I picked up a Zapco AG150 that someone else started working on. When testing it on a 2.2A current limiting supply it will try to draw the full 2.2A but in pulses. I removed the rectifiers and it only draws .25A with no pulses. The power supply transistors have a sine wave input and output. All of the output and power supply transistors have been replaced and I am not confident that they are correct as I had another amp from the same person with the incorrect fets installed. A schematic or part identification would be great. Any thoughts on the repair would be great also.
PS waveforms a 'sine' wave?
Did you try setting the bias lower to see if the amp drew less current?
Have you tried contacting Zapco for a diagram? They're hit and miss but you should try.
Did you try setting the bias lower to see if the amp drew less current?
Have you tried contacting Zapco for a diagram? They're hit and miss but you should try.
It has a SG3525 PWM that has a sine wave (modified sine wave) output instead of a square wave. Would that create pulses in the output of the rectifier?
The first image is the output of the IRFZ44 and the second image is after it goes through the transformer (input to the rectifiers). The rectifiers are removed. I am new to the amplifier circuitry but the other amps that I have worked on still had a nice square wave with the rectifiers removed.
Attachments
What was the ground for the scope connected to for the second photo?
Try connecting it to the secondary center tap.
Try connecting it to the secondary center tap.
With the rectifiers installed the amplifier will draw current to the current limit setting and then drop to zero and repeat. This happens faster the higher you adjust the current limiter.
Yes the waveform looks like what I expected. The amp will not idle within the 2.2A limits of my power supply. The manual gives an idle current of 0.84A. Can I turn the bias all the way down and then power it up?
Yes. If the transistors aren't tightly clamped to the heatsink, you must very closely watch their temperature.
As a side note on the ground point, if there is complete isolation from primary to secondary or a very high value coupling resistor, you must use the appropriate grounding point to get a reliable reading.
As a side note on the ground point, if there is complete isolation from primary to secondary or a very high value coupling resistor, you must use the appropriate grounding point to get a reliable reading.
If it powers up after that do I just adjust each bias 100mA or so until I get to the idle current?
I reinstalled the rectifiers and put it back into the heatsink with everything clamped down. I turned the bias to minimum and it still does not power up.
It is still tripping my supply and restarting it in pulses that get faster when you increase the current. I am just learning the amplifier circuitry and am unsure where to go from here.
Set the current limiter on your supply much higher and insert 2 ohms of resistance in the B+ line. Does the amp act any differently?
Have all semiconductors clamped tightly to the heatsink when doing this.
Post a photo of the amp's internals.
Have all semiconductors clamped tightly to the heatsink when doing this.
Post a photo of the amp's internals.