New Member, New Project, Here's my 400!

So I borrowed a rebuilt NAD preamp from a friend so I could power this bad boy up. Using my Klipsch KG4 speakers and my Kenwood turntable...the result is...

Holy mother of crap!! This sucker sounds amazing!! I barely had it working and everything in the house was shaking. Not sure these speakers are going to be up to the task, but I played one album through and was grinning from ear to ear.

Then I turned it off and the pop made me jump. Oh yeah, I forgot about that. Lol.

So I guess I should get to ordering the parts to make this a proper WOPL.

(Steely) Dan
 
Yep, just a small taste of what's in store. Expect much more clarity and soundstage with the WOPL version...Steely....
 
Well it seems I pushed this stock 400 beyond its limits today. All of a sudden...very very loud hum and static. Shit. Shut off as fast as possible and put on the bench. Sure enough, 75 volts DC on the left speaker output.

Guess it's time to start installing the new control board, caps, and yes, the DC protection kit I bought a while ago.

But 1st, a few questions...

1) what usually is the actual failure that causes this condition and should I address it before I start the mods?

2) should the dc protect kit I got come with wiring? Mine didn't but I think it should have. What guage of wiring should I be using with this?

Sad day here...
 
Danimal1969 said:
Well it seems I pushed this stock 400 beyond its limits today. All of a sudden...very very loud hum and static. Shit. Shut off as fast as possible and put on the bench. Sure enough, 75 volts DC on the left speaker output.

Guess it's time to start installing the new control board, caps, and yes, the DC protection kit I bought a while ago.

But 1st, a few questions...

1) what usually is the actual failure that causes this condition and should I address it before I start the mods?

2) should the dc protect kit I got come with wiring? Mine didn't but I think it should have. What guage of wiring should I be using with this?

Sad day here...
Click to expand...

It's complicated Dan...
 
Gepetto said:
It's complicated Dan...
Click to expand...


Well let me fine tune my real question. Should I address the problem with the amp as it sits or should I move forward with the new control board first and diagnose any problems later?
 
Danimal1969 said:
Well let me fine tune my real question. Should I address the problem with the amp as it sits or should I move forward with the new control board first and diagnose any problems later?
Click to expand...

You need to check all the output devices Dan. No doubt some of them if not all on the failed channel are shorted.

A new control board will not fix that.
 
I pulled all the outputs and tested them. They all test good, at least out of the circuit testing for shorts. I'll probably still wind up just replacing them all anyway, but that doesn't appear to be the root cause of the voltage on the speaker jacks.
 
Check the pre-drivers on the board for short. Q7,Q10
 
A quick in circuit of all transistors on the board can be done to cull out any shorted semi-conductors...
 
laatsch55 said:
A quick in circuit of all transistors on the board can be done to cull out any shorted semi-conductors...
Click to expand...

I will check the other 2 you mentioned 1st.

As far as giving them all a quick in circuit test, perhaps we can just pretend I'm an idiot that doesn't know how to do that? Lol. But seriously, am I just looking for shorts or for voltages or?? Can you give me a quick overview of this test?
 
Danimal1969 said:
I will check the other 2 you mentioned 1st.

As far as giving them all a quick in circuit test, perhaps we can just pretend I'm an idiot that doesn't know how to do that? Lol. But seriously, am I just looking for shorts or for voltages or?? Can you give me a quick overview of this test?
Click to expand...

Hi Dan
What process did you use to check all the output transistor devices?
 
Gepetto said:
Hi Dan
What process did you use to check all the output transistor devices?
Click to expand...

I pulled them out of the amp and connected red lead of the DMM to the base, black lead to emitter and then the collector. Both have around 625 when using the diode checker mode. No shorts/continuity in any other places.
 
Danimal1969 said:
I pulled them out of the amp and connected red lead of the DMM to the base, black lead to emitter and then the collector. Both have around 625 when using the diode checker mode. No shorts/continuity in any other places.
Click to expand...

Hi Dan
Now you need to do the opposite and put the black lead on the base and put the red lead to the emitter and collector and ensure OPEN in all these readings. Then finally you must measure collector to emitter on each device in both directions, red lead on collector, black on emitter and then black on collector and red on emitter.

Unless you do all these readings, you won't know.
 
Gepetto said:
Hi Dan
Now you need to do the opposite and put the black lead on the base and put the red lead to the emitter and collector and ensure OPEN in all these readings. Then finally you must measure collector to emitter on each device in both directions, red lead on collector, black on emitter and then black on collector and red on emitter.

Unless you do all these readings, you won't know.
Click to expand...

Yes, I did all that as well and actually just went back and rechecked them all. Those are all open.

I also went ahead and checked all the transistors on the board as best I could. I did find one that was faulty. The MPSA93 I'm holding in the photo is open from Base to Emitter. Base to Collector is ok and there are no shorts.

Could that open MPSA93 be the problem?
 
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