Digital Designs D4.90 in protection

[Perry Babin]

The red has a serious problem. It needs to be below about 2v before the yellow rises. As of now, it's overlapping.

 

[mild2wild]

What would be the cause of that? The SG3525, or something else?

 

[Perry Babin]

You need to look at the drive signal at every point...

output of the driver IC
base of the driver transistors
emitter of the driver transistors
both sides of the gate resistors of every FET if you don't see the problem above.

 

[mild2wild]

Drive signal at the base of each bank's driver transistors. I assume my problem is downstream of this point?

 

[mild2wild]

Drive signal at the emitters if each bank's driver transistors. I guess I need to change drivers?

 

[Perry Babin]

The gate drive was OK in post # 6. What happened?

 

[mild2wild]

ALOT happened since then, but in post #6, that drive signal was with capacitors loaded into each bank. As soon as I put FETs back in the amp, squealing and screeching... I tried several different FETs hoping that vwas the problem but they all had the transformer squealing. Two CS50Ns that I had lying around allowed the amp to power up almost silently, albeit with high current draw. Right now, the transformer is silent when the amp is on, but the mains limiters are glowing.

The non-square drive signal is coming out of the driver transistors connected to Pin 11 of the SG3525.

 

[Perry Babin]

Did you replace the driver?

 

[mild2wild]

Not yet, that is what I should do, right? 1GM and 2GM...

 

[Perry Babin]

Yes if those two have the bad drive signal on their emitters.

 

[mild2wild]

So, I changed those 2 driver transistors. The amp powers up with no noticeable current draw as the limiters are not illuminated. The drive signal however is showing both traces locked together. Is this right?

Amp is playing all channels. Reconnecting Q517 causes the protection light to illuminate but amp plays unaffected. Rail voltage is now +31/-26.

 

[Perry Babin]

Are you placing the probes precisely where you did when they were overlapping slightly?

 

[mild2wild]

Something weird is going on with the scope. The red trace is moving across the screen, but the yellow trace just shows a square wave without any movement. Swapping them doesn't make a difference...I will reboot and test again tomorrow.

How do I track down the protection and rail voltage issues?

Just asking, can this amp use IRF3205 and 47ohm resistors? I know the original IRF1404 has higher voltage and current ratings, but I put those ones from this amp into a different amp and I have no more IRF1404. Will IRF3205 not be strong enough for this amp?

 

[Perry Babin]

The user will determine if the 3205s will survive. If there's no absolute rush, order the right parts.

The red trace sounds like it's not triggering the scope. Try adjusting the trigger level.

Are you using the secondary ground for the black probe when measuring the rail voltage?

 

[mild2wild]

I was using GND input terminal when rail measurements were taken. I will try the secondary ground. I will also check the scope tomorrow. What about the protection LED?

 

[Perry Babin]

I have absolutely nothing on this amp so I may not be able to help with the protection LED issue. Let's get through all of the other issues first.

 

[mild2wild]

Picture is drive signal at gates of FETs. No significant current draw. When measured using secondary ground, rail voltage is +/-29v. Q517 is marked ASG which should be a KTA1504, of which I have none. It measures the same in either direction from base to collector so I replaced it with a MMBTA56 which stopped the protection LED. Amp powers up and operates properly when tested. Just to fully populate the power supply and remove the driver board jumpers and test again.