BD1000 noisy on startup

[brandes.cm]

I am working on a BD1000 that is really noisy on startup, after a couple minutes it seems to go away. The noise is seen only on the lower half of the signal, as you adjust the gain up the noise seems to head towards the zero cross point. I can see the noise on the input stages then it clears up. I'm not sure if one of the isolated supplies is failing or what.

cory

 

[Perry Babin]

Can you post a photo of the noisy waveform?

 

[brandes.cm]

While taking the waveform pictures, a bank of output fets failed. They seem to be a mix of IRF3415's (pos rail) and IRF6215's (neg rail). It looks like these were replaced once before, flux residue and excess solder on the tabs of the fets.

This amp has two drive circuits each covering a bank of 6 fets, 3 upper 3 lower. Unless the schematic is drawn wrong only one bank ties to the output, the other side goes the shutdown circuit. In my case the shutdown fets are fine so no protect light.

seems odd.

 

[Perry Babin]

The schematic diagram is, in general, correct (there may be minor errors). This amp has a floating rail. The transistors tied to the output inductor drive the speakers. The ones connected to ground drive the rail voltage for the transistors that drive the speakers.

For half of the time, the transistors drive the negative terminals of the rail caps to ground. The other half of the time the transistors drive the positive terminals of the rail caps to ground.

 

[brandes.cm]

Is it safe to operate after the output drive fet are removed? The Rail fets are still in place.

 

[Perry Babin]

It should be. What's the reason for powering it up before you install the outputs?

 

[brandes.cm]

Curious if the noise is still there. I found a gnd pin of the RCA jacks broke off, seems to be a common issue with the stacked variety.

 

[Perry Babin]

The amp won't produce any audio output. The preamp section up to U17 will work normally.

 

[brandes.cm]

That's all I am looking for, the noise was on the U2 stage.

 

[brandes.cm]

Here is the gate waveform on q104-106 and q110-112, a bunch of ringing on the rise and fall. The gate signal on the bank of removed fets looks clean. Could be the digital scope giving me false noise too.

The noise on the input circuit seems to have disappeared.

 

[Perry Babin]

Try grounding the legs of the FETs that connect to the inductor to the negative speaker terminal. That should clean it up. Be careful. It shouldn't cause any problems but you should be ready to remove the jumper or shut the amp off quickly.

This won't work if you short the output side of the inductor to the - speaker terminal.