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$) Speaker system flat compared to these MAgnepan. When you listen to it once it's hard to get back to traditional speaker systems...
When I connected correctly the speakers, sound was Ok on left Chanel. But distorsion is still present on the right. Furthermore right lacks power compared to left.
I tested preamp section of the 7240 by connecting it to another amp section of a NAD 304. It sounded great. No problem. I reversed the process, connecting the preamp of the NAD 304 to the amp of the 7240, distorsion and weakness are present on right chanel.
Conclusion: The problem is located on the right amp of 7240PE.
Any idea on how I should proceed to identify the worn out components?
Thanx for your help.
A friend of mine asked me to take care of her NAD 7240PE. She bought this integrated amp along with a pair of MAGNEPAN MG-.5QR (and also NAD CD DECK) about 25 years ago in the US.
First time I listen to it (about 4 mounths ago) the sound was awfull, distorded. First thing I found was that the speakers were no correctly connected to the amp. Imagine: both werre connected to the same chanel, one on Right-A and the other on Right-B. My friend never wondered why she had to select A+B on the front panel to get sound on both speakers. To bad, she might have save her 7240...
Magnepan are 4ohms speakers. Connected in parallel on one channel makes 2ohms. With high volume level the amp suffered a lot.
These Magnepan deliver magnificent music. Now I find my brand new 1000€ (1300$) 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.
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.
Thanx for your help J.C.
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.
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.
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.
Thankyou.
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 ?
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 ?
one step at a time...
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
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
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 ?
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 ?
getting there...
O.K. (thanking you for your patience Mooly) lets continue...
Voltage accorss Q427 (collector to emitter) 71.0V DC
Voltage accross Q429 (collector to emitter) 36.5V DC
BK-1 (relay contact) with amp OFF does measure short circuit.
Hmmm, so where does this leave us now?
Looking forward to your reply
O.K. (thanking you for your patience Mooly) lets continue...
Voltage accorss Q427 (collector to emitter) 71.0V DC
Voltage accross Q429 (collector to emitter) 36.5V DC
BK-1 (relay contact) with amp OFF does measure short circuit.
Hmmm, so where does this leave us now?
Looking forward to your reply
Only just seen your post... guess you are still under moderation as the thread notifier isn't being triggered. The voltage readings suggest a problem... perhaps another fault as well.
This amp uses a two supply rails for the outputs... class G configuration. The voltage of 36 volts sounds OK... the 71 volts suggests that the circuitry around Q431 is faulty. Is Q431 short circuit ?
However... none of this would give your (distorted audio) fault... the amp would still work as "normal". The only effect would be a higher power dissipation (heat) in the outputs under low signal conditions.
Just recheck those readings and compare against the other channel. The output transistors should run at the lower 40 volts or so under anything but high signal levels.
I just happen to have a 7240PE in the closet. Output is fried, but the pre is still alive. I don't need the output stage, but the pre could be useful, so I'm going to recap the electrolytics it's power supply and everything surrounding the op-amp and such. The opamps are lousy jellybeans so I'm doing a dive into my stash and may replace them with TI OPA2604 or browndogged OPA827's.
I replaced most of the electrolytics on the preamp/tuner/phono side of the amp. Most of the larger capacitors had released their innards so that was a worthwhile effort. I also replaced the DIP8 opamps with sockets so I could roll opamps. The main preamp didn't like browndog'ed OPA827's, but was happy with an OPA604. Not the best sound so I swapped out for OPA602's. Much better! OPA2107's are good too. In the tuner section there seems to be three opamps. One appears to only use half of the opamp. I left the NJM2043 in there. The other two didn't like the OPA604's at all. They're happy with the OPA2107's though.
Just for fun I tried a 1Ghz opamp in the main pre section. It worked, but shut down once a stronger signal was applied.
I haven't tried the phono section. Hopefully soon.
Sounds a whole heck of a lot better!
When I have more time/money, I'll work on the amp section. I just needed a preamp though, so I'm in no hurry.
Just for fun I tried a 1Ghz opamp in the main pre section. It worked, but shut down once a stronger signal was applied.
I haven't tried the phono section. Hopefully soon.
Sounds a whole heck of a lot better!
When I have more time/money, I'll work on the amp section. I just needed a preamp though, so I'm in no hurry.
I'll just add to this thread as it may help in the future. The impedance switch on the rear panel switches between two voltage tapping levels off the main transformer, and supplies the mid-level power rail for the output stages. My amp had tarnished contacts on the impedance slide switch, and the 6,800uF 50V caps were bulging, and one channel had damaged driver and output semis.
I reckon the tarnished contacts have caused intermittent and half wave rectification, stressing the caps from ripple and obviously the semis as well. I've removed the switch wipers and just soldered for one impedance setting.
Ciao, Tim
I reckon the tarnished contacts have caused intermittent and half wave rectification, stressing the caps from ripple and obviously the semis as well. I've removed the switch wipers and just soldered for one impedance setting.
Ciao, Tim
NAD 7240 Service Horror...
So I got such a thing to my hand, but have some problems: One channel is said to be dead. Luckily I believed, and measured for DC offset. DC offset is around 44 Volts. Then I compared the collector voltages of the output transistors.
The correct one had 38V each rail, the dead one 38V on the PNP output collector, but aound 74 on the NPN output collector.
My question: How can this result in an offset voltage??? I thought, there is high isolation on Powersupply in follower configuration and the feedback loop would care for dc stability as well...???
I guess, I have to fix Q432, maybe Q436 as well. (For voltage switching)
Next: Q432 is BD912, not 2SB826, Q436 is ??? not 2SB649. Can I search the Web for replacement types????
I need some adwise to continue. I could also imagine the high offset to turn the higher voltage on. There is no smell, no burnt parts...
Is it possible that the NPN output transistor is faulty or the drivers? Can I measure the transistors without unsoldering them with multimeter option "diode test"???
And a stupid "final(?)" question. After fixing all issues, I have to adjust the pot VR104 for the idle current.
- There are no emitter resistors -how can I measere the idle current??? Measure the drivers??? Which value???
- How can I make sure, that I will not kill the amp during adjustment??? Turn put utmost right/left??? Then start???
HELP!!!!!!!!!! Thanks!!!
So I got such a thing to my hand, but have some problems: One channel is said to be dead. Luckily I believed, and measured for DC offset. DC offset is around 44 Volts. Then I compared the collector voltages of the output transistors.
The correct one had 38V each rail, the dead one 38V on the PNP output collector, but aound 74 on the NPN output collector.
My question: How can this result in an offset voltage??? I thought, there is high isolation on Powersupply in follower configuration and the feedback loop would care for dc stability as well...???
I guess, I have to fix Q432, maybe Q436 as well. (For voltage switching)
Next: Q432 is BD912, not 2SB826, Q436 is ??? not 2SB649. Can I search the Web for replacement types????
I need some adwise to continue. I could also imagine the high offset to turn the higher voltage on. There is no smell, no burnt parts...
Is it possible that the NPN output transistor is faulty or the drivers? Can I measure the transistors without unsoldering them with multimeter option "diode test"???
And a stupid "final(?)" question. After fixing all issues, I have to adjust the pot VR104 for the idle current.
- There are no emitter resistors -how can I measere the idle current??? Measure the drivers??? Which value???
- How can I make sure, that I will not kill the amp during adjustment??? Turn put utmost right/left??? Then start???
HELP!!!!!!!!!! Thanks!!!
Turn the preset to give max resistance (so vbe multiplier is turned fully on). The circuit shows a 1 ohm resistor with a link across it (R472). Open the link and measure volt drop across the resistor.
Bias adjustment...
Hi Mooly,
I got the point. Instead of measuring the emitter resistance, I could put any resistance in series. So, open the shortcircuit on the resistor, measure and adjust, and reinstall short circuit again.
The pot VR104 is 1K??? How can I know, that I put it on max setting? Measure resistance across it in both conditions, choose max setting???
Does this work with the pot build in??? Can I take the current from the working state as reference??? (Value???)
Next:
How can I find out, what is defective? Do I have to unsolder trannies for testing? Can I check PN junctions in built in state???
Thanks for any info, I want to learn and improve my skills...
Regs, Dirk
Hi Mooly,
I got the point. Instead of measuring the emitter resistance, I could put any resistance in series. So, open the shortcircuit on the resistor, measure and adjust, and reinstall short circuit again.
The pot VR104 is 1K??? How can I know, that I put it on max setting? Measure resistance across it in both conditions, choose max setting???
Does this work with the pot build in??? Can I take the current from the working state as reference??? (Value???)
Next:
How can I find out, what is defective? Do I have to unsolder trannies for testing? Can I check PN junctions in built in state???
Thanks for any info, I want to learn and improve my skills...
Regs, Dirk
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