Slow failure of NAD 7225PE. Suggestions?

riotpack · 22nd October 2011, 09:27 AM

The fault is that the right hand channel will randomly make a loud crackling sound and blow my 1 Amp fuse connected inline with the speaker (I didn't trust this from the start and made speaker leads with 1A fuses!).

I may get a week of listening before it happens, or maybe 2 days, I spent an hour going over suspect joints (actually sucking up all the old solder and replacing it) and testing caps with my ESR meter. Does anyone have an idea what is going on??

R438 is burning up when this fault occurs, and now has a black ring in the centre but it still measures ok and I am listening to the receiver now. I think Q410 may be going leaky when hot, but could be Q408 or anything really causing this chain reaction.

Has anyone spent time servicing this model that may be willing to throw some ideas at me? Or is the idea to throw this at a wall?

riotpack · 22nd October 2011, 09:29 AM

Oh just to add something I have noticed, threads I have read from the past assumed the OP stage went faulty and took out R438 and pre stage transistors, this looks like the opposite is happening and my fuses are saving the OP stage!

Mooly · 22nd October 2011, 10:28 AM

My instincts say that if this is not a dry joint etc then you should look at replacing the driver transistors (the 2SD669 and 2SB649). For no explainable reason these type of Japanese devices do fail open circuit base-emitter intermitently. MJE340/350 would be fine.

Also do check for dj's by actually resoldering the board. Sometimes it can be hard to spot a bad joint. Make sure the solder "takes" to the component as oxidation is a problem with many parts.

riotpack · 22nd October 2011, 10:42 AM

Thank you for your reply, I have just spent a while removing solder and reapplying solder to all main and suspect joints in the power amplifier section. I will thrash it for the next few days and see what happens. As a side note the top of the PCB actually measures a few megohm! Ah yes the classic NAD glue that conducts.... I made sure to clean it up, as where resistors are close it may just make a difference. I don't think this is the kind of amp you ever gain the confidence to remove your fused speaker leads!

Mooly · 22nd October 2011, 11:05 AM

Faults like this are the kind of thing you don't want for the safety of your speakers...

Personally I would replace the drivers on both channels and adjust the quiescent current to the correct value unless you are 100% sure you have found a problem joint... I have seen these fail (this type of TO126 packaged device) so many times and it's just not worth the risk. The very act of soldering them may temporarily "fix" them. I would also perhaps add the BD139 to the list.

If you can catch it in a faulty condition measuring across the base emitter junction (voltage) would reveal the problem.

It's just not worth the risk to your speakers.

riotpack · 22nd October 2011, 11:14 AM

Ah yes, the quiescent current. The service manual advises removing the short across a couple of resistors (one each channel) and measuring the voltage across them; I haven't found the bridge yet, but may have to take the bottom off tomorrow. I have set the amp to 8 Ohm mode to lower the supply rails. Tonight I will listen to music... It does throw a lot of heat out to be honest, but all NAD amps do this IMO.

sreten · 22nd October 2011, 12:11 PM

Quote:

Originally Posted by riotpack

I have set the amp to 8 Ohm mode to lower the supply rails.

Hi, AFAIK its the 4ohm setting that lowers the supply rails, rgds, sreten.

riotpack · 22nd October 2011, 12:57 PM

Sorry yes you are right, switching to 8 ohm assumes less load and so a higher rail voltage should not pass the SOAR of the transistors.

riotpack · 22nd October 2011, 01:41 PM

NAD 7225PE test - YouTube

Just for kicks here is a link to the amp going through a burn in test

Hugo Rodriguez · 23rd October 2011, 03:27 AM

Hi Riotpack:

Some minutes ago, I finally finish fixing my 3225PE amp, that has exactly the same output stage, that you show in the schematic.

My amp also showed the hi-heating of R438, but was produced by a mistake made by me. I forgot to put the mica insulator below Q408 when I replaced it. After putting it again, the heat problem finished, but the output showed a 24 V voltage instead of 0 V d. c., and having a light bulb connected in series with the amp, as recommended in the service manual.

This other problem was caused by a leakage in Q402 between base and colector. Testing the transistor as two diodes, showed be in good shape, but when I measured resistance between leads, I found a strange value (1.8 kohm instead "OL" indication). This leakage moved the voltage at several transistor bases, moving to conduction some that should be in cut. After changing Q402 with a replacement (MPSA-06G, as I have not the BC550C at hand), the problem disappear and then I could do the center voltage and iddle current adjustments indicated in the service manual.

The solder bridge you need to remove to do the adjustments is in parallel with R456 (and R455 for the left channel).

I tested the amp for about one hour, listening some vinils and everithing was OK. (After two weeks of one or two hours night working).

Check the voltage at the colectors junction of Q402 and Q404. Probably will give you an indication of the cause of the failure.

Good look, and cheers.

Hugo.

22nd October 2011, 10:28 AM	#3
Mooly diyAudio Member Join Date: Sep 2007	My instincts say that if this is not a dry joint etc then you should look at replacing the driver transistors (the 2SD669 and 2SB649). For no explainable reason these type of Japanese devices do fail open circuit base-emitter intermitently. MJE340/350 would be fine. Also do check for dj's by actually resoldering the board. Sometimes it can be hard to spot a bad joint. Make sure the solder "takes" to the component as oxidation is a problem with many parts. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

22nd October 2011, 11:05 AM	#5
Mooly diyAudio Member Join Date: Sep 2007	Faults like this are the kind of thing you don't want for the safety of your speakers... Personally I would replace the drivers on both channels and adjust the quiescent current to the correct value unless you are 100% sure you have found a problem joint... I have seen these fail (this type of TO126 packaged device) so many times and it's just not worth the risk. The very act of soldering them may temporarily "fix" them. I would also perhaps add the BD139 to the list. If you can catch it in a faulty condition measuring *across* the base emitter junction (voltage) would reveal the problem. It's just not worth the risk to your speakers. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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22nd October 2011, 09:29 AM	#2
riotpack diyAudio Member Join Date: Sep 2011 Location: Brisbane	Oh just to add something I have noticed, threads I have read from the past assumed the OP stage went faulty and took out R438 and pre stage transistors, this looks like the opposite is happening and my fuses are saving the OP stage!

22nd October 2011, 10:42 AM	#4
riotpack diyAudio Member Join Date: Sep 2011 Location: Brisbane	Thank you for your reply, I have just spent a while removing solder and reapplying solder to all main and suspect joints in the power amplifier section. I will thrash it for the next few days and see what happens. As a side note the top of the PCB actually measures a few megohm! Ah yes the classic NAD glue that conducts.... I made sure to clean it up, as where resistors are close it may just make a difference. I don't think this is the kind of amp you ever gain the confidence to remove your fused speaker leads!

22nd October 2011, 11:14 AM	#6
riotpack diyAudio Member Join Date: Sep 2011 Location: Brisbane	Ah yes, the quiescent current. The service manual advises removing the short across a couple of resistors (one each channel) and measuring the voltage across them; I haven't found the bridge yet, but may have to take the bottom off tomorrow. I have set the amp to 8 Ohm mode to lower the supply rails. Tonight I will listen to music... It does throw a lot of heat out to be honest, but all NAD amps do this IMO.

22nd October 2011, 12:57 PM	#8
riotpack diyAudio Member Join Date: Sep 2011 Location: Brisbane	Sorry yes you are right, switching to 8 ohm assumes less load and so a higher rail voltage should not pass the SOAR of the transistors.

22nd October 2011, 01:41 PM	#9
riotpack diyAudio Member Join Date: Sep 2011 Location: Brisbane	NAD 7225PE test - YouTube Just for kicks here is a link to the amp going through a burn in test

23rd October 2011, 03:27 AM	#10
Hugo Rodriguez diyAudio Member Join Date: Dec 2003 Location: Queretaro, Mexico	Hi Riotpack: Some minutes ago, I finally finish fixing my 3225PE amp, that has exactly the same output stage, that you show in the schematic. My amp also showed the hi-heating of R438, but was produced by a mistake made by me. I forgot to put the mica insulator below Q408 when I replaced it. After putting it again, the heat problem finished, but the output showed a 24 V voltage instead of 0 V d. c., and having a light bulb connected in series with the amp, as recommended in the service manual. This other problem was caused by a leakage in Q402 between base and colector. Testing the transistor as two diodes, showed be in good shape, but when I measured resistance between leads, I found a strange value (1.8 kohm instead "OL" indication). This leakage moved the voltage at several transistor bases, moving to conduction some that should be in cut. After changing Q402 with a replacement (MPSA-06G, as I have not the BC550C at hand), the problem disappear and then I could do the center voltage and iddle current adjustments indicated in the service manual. The solder bridge you need to remove to do the adjustments is in parallel with R456 (and R455 for the left channel). I tested the amp for about one hour, listening some vinils and everithing was OK. (After two weeks of one or two hours night working). Check the voltage at the colectors junction of Q402 and Q404. Probably will give you an indication of the cause of the failure. Good look, and cheers. Hugo.

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