a dead output transistor. Pulled it (and the rest of the output transistors) the dead zobel resistors, replaced the rectifier diodes, and the amp fires up and i have rail voltage on all output collector pins!!
Thak you again for all the help!