• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

NAD 7240 PE Multiple Transistors Dead

R

Rocks

AK Subscriber
Subscriber
Hi all,

Recently purchased a known bad NAD 7240 PE owner saying the fuse blew. He said previous owner said it was working, he plugged it in and said a fuse blew and his left speaker blew. I recently purchased a working NAD 7130 replaced all display digits and full recap except the large capacitors (didn't sound any better but now tuning speed is 50 times faster). Liked it so much I wanted to try and fix this 7240 PE.

Opened it up for a visual check and fuses were fine but R459 was completely burnt. Also noticed the emitter of Q25 was detached from the solder pad. Figured this was the problem so replaced R459 and resoldered Q25 emitter. RH channel was already working ok.

- Power on and R459 smoked again. Decided to dig deeper (not deep enough I would realize). Found Q427 shorted base-collector. Replaced R459 and put in something similar for Q427 (was hoping I could confirm there were no other problems).
- Power on and smoked R459 again. Decided to check all transistors in the LH channel. Found Q423 B-C short, Q427 B-C short (again), Q429 C-E short, Q433 all 3 legs shorted. All other transistors and diodes were OK using a diode check on my meter and comparing with RH channel. Ordered replacements and waiting.

Could the open emitter on Q25 have caused all this?
Can I power up with these transistors removed to check some voltages or do I risk blowing up more components? I assume power supply is fine since RH channel works well.
Will the light bulb limiter help to not blow up new transistors? Or does that only work to avoid fuses blowing?
My power amp section has multi-turn trim pots in place of the single turn ones. Is it possible this is factory or did someone change them.

Any advice would be appreciated before I blow up more transistors. I really like the 7130 and this is newer and a bit more powerful so I'd really like to fix it.

Thanks,
 
Register to hide this ad
Rocks said:
Hi all,

Recently purchased a known bad NAD 7240 PE owner saying the fuse blew. He said previous owner said it was working, he plugged it in and said a fuse blew and his left speaker blew. I recently purchased a working NAD 7130 replaced all display digits and full recap except the large capacitors (didn't sound any better but now tuning speed is 50 times faster). Liked it so much I wanted to try and fix this 7240 PE.

Opened it up for a visual check and fuses were fine but R459 was completely burnt. Also noticed the emitter of Q25 was detached from the solder pad. Figured this was the problem so replaced R459 and resoldered Q25 emitter. RH channel was already working ok.

- Power on and R459 smoked again. Decided to dig deeper (not deep enough I would realize). Found Q427 shorted base-collector. Replaced R459 and put in something similar for Q427 (was hoping I could confirm there were no other problems).
- Power on and smoked R459 again. Decided to check all transistors in the LH channel. Found Q423 B-C short, Q427 B-C short (again), Q429 C-E short, Q433 all 3 legs shorted. All other transistors and diodes were OK using a diode check on my meter and comparing with RH channel. Ordered replacements and waiting.

Could the open emitter on Q25 have caused all this?
Can I power up with these transistors removed to check some voltages or do I risk blowing up more components? I assume power supply is fine since RH channel works well.
Will the light bulb limiter help to not blow up new transistors? Or does that only work to avoid fuses blowing?
My power amp section has multi-turn trim pots in place of the single turn ones. Is it possible this is factory or did someone change them.

Any advice would be appreciated before I blow up more transistors. I really like the 7130 and this is newer and a bit more powerful so I'd really like to fix it.

Thanks,
Click to expand...
This is why you need to use a dim bulb tester, to limit current, give a visual indication of trouble and save burning up replaced parts unnecessarily.
 
Rocks said:
Could the open emitter on Q25 have caused all this?
Click to expand...
Nah, probably just a casualty of war started in the PE area, eg, Q433.
As per ray, do the DBT thing, it will become your best friend, especially on these PE amps.

Suggest disable the PE stage and get the basic amp working first, need to verify driver and output transistors (Q423, 425, 427, 429) are working under load, then sort out the PE stage. Looks like you can disable the PE by removing PTC1 (positive temp coefficient resistor?)
 
Already made and tested the DBT. Others have suggested removing Q431, Q433 to isolate PE stage. I think this is better since it won’t touch the right channel. Just waiting for parts now. Patience is not my strength….
 
Rocks said:
Others have suggested removing Q431, Q433 to isolate PE stage
Click to expand...
Yeah, I thought about that but was unsure about a c-e short on Q439, Q441 would cloud the fault picture. Certainly Q431, 433 are candidates.
 
Ray Gianelli said:
This is why you need to use a dim bulb tester, to limit current, give a visual indication of trouble and save burning up replaced parts unnecessarily.
Click to expand...

Agreed.

I would also invest in a FLIR. You can get one now that plugs in to an ipad or phone. When you give it some juice with the dim bulb tester, you look at the board to see what gets hot. Its useful in many ways beyond stereo. Finding leaks in exterior doors, a leaking fridge, etc, its handy.
 
Update. Thanks for all the tips. This is getting more fun since I didn't blow anything up. Yet....
I checked every capacitor, resistor, potentiometer, diode, and transistor in the LH channel with a multimeter and all seems good except what I previously found as bad.

Received and replaced Q423, Q427, and Q429. Q433 was shorted and already removed. I removed Q431 to remove the high voltage (+/-71V) from the equation.
Powered it up with my dim bulb tester and it powered on and bulb went bright then dim. No smoke, no bad sounds.
I set it to 8 ohms, soft clipping OFF, speaker selector to A, input to CD, volume 0, all other buttons out. Measured DC on the speaker A-Left terminals and it was about 36mV (RH channel is about 10mV). I adjusted VR401 but it seems to have no effect. It is a multi-turn (someone changed it?) so I tried multiple turns. No change. The pot resistance seems good including the wiper.

I'll try again when I get my replacement Q433. Should I still be able to adjust my output zero with Q431 and Q433 removed? Should I not have powered it without Q431 and 432? Should I try to adjust my idle current first? this is not what the service manual says.

Thanks again. I'll post when I get my other transistor.
 
Strange. my power supplies are about half what they should be (+/- 23VDC and (+/-43VDC). I checked and it's a US 120V input. But I checked the voltage input to the amp and it's about 80V RMS. The 40 Watt bulb is dropping about 40V. Is that normal? Seems like it to me, about 1 A. How do I check my voltages with the dim bulb tester in the loop? Does this mean I still have an issue? Can I remove dim bulb tester?

Any help would be appreciated.
 
Ok big update. The offset was actually 36V and not 36mV! Any way. Checking some voltages and noticed when I was pushing on the board with my probes, voltages would change. While monitoring offset i pushed around the area that was causing the change. By some miracle I found the trace between the base of Q421 and (Q415 collector+C419+C425) was broken and making intermittent contact. Couldn't even see the break but could measure it with a voltmeter. Cleaned up the trace and made a solder bridge and now voltages are good and can easily adjust offset.

The voltages are still about half due to the bulb I think. Does it make sense to adjust offset and idle current in this condition? I'm confident to plug it in directly. If I adjust offset and idle current with 71V disconnected from output stage will I have to redo it when I reconnect 71V?

I think the dim bulb tester saved my butt. Probably would have blown up a few more transistors.
 
Rocks said:
The 40 Watt bulb is dropping about 40V. Is that normal? Seems like it to me, about 1 A. How do I check my voltages with the dim bulb tester in the loop? Does this mean I still have an issue? Can I remove dim bulb tester?
Click to expand...
A 40W DBT will choke the amp supply so low rail voltages are expected.
If the bulb dims then these is no short. With a 40W bulb it could be faintly lit again indicating no near short. If the bulb is reasonably bright (70% + of full brightness) then you still have an issue. You could try changing to a 60-100W bulb if you feel uncertain.

Rocks said:
Does it make sense to adjust offset and idle current in this condition? I'm confident to plug it in directly.
Click to expand...
While on DBT to could tweak the bias VR's a little to confirm the bias current does respond as expected. Once confirmed, reset the VR's to minimum/zero bias current, this step is very important.

If DBT is dim and bias has been reset to near zero then you could power up without DBT then set dc offset and bias as per the sm.

Rocks said:
If I adjust offset and idle current with 71V disconnected from output stage will I have to redo it when I reconnect 71V?
Click to expand...
No but I would certainly be rechecking the bias after repairs to the 71V/pe stage.

The 71V/PE stage was disabled so you could sort out the basic amp. Now that appears to be working you need to test the PE components and replace any faulty.
You should then reconnect the 71V/PE stage and DBT power up, go with 40-75W. Then investigate why the DBT is bright... ie, check voltages...
 
mbz said:
A 40W DBT will choke the amp supply so low rail voltages are expected.
If the bulb dims then these is no short. With a 40W bulb it could be faintly lit again indicating no near short. If the bulb is reasonably bright (70% + of full brightness) then you still have an issue. You could try changing to a 60-100W bulb if you feel uncertain.


While on DBT to could tweak the bias VR's a little to confirm the bias current does respond as expected. Once confirmed, reset the VR's to minimum/zero bias current, this step is very important.

If DBT is dim and bias has been reset to near zero then you could power up without DBT then set dc offset and bias as per the sm.


No but I would certainly be rechecking the bias after repairs to the 71V/pe stage.

The 71V/PE stage was disabled so you could sort out the basic amp. Now that appears to be working you need to test the PE components and replace any faulty.
You should then reconnect the 71V/PE stage and DBT power up, go with 40-75W. Then investigate why the DBT is bright... ie, check voltages...
Click to expand...
Thanks for the tips. All makes sense. I’ll continue when I have some time.
 
Success! All parts changed, amp adjusted and working great. I suspect the broken trace caused all the failures. I didn’t do any work in that area so maybe shock, vibration, or previous repair. Only issue now is erratic tuning in the tuner. if anyone has a simple solution for suggestion I’ll give it a try. Otherwise I’ll just be using 5ge phono input.

Thanks again for all the help. Hopefully I can give back in the future.
 
Don’t know whether to laugh or cry. Was doing back to back audition of my NAD 7130 and this 7240 PE I just repaired to see which I should keep. Was listening for about 15 minutes at about 1/3 volume. All of a sudden pop, a flash, and smoke. I think I saved my speaker but it took out almost all the transistors in LH channel. What the heck are the fuses for! I don’t give up easy and transistors are cheap, so I´ll spend a bit more time on it. Was there maybe some transistors from previous disaster that were weakened? Was it the cheap replacements from China? Is there other hidden broken traces or cold joints.

A shame because I liked the 7240 better. More neutral which is what I like.

Oh well. Maybe I’ll post again. Maybe not.
 
Latest casualties: Q417, Q419, Q421, Q423, Q425, Q427, Q429, R459, R467. Also, when I did my offset and idle current adjustments, I was in 8 ohm position as per the manual. This was also the case for initial sound tests with my speakers. Realized I should be in 4 ohm normal position. After this switch it did work for awhile then blew up. Did some more checking and SW502-2 is showing high resistance in 4 ohm position. Ordered all parts including switch and will start over. Is it possible high resistance switch caused all this destruction?
 
A little odd the Q417 smoked. Have you replaced the bias trimmer VR403?
NAD's of this vintage typically used very cheap trimmers, maybe intermittent failed open?? Was bias adjust smooth or was it difficult to adjust. Go with a multitunr type eg, Bourns 3296Y???series
 
Whoever fixed it previously changed all the pots on the amp board to multi turn. I don’t remember having any particular difficulty adjusting bias. Not sure about my switch theory either. When I unsoldered the xformer wires the switch tested good. I’ll change it anyway but if it was an issue it probably would have damaged RH channel as well.
 
This is an example of where comparing the signatures of the good channel with the bad one using a Huntron Tracker™ might be very productive.

I haven't repaired a lot of DC-coupled SS amps with blown channels like this, but the few I have seen have leakage or other "one signature doesn't look like the other" subtle problem, typically in the transistors at the very beginning of the amplifier stage.
 
You must log in or register to reply here.
Back
Top Bottom