I need to replace my Nad M25 Power amplifier.

[Verdinut]

I had a comparable event happen recently with a Phillips CD player. It was a power supply cap. Along with the cap, it took out a regulator and the fuse. While not a very big cap, it cost me about $100 to get fixed. I only paid $15 for the diagnosis.

So, if you blew a power supply cap, expect you also (at least) need a fuse too. Also, if you either hope to get it back or sell it, would urge you to take it to a repair shop. The primary rule of blown fuses is not to replace them without knowing what caused them to blow. Sounds as though you have some other equipment around to handle the time it takes to service. If you just wing it, you may cause more damage.

Hope this helps...

Ww

Sometimes a fuse will blow not because of a defective equipment, but because the fuse has been in the apparatus for a long period. I don't think there is much risk in replacing a blown fuse with a new one of the same type and amperage, before spending time and money in paying for a diagnosis testing which might not have been necessary.

 

[mtrycrafts]

...
Upon farther inspection, I found out that it blew a capacitor.
....

Why not just replace or have someone replace those caps? Should cost way less.

 

[TLS Guy]

Thank you for the replay.
I noticed that the capacitor blew a tiny hole on the side. That’s where the sparks came from.
Right next to the hole you can see all the black dust on the board.
I know what you mean and I’m afraid that maybe something else cause the cap to failed.
Maybe it won’t be as an easy fix, that’s why I’m getting ready in case I need to replace the amp.
I’m hoping that it will get fix with just that.

I have had a look at the service manual and the circuit.

By the value of the cap, the cap has to be C 202 or 203. They are the same value and paired.

I suspect that the primary culprit is Bridge rectifier BR 201 DB 104G.That is switch diode 1N4148 . I would replace that and C202 and 203 both the same value. I would also replace R 2H1 Metal film resistor 100Ω, 1/6W, 5% and C 204 and 206 both Polyester caps 10nF, 10%, 100V . I would suspect fuses 201 and or fuse 203 are blown. They are T1.6AL/250V .

After replacing those components it should be started up on a variable voltage auto transformer, know as a Variac and observed closely.

This should be repairable at modest cost.

 

[Pablo2k]

I have had a look at the service manual and the circuit.

By the value of the cap, the cap has to be C 202 or 203. They are the same value and paired.

I suspect that the primary culprit is Bridge rectifier BR 201 DB 104G.That is switch diode 1N4148 . I would replace that and C202 and 203 both the same value. I would also replace R 2H1 Metal film resistor 100Ω, 1/6W, 5% and C 204 and 206 both Polyester caps 10nF, 10%, 100V . I would suspect fuses 201 and or fuse 203 are blown. They are T1.6AL/250V .

After replacing those components it should be started up on a variable voltage auto transformer, know as a Variac and observed closely.

This should be repairable at modest cost.

Thank you for all the info!
The capacitor says CD294 on it. There is another one next to it with same numbers, CD294 35v 3300uf Jianghai on the side and on top 590065.
In C202 and C203 just like you said!

 

[Pablo2k]

Thank you for all the info!
The capacitor says CD294 on it. There is another one next to it with same numbers, CD294 35v 3300uf Jianghai on the side and on top 590065.
In C202 and C203 just like you said!

Here are some photos, there is also some rust? Or goo? Is that going to be Noe problems in the future?

http://imgur.com/a/hujMvdQ

 

[Pablo2k]

http://imgur.com/a/Lq7LSan

 

[TLS Guy]

Here are some photos, there is also some rust? Or goo? Is that going to be Noe problems in the future?

http://imgur.com/a/hujMvdQ

I can't forecast future problems. I should have pointed out that you will need to four of those diodes, to make the bridge. All the components are common and available from sources like Mouser.

I suspect what happened was a failure of the bridge rectifier so that the caps were exposed to AC, which would have resulted in high current passing though the cap and hence the exploded cap. I think I have identified all the components which would have been exposed to this disaster and need replacing.

 

[Winkleswizard]

http://imgur.com/a/Lq7LSan

If nothing has been dumped on it and it has not been exposed to smoke, you are likely seeing leftover residue from the capacitor failing and/or solder flux residue. If this is the case, there are 2 primary issues. One is how conductive the goo might be and the other is increased heat. Neither are likely catastrophic in the short term but can shorten electronics longevity. Solder flux residue can be from original manufacture or repair. Good practice is to clean with a solvent. These solvents are usually environmentally nasty, but have been replaced by most reputable manufacturers in recent years.

I have an old Proton amp, and it was shocked to see how much flux residue it has. I only noticed after some problems with the output relays and it is much older than your NAD. If you are taking it to a shop, would have them check. In manufacturing, the circuit boards are bathed in a solvent and thoroughly dried. The trick in repair is to clean judiciously and allow time to dry. You do not want to bathe the board as this may shift the goo from being a thin layer to accumulating into a thicker one.

As you might surmise, have done a bit of my own repair and taken the sort of risks that you are considering. Usually, I only do this when I know the repair cost may exceed the value of the item. From a quick check, your NAD is worth much more than what it sounds like it will cost to repair. Notably, if the shop repairs to like new condition, your outcome will likely be better. Would also ask how they test the repair. This can be a key difference between a DIY repair and a factory authorized shop. They have equipment to test the amp that can ensure it sounds like new. In the end, your call. Good luck!

Ww

 

[PENG]

Here are some photos, there is also some rust? Or goo? Is that going to be Noe problems in the future?

http://imgur.com/a/hujMvdQ

As mentioned, those were likely from soldering flux, glue related corrosion over time. I agree with TLSGuy, no one can forecast future problems, other than to say nothing will last forever. Having said that, the unit is only 13 years old so if properly repaired, then well protected from voltage surge and not abused, should be good for a number of years.

It is highly recommended that you replace all the components of that 12 V PS rectifier, including the diodes. Also, there is no guarantee that the other bridge rectifier (18V) has survived that same spike/surge, but if there are no visible signs then it may be fine. 35 V for those caps seems a little low, for such relatively high end NAD. You are going to have trouble getting the exact replacement caps anyway, so may as well get some higher voltage ones whether it is necessary or not.

 

[TLS Guy]

As mentioned, those were likely from soldering flux, glue related corrosion over time. I agree with TLSGuy, no one can forecast future problems, other than to say nothing will last forever. Having said that, the unit is only 13 years old so if properly repaired, then well protected from voltage surge and not abused, should be good for a number of years.

It is highly recommended that you replace all the components of that 12 V PS rectifier, including the diodes. Also, there is no guarantee that the other bridge rectifier (18V) has survived that same spike/surge, but if there are no visible signs then it may be fine. 35 V for those caps seems a little low, for such relatively high end NAD. You are going to have trouble getting the exact replacement caps anyway, so may as well get some higher voltage ones whether it is necessary or not.

You have to be careful using caps of higher voltage, as they hold their specified charge at the specked voltage.

Really this job should be done in a shop for a unit of this value. The components can be tested and instruments put round it on start up with the Variac. This is only a DIY job with experience and with someone owning the right test instruments.

 

[PENG]

You have to be careful using caps of higher voltage, as they hold their specified charge at the specked voltage.

Not really, the highest voltage is determined by the transformer secondary voltage. For example, if the transformer secondary voltage, i.e. the supply voltage to the cap is, say 10 V, then the maximum it would hold should be 14.14 VDC. You can use a 80 VDC cap, it will still only be charged to the peak of the supply voltage. For this application, 35 VDC is in fact adequate, but for all the labor work involved, may as well use 50 VDC caps for potentially better reliability/longevity, and it most likely would cost just a little more.

Really this job should be done in a shop for a unit of this value. The components can be tested and instruments put round it on start up with the Variac. This is only a DIY job with experience and with someone owning the right test instruments.

I tend to agree, all the caps and diodes should be replaced, in least those for the 12 V bridge rectifier, and something else could have been damaged or weakened by the spike/surge event, a qualified audio repair shop should have the equipment to properly bench test the repaired unit.

 

[TLS Guy]

Not really, the highest voltage is determined by the transformer secondary voltage. For example, if the transformer secondary voltage, i.e. the supply voltage to the cap is, say 10 V, then the maximum it would hold should be 14.14 VDC. You can use a 80 VDC cap, it will still only be charged to the peak of the supply voltage. For this application, 35 VDC is in fact adequate, but for all the labor work involved, may as well use 50 VDC caps for potentially better reliability/longevity, and it most likely would cost just a little more.



I tend to agree, all the caps and diodes should be replaced, in least those for the 12 V bridge rectifier, and something else could have been damaged or weakened by the spike/surge event, a qualified audio repair shop should have the equipment to properly bench test the repaired unit.

Increasing the voltage of the cap a small amount will not hurt. Any cap will charge to the same voltage, but that misses the point. If you have two 20 mfd caps. and one takes 30 volts to charge it to 20 mfd, and the other takes 100 volts to charge it to 20 mfd, sure the voltage of the charge will be the same, but the 30 volt cap will have a larger electric charge than the 100 volt cap charged to 30 volts. So the 100 volt cap will not smooth as well as the 30 volt cap at that voltage.

 

[PENG]

Increasing the voltage of the cap a small amount will not hurt. Any cap will charge to the same voltage, but that misses the point. If you have two 20 mfd caps. and one takes 30 volts to charge it to 20 mfd, and the other takes 100 volts to charge it to 20 mfd, sure the voltage of the charge will be the same, but the 30 volt cap will have a larger electric charge than the 100 volt cap charged to 30 volts. So the 100 volt cap will not smooth as well as the 30 volt cap at that voltage.

It seems to me perhaps you are the one missing the point on this. In the example you used, a 20 mfd capacitor's rated capacitance is just 20 mfd (obviously subject to the specified tolerances),this will be the same whether the applied voltage is 30 V or 100 V, as long as the applied voltage is within it's specified voltage rating. The voltage rating is just the rated maximum voltage. If the operating voltage is lower than the rated voltage, it is good. All else being equal, a 50 V 3300uf should last longer than a 35 V 3300 uf in that bridge rectifier as shown in the M25 service manual. 85 degree C temperature rating would be fine.

I highly recommend he replaces all of those 3300 uf 35v caps with 50v rated caps. No I would not go higher than 50v as that would borderline on being silly. Again, it is not necessary, but if one is to go through the trouble of taking things apart and all the soldering, one may as well put something back in that is better, for just a little money, likely less than $1 difference.

 

[KenM10759]

I'll offer a couple hunert bucks for the M25 as-is, and have my friend fix it. He's primarily a factory-authorized repair station for pro musician equipment (Mesa, Boogie, Marshall, etc.) but helps me out with stuff like this. Did a nice job finding a bad diode on a Marantz 2270 and got it running perfect.

 

[PENG]

I'll offer a couple hunert bucks for the M25 as-is, and have my friend fix it. He's primarily a factory-authorized repair station for pro musician equipment (Mesa, Boogie, Marshall, etc.) but helps me out with stuff like this. Did a nice job finding a bad diode on a Marantz 2270 and got it running perfect.

For a couple hundred bucks as is, sound like a good deal. That is one powerful amp but by today's standard the design sucks, for one thing 98 lbs for that 200/400 W output, no thanks!

 

[TLS Guy]

It seems to me perhaps you are the one missing the point on this. In the example you used, a 20 mfd capacitor's rated capacitance is just 20 mfd (obviously subject to the specified tolerances),this will be the same whether the applied voltage is 30 V or 100 V, as long as the applied voltage is within it's specified voltage rating. The voltage rating is just the rated maximum voltage. If the operating voltage is lower than the rated voltage, it is good. All else being equal, a 50 V 3300uf should last longer than a 35 V 3300 uf in that bridge rectifier as shown in the M25 service manual. 85 degree C temperature rating would be fine.

I highly recommend he replaces all of those 3300 uf 35v caps with 50v rated caps. No I would not go higher than 50v as that would borderline on being silly. Again, it is not necessary, but if one is to go through the trouble of taking things apart and all the soldering, one may as well put something back in that is better, for just a little money, likely less than $1 difference.

No! The charge held on a capacitor is directly proportional to charging voltage. So if we take a 50 mfd cap rated 50 volts and charge it with 50 volts it will store 50 mfd. Now if we take a 50 mfd cap rated to 100 volts and charge it to 50 volts it will only be storing 25 mfd. However that voltage at which the charge is stored will be the same. The amount of charge held on the capacitor will only be at specification of the cap at the specified voltage. This is a common error to think that the voltage rating of a cap is just its safety voltage. It is that and more. So when replacing caps you have to keep to the charging voltage to store the specified charge. So if in a lower supply you replaced with caps of higher voltage you would reduce the smoothing and cause trouble.

 

[Beave]

 

[slipperybidness]

No! The charge held on a capacitor is directly proportional to charging voltage. So if we take a 50 mfd cap rated 50 volts and charge it with 50 volts it will store 50 mfd. Now if we take a 50 mfd cap rated to 100 volts and charge it to 50 volts it will only be storing 25 mfd. However that voltage at which the charge is stored will be the same. The amount of charge held on the capacitor will only be at specification of the cap at the specified voltage. This is a common error to think that the voltage rating of a cap is just its safety voltage. It is that and more. So when replacing caps you have to keep to the charging voltage to store the specified charge. So if in a lower supply you replaced with caps of higher voltage you would reduce the smoothing and cause trouble.

I was in agreement with @PENG on this. I also thought that this was basically a "max voltage rating", and nothing more. How does this relate to "derating capacitors" to get a safety margin? You must also consider the charging capacity with the derating calculations???

I have never heard of this before! If this is indeed correct, then "common error" is a vast understatement! I think "proliferated error" or "ubiquitous error" would be a better description.

@TLS Guy do you happen to have any literature references for this? I do believe your technical abilities, I'm just surprised that I seem to have also made this error and never have seen any literature to the contrary!

 

[PENG]

No! The charge held on a capacitor is directly proportional to charging voltage.

This part is correct, who is arguing?? Now, Q=CV, Q is the charge in Coulomb, C is the capacitance in Farad. It should be clear from this formula that the "Charge held.." is proportional to C (unit in mfd) and the applied voltage (unit in V, i.e. volt).

So if we take a 50 mfd cap rated 50 volts and charge it with 50 volts it will store 50 mfd.

No, are you mixing up "Charge" with "Capacitance" ? You don't store mfd, you store Coulombs, higher mfd value means more Coulombs you can store, at a given applied voltage.

Now if we take a 50 mfd cap rated to 100 volts and charge it to 50 volts it will only be storing 25 mfd.

Incorrect ! As mentioned above, you don't store 25 mfd, 25 mfd is microfarad, or uF, that is the capacitance, and is equal to Q/V. So you can see clearly that since Q=CV, so as you said, the charge is proportional to voltage, but it is also proportional to the capacitance. If the applied voltage is the same, then the charge held in the example you are using will be the same, as shown below.

In your example of 25 mfd, charged to 50 volts:

Q=CV = 25 X 10−6 X 50 = 0.00125 Coulomb.

However that voltage at which the charge is stored will be the same. The amount of charge held on the capacitor will only be at specification of the cap at the specified voltage. This is a common error to think that the voltage rating of a cap is just its safety voltage. It is that and more. So when replacing caps you have to keep to the charging voltage to store the specified charge. So if in a lower supply you replaced with caps of higher voltage you would reduce the smoothing and cause trouble.

That is so incorrect! TLS Guy, I do respect your knowledge. I called you out before (rarely) on certain things, usually on the power and electronic side, and you never had problem with it. For that you have my respect too. This is about electrical principles, definitions, theories etc., so it is a matter of facts, not opinions. I hope you will do your research properly, admit the error and move on, so it won't stay on record that may mislead people inadvertently.

 

[AcuDefTechGuy]

You guys make me feel like I wish I had gotten a degree in Electrical Engineering so I could join the debate.

My EE degree from the University of Google just won't do.

Too bad my younger brother (who has a degree in Electrical Engineering with Computer Science Option) doesn't even care about Audio stuff.