Nad 7240pe damaged amp section

[SAUDADE]

Hello,

A friend of mine asked me to take care of her NAD 7240PE. She bought this integrated amp along with a pair of MAGNEPAN MG-.5QR (and also NAD CD DECK) about 25 years ago in the US.

First time I listen to it (about 4 mounths ago) the sound was awfull, distorded. First thing I found was that the speakers were no correctly connected to the amp. Imagine: both werre connected to the same chanel, one on Right-A and the other on Right-B. My friend never wondered why she had to select A+B on the front panel to get sound on both speakers. To bad, she might have save her 7240...

Magnepan are 4ohms speakers. Connected in parallel on one channel makes 2ohms. With high volume level the amp suffered a lot.

These Magnepan deliver magnificent music. Now I find my brand new 1000€ (1300$) Speaker system flat compared to these MAgnepan. When you listen to it once it's hard to get back to traditional speaker systems...

When I connected correctly the speakers, sound was Ok on left Chanel. But distorsion is still present on the right. Furthermore right lacks power compared to left.

I tested preamp section of the 7240 by connecting it to another amp section of a NAD 304. It sounded great. No problem. I reversed the process, connecting the preamp of the NAD 304 to the amp of the 7240, distorsion and weakness are present on right chanel.

Conclusion: The problem is located on the right amp of 7240PE.

Any idea on how I should proceed to identify the worn out components?

Thanx for your help.

[SAUDADE]

Here are the schematics...

The right amp section is at the bottom of the page.

[jaycee]

You have a good channel to compare to which helps as the circuits are identical.
What voltages are being used? I mean, whats the setting of SW502? If it does it with both settings then it is likely the output transistors and/or their drivers are faulty.

[SAUDADE]

I will check the voltages ASAP. I'll check SW502 and give the position. I guess SW502 switches between 110-120 VAC and 220-240 VAC. This amp works perfectly (except for the right channel of course) so it has already been switched to 220-240VAC when my friend brought it back from the US to France (She doesn't remember that...ah women!!!.)

Thanx for your help J.C.

[jaycee]

No, SW502 is probably labelled something like "Load setting" and has 4 ohm or 8 ohm options. What it changes is what power rails are used by the amplifier. This amp is also a class G type which switches to higher output rails if the load requires it.

[SAUDADE]

Ok.

I found the 4ohm 8ohm switch on the back panel. Originally it was set on 4 ohms for the Magnepan speakers. I had to switch it to 8 when I connected my own 8 ohm speakers. This didn't solve anything. The sound was still distorted on right channel. I even noticed that there was an important loss of volume on the right channel compared to the left (I made the test with a 1kHz input into the 7240 at same level on right and left and the balance level stricly in the middle). I guess this difference was also noticeable mith the Magnepan, but I wasn't able to perform tests in good conditions at my friends home.

I'll check ASAP SW502.

[RetroNAD]

G'Day, after reading several posts on here regarding the NAD 7240PE I was hoping someone might be able to 'spell' out a solution for me ragarding this amp. I have an 7240PE amp/tuner which no longer works on the right speaker channel. Pre-amp works on both left and right outputs. The only time I can get the right speaker output to work is with the balance turned all the way over to the right and volume at high level. The result is a very distorted and muffled output on the right speaker channel. I have the service manual for this amp (attached) but being only new to the DIY electronics scene, I am a little unsure where to begin. Any help with this would be greatly appreciated and remember to keep it pretty simple for me

Thankyou.

[Mooly]

Well you are probably better starting a new thread for this, although it follows the original pretty well

It sounds like you haven't much experience, and this sounds like a slightly out of the ordinary fault... power amps usually self destruct giving no output.

1. You need to be 100% sure that the input to both power amps is really OK... in other words is the fault definitely in the power amp stages.

2. Measure the DC volts on the output of the amp before any output relays etc... so that's the junction of Q427 and Q429 assuming it's that channel that's faulty... if not it's the same point on the other channel.
Remember you have a good channel to compare against.
That voltage should be zero.

3. What is the DC voltage across R459 ?
4. What is the DC voltage across R435 and R439 ?

[RetroNAD]

Thankyou Mooly,

In reply:

Q1. "You need to be 100% sure that the input to both power amps is really OK... in other words is the fault definitely in the power amp stages."

A1. Yes tested crossing over pre-amp outs to both main-in, problem is in the right amp/speaker output.

Q2. "Measure the DC volts on the output of the amp before any output relays etc... so that's the junction of Q427 and Q429 assuming it's that channel that's faulty... if not it's the same point on the other channel.
Remember you have a good channel to compare against.
That voltage should be zero."

A2. O.K. not 100% sure what you mean by this BUT did measure across Q427 (0.54V DC) and Q429 (0.56V DC)


Q3. "What is the DC voltage across R459 ?"

A3. R459 (1.10V DC)


Q4. "What is the DC voltage across R435 and R439 ?"

A4. R435 (3.02V DC), R439 (2.93V DC).


Sorry for being such a newbie, but its good to start somewhere

Look forward to your reply,

retroNAD

[Mooly]

Voltage ACROSS Q427 and Q429. Well that should be around plus and minus 45 volts or so. That's measured from the middle leg (collector) to the emitter which is the "output". You may have measured from the end two legs which is base to emitter and if so that voltage of around 0.55 sounds about right if a little low... are the output transistors warm at all... compare with the other channel.
I actually meant to measure the DC voltage at the output of the amp... at that "thing" called BK-1 on the circuit... read on

Voltage across R459 sounds reasonable.

Next to the output transistor on the diagram is something called BK-1 What's that ?
Is it a relay contact that the output passes through or is it some form of fuse etc fitted on the PCB. Whatever it is it should (With the amp OFF if it's not a powered relay) measure short circuit. Does it ?