I have a NAD C326BEE stereo amp that screeches in both channels (though not exactly the same screech in both channel) when I power it on, and then again after I power it off, using the front panel power switch. It sounds like a needle scratching across an LP, and is independent of the volume pot level.
The service manual (with schematics) for this amp is
here.
The amp also has an occasional pop in the left channel (independent of volume, source, or whether there's a signal), but that symptom is very intermittent and difficult to troubleshoot. The pop will sometimes activate the protection circuit of the amp, which makes it go offline (red light on front panel)
The amp has a protection relay that turns off the speaker outputs until a second after the amp powers on (and immediately when I power it off), so the screech doesn't actually come out of the speakers, but it does come out of the headphone jack.
With this easily reproduceable screech, and a bit of help, I'm hoping to be able to finally fix this
The screeches are present on both output channels before the headphone/speakers split, so it's not just on the headphone circuit.
The power amp area of the PCB is a bit discolored from heat. The amp was on continuously for many years, but used rarely.
I have disconnected the pre-amp and power amp sections by removing the RCA jumpers at the back, and the noise is still present on the output channels, which I believe rules out the pre-amp.
I have:
- Replaced a seized trim pot
- Resoldered every joints that looked suspicious
- Tested the large capacitors of the power supply board
- Carefully inspected the catalytic capacitors
- Tapped around with a chopstick (no noise)
- Scanned around with an infrared thermometer and ended up plucking out C207 because it was hotter than C408 (equivalent in the other channel), but it tested fine.
- Sniffed around with a Carlson Super Probe type signal tracer, which sniffed the screeches coming out of most of the power amp section, both sides of the PCB, but otherwise no randomly noisy parts that I could find.
I wish the signal tracer probe was giving me a hot or cold indication when sniffing around, but the screech seems to permeate the power-amp circuit when it happens, so no areas stand out as particularly suspicious.
I'm hoping the turn on/off symptom can help narrow down which parts to pluck and test. Insights greatly appreciated.
Thanks!