NAD 7250PE DC Offset Problem

[BMW2002]

I'm troubleshooting a NAD 7250PE power amplifier. The output of the left channel has a -65 VDC offset and opens the protection relay RY801 via IC801. Does anyony have a suggestion on what component in this exotic amplifier might be the cause of the offset?

Here's what I have tried:

Removed R608 and RR617 to isolate 1st stage amp from last stage. Still has DC offset.

Traded Q619 with Q669 - problem stays with left channel.

Traded Q618 with Q668 - problem stays with left channel.

Any other areas to suggest?

Schematic attached for your consideration.

Thanks,

 

[BMW2002]

Soory, but the schemo was stripped. If you need it send me an e-mail.

 

[clovi_kate]

schematic does have pdf files?.. much better if that is clear..

 

[BMW2002]

Found the problem!!! The culprit was R618 potentiometer (top middle), which the NAD service manual says is used to adjust for minimum distortion. It measured 1.5 kOhm instead of 200 Ohms. The little rivets had evidently lost their spring pressure. Now it is back to sounding great, and loud!

 

[Analogwar]

odd problem

I also have a NAD 7250PE that I love. I had to send it in to get it fixed a few years back. After I sent it in, the place I sent it to closed and a different place bought it out. They did a terrible repair job. Now the left channel speaker moves in and out with the lower frequency music and mid to high volumes. I cannot turn the volume up much because it will bottom out the speaker. Have any of you seen this before? Since I am a new member, I cannot email anyone, can someone email me the Schematics? That would be a great start. Also the area of D169, C234, C235, D171 and IC114 looks to be getting very hot. What can I do to fix this? Replace C231, C244, C234, C235, and maybe the diodes as well? What are their values?
Sorry for the barrage of questions.

 

[taj]

These are pretty much Leach amps with a different bias circuit and lower power than the original. Nothing exotic really. I have a 7175PE, same circuit, slightly more power.

..Todd

 

[gutekag]

I also have NAD 7250 PE and i am looking for schematic diagram.
BMW2002 can you send full diagram ?
Thanks

 

[MinesJA]

I'm also having a problem with my NAD 7250PE. I'm not getting any signal from the speaker outputs or from the headphone jack, but the inputs all work and if I hook up speakers to the tape out that works. Anyone have any idea what the issue might be? Just from doing some light research I saw a couple people with similar problems mention faulty potentiometers, but I'm not sure.

Thanks!

 

[troubadour1242]

Hello,
Can someone send me a message if you have the service manual, or any schematic diagram?
I have a file server here, so if you have a big file you can upload it to my server quickly.

I have a NAD 7250PE amplifier with DC voltage swinging in the right channel output. There is some peaks of 25-30V, the protection relays goes off, then it returns to normal a few seconds later. I am searching for a leaking part, intermittent part, etc. and when i will solve it i will post on forum. Any help, please tell me!

Thanks for the forum

 

[troubadour1242]

Solved!

I found that the DC problem i have comes really from these little variable resistors, R618 for left and R668 for Right. Replacing with a new fixed the problem. I tried and you can just add a simple 100 ohm resistor to replace this control. Then adjust DC offset with R645, R695.

Works perfectly! Thanks again for the tips!

 

[2dbit]

I am very proficient in soldering. So, removing the components is no problem, testing is also no problem. I mainly run into issues with testing components while they're still in-circuit. I've been lead astray before by failed resistors that show no visual signs of failure, and when checked for voltage give no indication of failure...

So, for me it'll likely be ...start with the output transistors, find the failed component, then determine what associated driving transistors could be failed along with that. Beyond that, figuring out where the failed resistors are, and any possible failed diodes, that's where I see myself falling apart. I don't want to remove the entire circuit, but also don't want to miss one component that could cause the whole fall-out over again.

So, tonight, i'll locate the failed power transistor(s) and if I find any local damage, i'll report that as well. Hopefully we can use the fragmented 7250 schematic that I have available to determine any additional checks....