Mosconi AS200.2

Hello all,
Looking for some advice on the repair of a Mosconi AS200.2. It powers on and does not go into protection but there is no sound output. I do not detect any abnormal heat and my power supply indicates a steady 0.3A draw.

I have checked the rail voltage and it is low, it was around +/- 9V. All the power supply MOSFETS look okay visually. I ohm checked them in circuit each gate to drain has a low ohm reading (~0.2ohms), gate to source behaves like a cap charging up over 200 ohms, drain to source also behaves like a cap charging.

I checked the SG3525A output on pins 11 and 14 and it starts out clean at 50kHz but then it becomes noisy at about 10 seconds after power up. Here is a video of it.

Both outputs behave the same. Output A and B are 180 out of phase as they should be.

So my question is, is this indicative of a failed SG3525A? Or do I need to pull the MOSFETS and recheck?

One other question the SG3525A is an On Semiconductor, is the TI UC3525ADW a drop in replacement? The SG3525A are out of stock everywhere I checked.
That is the same diagram I found.

No DC on the outputs. I noticed that today it took a much longer time for the outputs of the SG3525A to become noisy after being off all night. I also measured that the temperature of the SG3525A gets to 140F. All the MOSFETS stay at room temperature.

I see the mute circuit in the schematic, does it control the HV power supply section at all (I didn't see where it does, but wanted to ask to confirm).
The mute circuit controls the power supply to the final op-amps only.

The waveform issue appears to be a trigger problem with the scope.

When posting waveforms, you need to post, vertical amplifier, timebase, note whether the scope was set to AC or DC coupling (DC is right virtually all of the time) and whether the probes are set to 1x or 10x. If you are going to post multiple photos with the same settings as before, you can just note that the settings are the same. Align the trace to the center reference line before probing the test point.

For myself, I MUCH prefer still images to video.
I just double checked the outputs of the SG3525A and I get the same result. It starts out a clean square wave and then without changing anything a second square wave intermittently appears as seen in the video.
It takes longer for this to happen if the amp has been off for an extended period of time, and I can make it stop if I cool the SG3525A by blowing air on it. When I stop blowing on it it comes back after a several seconds.

If it is a triggering issue, I am not sure what to change since it triggers fine initially and the square wave only seems to change with chip temperature.
Adjusting the trigger level does not change the behavior. If the level is set beyond the square wave's amplitude peak the waveform disappears as expected.

Good call the probe did need to be adjusted, I went back and calibrated it on both the 1kHz and 1MHz calibration settings.
Behavior still present after adjusting the capacitance.


Pin 14 of SG3525A, second squarewave present.


Blew on SG3535A area and second square wave disappeared. Did not lift the probe, or change any settings on the scope.


This is pin 4 of the SG3525A (oscillator input), 100kHz. Have not observed any anomalies with this signal.

I also tried a few other probes (another Hameg HZ51 (10X probe) and a Tektronix 100MHZ probe I have), and the behavior of the SG3525A outputs is the same with all three probes.
That makes sense, since the MOSFETS would be nearly always on. My digital "scope" (Ni Mydaq) is limited to 20kHz. Thank you for walking me through that, I will move on and continue troubleshooting.

This is really the first time I have used this scope, what is your opinion should I sell it and get a digital Rigol, Hantek, or Siglent?
My signal generator (NI Mydaq) only goes to 20kHz, and the waveform as displayed on the analog scope was funky. I suspect the Mydaq output is to blame for the funky waveform, it is an all in one unit with very limited capabilities.

Funky waveform aside, moving the trigger level up and down moved the waveform left and right. And the triggering was stable within the range of the waveform's amplitude.



Since I had the Mydaq hooked up I grabbed a capture of the SG3525A's output. But the limited bandwidth hindered the capture/displaying of the waveform.
Took me a bit to figure out how to switch it to digital, ended up having to translate a Spanish manual section.

I see the same behavior in digital mode.



Same signal at 10us time base
And as before if I cool the chip slightly it goes away.
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