NAD C370 protect mode because right channel

[Lynyrd]

Hello.
NAD C370 INTEGRATED Amplifier.
Protect mode.
If I deconnect CZ404 no protect mode.
Amplifier works fine,but only left channel.
I desoldered all power transistors all 6 transistors are ok.
I replaced bad caps on protect circuit,no results.
+64v & -64v are ok.
Q413 Q415 Q417 Q410.....OK
Q411 Q418 Q414 all ok.

No damaged parts.

SM

https://elektrotanya.com/nad_c370_sm.pdf/download.html

 

[jhjove]

can you check the dc offset on the right channel by measuring at the "VOP R" point?
I wonder if it's possible the VR401 trim-pot for dc offset adjust is wonky.....

--Joe

 

[Lynyrd]

-2.64v on pin 1 of CZ404
Trimmer on Minimum position.
On max position -3.35v

 

[jhjove]

Yes, pin1. So pretty damn high DC.

What about the main outputs (Q421/Q423/Q427 + Q422/Q424/Q428)?

 

[jhjove]

Sorry I re-read you checked these....

 

[Lynyrd]

Yes all power transistors desoldered and tested

 

[jhjove]

Does VR401 have any affect on that 2.64V DC value?

 

[Lynyrd]

Position min -2.64v
Position max - 3.56v

 

[jhjove]

I was gonna suggest lifting the leg of R428 (100ohm), which disconnects the input to the power amp and check VOP R. This should isolate the poweramp stage and result in low offset (since the active devices tested OK).

But it seems like it's the input to the comparator stage or the stage itself causing offset.

I'd check into Q409/Q410/Q411...... Q408+Q412 [must be matching].
Are zeners D401 and D402 holding ~18V ?
Have D403/D404 D405/D406 failed?

 

[Lynyrd]

О,i will check all

 

[Lynyrd]

Its difficult,but I checked all Q and diodes.
No bad parts......😥

 

[floriant]

I was gonna suggest lifting the leg of R428 (100ohm)

Yes, do what jhjove suggested, unsolder the pin of R428 that is connected to C408 and connect it (with some wire) to the ground.
Then power on and if you still have protection issues then the problem is in the final stages;
If not, then is the first stage and we can do some measurements (with the power on, be careful!) like

• check voltage VOP R to ground (should be close to 0)
• voltage on R402 (should be 0)
• voltage on R427
• compare voltages from nonworking R channel to working L channel, like compare voltages on R423, R323; on R421, R321; on R416, R316, voltages on on D401, D402, D301, D302, voltages on on C403, C404, C303, C304, voltages in emiter Q401, Q403, Q404 to ground and so on...and please report what you found the same / different (maybe is not a bad idea to write them on the schematic as you go and then share the schematic with measurements here)

 

[Lynyrd]

Ok thanks

 

[Lynyrd]

I have not yet desoldered the resistance leg, but I noticed that now it has become minus 30v on VOP R. Resistor R334 of left channel one side 65v,other side 5v.
On bad right channel R434 one side 65v other side minus 25v,

 

[Lynyrd]

On R402....0v.
On R427.....minus 30.3v

 

[Lynyrd]

R423 R323 same value -30v
R416 R316 same values
One side minus 17v other side minus 30v.
D403. D303 same values....26...27v
D402 D302....ok.
D404 D403...ok
No test D401 & D301,very difficult access.

C403 C303....ok...17v
C404 С403.....dufficult Access.
Q404 emitter.....17v.
Q401 emitter......0.6v.

 

[Lynyrd]

Q403 emitter.......0v


R434 very hot.

TL431 dead?

 

[Lynyrd]

I replaced TL431,no results

 

[Lynyrd]

Its my mistake.
I soldered Q409 vice versa
Now I solder Q409 and same history,minus 2.5v on VOP R.
Now on R434 65V and 2.6v other side.
On R334 65v and 5v other side.

 

[Lynyrd]

Q418?