Crunch CR150 with DC at output

[gisewhcs]

When installing my CR150 I made a mistake and shorted one channel to ground.
Now this channel ist producing ~4,75V DC, before it triggers the protection circuit.
When I power it up without anything connected but power it "works" for ~2s and then goes into protection.

I measured all semiconductors in both channels, all measure about the same.

Muting FETs are obviously okay, and I already changed the TL072.

I've been looking for burned Rs or open traces all over the board, but didn't find any!

It's one of the later revisions (Rev. CC) with those annoying black networks in it. I hope i didn't destroy one of those...


Any help is appreciated!

[Perry Babin]

Sometimes a transistor will open or become leaky. Either can cause DC to appear on the speaker outputs. You may have to pull the outputs to find the defective one. Also pull and check the driver transistors to see if any are leaking, shorted or open.

[gisewhcs]

Thanks for your help!

But damn!
I just resoldered the outputs because I measured them out of circuit and all were fine in my eyes.

Okay, so I have to desolder them again.
But how do "leaking" semis behave?
Shorted or open is clear to me, but leaking?

But desoldering the ouptputs is a good idea, because the protection circuit is beyond them, so the amp probably won't trip protect if they are out of circuit!
Perhaps I will then find the fault without desoldering all the drivers and so on!

[Perry Babin]

What output transistors does this amp use?

[gisewhcs]

The outputs are each three TIP35/36C, driven by a pair of BD911/912.
What drives the BDs? I'm not sure at the moment!
But I will look that up.

[Perry Babin]

For the TIP35 and 36, with your meter set to ohms, you should read ~2-3M ohms from legs 1-2 and 1-3 with the probes one way and OL when you reverse the probes (assuming your meter reads OL for an open circuit). They should read OL when placed across legs 2 and 3, no matter the probe orientation. If you read anything other than OL for any test that should result in an OL reading, the transistor is very likely defective.

If the outputs are not defective, you'll need to check the drivers next.

Have you confirmed that the emitter resistors are within tolerance?

[gisewhcs]

Now over the holidays I found some time to check all you've mentioned.

One of the emitter Rs was out of tolerance, that was probably a mechanical fault.
I measured something between its normal .1Ohm and 15Ohm when twisting it.
So I changed all in the bad channel.

But that didn't solve the whole problem.
It got better but there's still ~1V DC left.
At least the amp doesn't go into protect anymore!

I checked all semis as you've mentioned.
The TIPs read 5-6MOhm with my meter, but all as you said.

Now I really believe that I toasted one of those networks.
They are labeled ANW6 and ANW7, does anybody know what's in there?
I just sold the older CR150 with the colored networks where I could take a look!

[jol50]

If the transistors are flat with the board, some you can lift just the leg on the speaker outs. Then carefully fire it up and see what ones have voltage. I have had that work for me to check the outputs if I can't tell by the voltage on the bases of the outputs. Not sure it is recommended way, but simple. Quite often if you have limited DC then it is the drivers or differential amps/bias/etc.

[Perry Babin]

I've seen quite a few of the ANW7 modules fail. They tend to leak DC internally. Check with ZED audio to see if they can supply the internal schematic for the ANW7. If they can't, I can probably produce a schematic that would work but check with ZED first.

[gisewhcs]

Doh!
I just forgot my CR50!
It's one with those colored networks with the values of the resistors underneath it!
I've had one were I rebuilt one of the networks für the PSU...
The only diodes in there were 1N4148, hope that will be same in the CR150!

I will take it apart and try to rebuilt the networks with this one for reference.
If I get a working solution I will post it here!