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Old 3rd July 2008, 11:59 AM   #1
AlexRox is offline AlexRox  Singapore
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Angry Problem with a NAD 3130

Hey everyone, this is my first post.
I may not be an expert at audio, but Im learning, just keep in mind what you are about to read will make you wonder how stupid I am.

A needle fell through the top of the NAD 3130's case and bridged VR401 with Q407's E contact.
After this, I think the variable resistor caught fire, because I could smell smoke. So now it doesn't work at all, well, it turns on, but all I get is static and audible hiss.

Im clueless as to what to do for lack of knowledge of what parts to buy.
I can solder the new parts to the board etc and do necessary repairs...
I was just hoping someone could push me in the right direction.

Attached is a picture showing what I see, (And some obvious blackening from the short circuiting of VR401 and Q407's E contact), just for reference purposes.

Again, I cannot stress how stupid I already feel, Im just looking for a solution.

Thanks in advance for any help.

Alex
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Old 4th July 2008, 06:34 AM   #2
Mooly is offline Mooly  United Kingdom
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Hi,
It is almost certainly easily fixable but you will need to post a circuit. Otherwise you need to check semiconductors OUT OF CIRCUIT , replace any obviously damaged parts, and have the confidence in your measurements and interpretation of those readings.
Regards Karl
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Old 4th July 2008, 02:11 PM   #3
AlexRox is offline AlexRox  Singapore
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Kar, I have enough confidence that its fixable, but when you say post a circuit, are you asking for the whole diagram? Im hoping theres someone on here with the same AMP as me to tell me which parts exactly I've damaged, because, while theres visible damage to the variable resistor, I don't have confidence that replacing just that will make it work as good as new.
Infact, I've bought the replacement variable resistor, and put it back in, and Im still getting some very unstable output. Like I said, I need someone who has experience with this amp 'intimately' so to speak. Any help will not go un-noticed.

Alex
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Old 4th July 2008, 02:31 PM   #4
jaycee is offline jaycee  United Kingdom
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VR401 is probably the bias adjust pot. If you have shorted that then it is highly likely that the output transistors are also dead. This could turn into an expensive repair.
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Old 4th July 2008, 03:26 PM   #5
djk is offline djk
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Q407 looks like the Vas, the emitter would connect to the rail.

I would check every semiconductor in that channel, and probably replace them all. Parts are cheap.
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Old 4th July 2008, 04:06 PM   #6
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It is extremely rare to just replace visably faulty components and have things work perfectly as before, please bear this in mind, ill try to give you some advise but you would normally need a few essential items of equipment before attempting a fix.
firstly see if your output transistors are dead, do this by connecting a multimeter to your speaker outputs on the faulty channel if your measuring around 30volts, which is around the rail voltage then the transistors are toast! if you are reading millivolts idealy under 30mv then the problem lies with the damaged components (doh!) and very probably the drivers (the squarer looking transistors. please post your findings so further help my come.

also a word of advise if you havn't got a meter or dont know how to use one, i would suggest you find someone who is qualified in such things to have a look
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Old 4th July 2008, 05:08 PM   #7
Mooly is offline Mooly  United Kingdom
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Agree with most of the comments so far. You can use a 60 watt in series with the mains to limit any further damage.
A lot depends on your experience, if you were handed a bag of unmarked transistors some faulty (leaky junctions, O/C junctions S/C junctions etc) and some perfect could YOU reliably and confidently say which were good or bad and even identify their polarity .
Sorry to sound a bit harsh but it's not as simple as saying that such and such a part will be faulty -- replace it and all will be well.
If you replace the bias pot do you know how to adjust it, whether to have it on min or max resistance to begin with etc.
If you can come up with a circuit of the output stage I am sure you can fix it.
You have to remember how the fault occured, you may have zapped even the input stage as well-- you need a game plan to fix it.
Regards Karl
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Old 4th July 2008, 09:03 PM   #8
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Quote:
Originally posted by Mooly
Agree with most of the comments so far. You can use a 60 watt in series with the mains to limit any further damage.
A lot depends on your experience, if you were handed a bag of unmarked transistors some faulty (leaky junctions, O/C junctions S/C junctions etc) and some perfect could YOU reliably and confidently say which were good or bad and even identify their polarity .
Sorry to sound a bit harsh but it's not as simple as saying that such and such a part will be faulty -- replace it and all will be well.
If you replace the bias pot do you know how to adjust it, whether to have it on min or max resistance to begin with etc.
If you can come up with a circuit of the output stage I am sure you can fix it.
You have to remember how the fault occured, you may have zapped even the input stage as well-- you need a game plan to fix it.
Regards Karl

Personally I would remove the output transistors and link the output back into the LTP. Only once had I got the driver working and set the bias to minimum would I even consider refitting output transistors (after checking they are not faulty !)

Even then I sometimes only fit one pair of output transistors to reduce costs if they should still blow.

Costs is one reason I go for cheapo nasty MOSFETS IRFP240/9240.
At 2 each on ebay they are almost peanuts.

I also always put a zener across the bias cct to ensure it cant go way out of bounds. On a MOSFET amp I put in a 7 volt zener and the MOSFETs wont blow at that voltage.
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Old 4th July 2008, 11:22 PM   #9
djk is offline djk
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"Even then I sometimes only fit one pair of output transistors to reduce costs if they should still blow."

Good advice on a larger amplifier, this one only has one pair.

"Costs is one reason I go for cheapo nasty MOSFETS IRFP240/9240.
At 2 each on ebay they are almost peanuts."

Would require a re-design of the bias circuit, reduce amplifier output power, and cost too much (3055/2955 can be had for about USD$1 each).

"I also always put a zener across the bias cct to ensure it cant go way out of bounds. On a MOSFET amp I put in a 7 volt zener and the MOSFETs wont blow at that voltage."

That may work on lateral FETs, but not on the IRF types you suggested.

http://www.vishay.com/docs/90274/90274irf.pdf

According to the data, a 7V zener will hold it down to about 20A.
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Old 4th July 2008, 11:30 PM   #10
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Quote:
Originally posted by djk
"Even then I sometimes only fit one pair of output transistors to reduce costs if they should still blow."

Good advice on a larger amplifier, this one only has one pair.

"Costs is one reason I go for cheapo nasty MOSFETS IRFP240/9240.
At 2 each on ebay they are almost peanuts."

Would require a re-design of the bias circuit, reduce amplifier output power, and cost too much (3055/2955 can be had for about USD$1 each).

"I also always put a zener across the bias cct to ensure it cant go way out of bounds. On a MOSFET amp I put in a 7 volt zener and the MOSFETs wont blow at that voltage."

That may work on lateral FETs, but not on the IRF types you suggested.

http://www.vishay.com/docs/90274/90274irf.pdf

According to the data, a 7V zener will hold it down to about 20A.

Not in my experience !

I have opened up the pot until I got 6.9 volts on it and get 25mA bias current per MOSFET on average. I have opened the bias pot right up until the zener kicked in and it was still barely more than 30mA.

Because I used multiple sets of output transistors the bias through each MOSFET does vary a bit due to each having slightly different turn on voltages.

The datasheet says 3 to 4 volts to switch on so my 3.5 volts per MOSFET sound sabout right.

I use 0R22 output resistors so even at 4 volts you are only pulling 2.3 amps.


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