Nad 310 amplifier - blowing fuse in plug

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Hi collective!

Am hoping someone might be able to help out with a problem I've encountered with my Nad 310 amplifier. After years of faithful service without so much as a hint of a problem, it's all gone Pete Tong.

Turned it on this morning, got the familiar 'pop' through the speakers, played teh CD and the sound just faded out and went crackly - like the amp had been turned off. Checked the front panel and, yep, the power light was fading.

Checked the fuse in the plug (3A fuse) and it was blown, so replaced it, and tried again. Familiar 'pop', power light glows bright then fades away. Fuse had blown again.

Tried a third time with all interconnects and cables removed (just on the off chance....) and still no joy.

Obviously it is blowing the fuse as the amp fires up, and I've taken the top off and checked the two glass fuses on the board beside the transformer winding and they are both fine (checked in situ, not removed).

Any suggestions what to check next? Or is it dead (ie, not really worth spending the money on repairing)? I know it's quite an old amp, and not exactly high-end, but it's served me well and I don't particularly have the cash to go out and buy a new amp at the moment!

Many thanks in advance for your help.

Andrew
 

stevieg330

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Hi Andrew,

From a quick look at the schematic the two internal fuses you can see are to protect the left and right sides of the circuit. From the fuses the next component in line are two rectifiers, one for each part of the circuit.

I would unplug the amp from the mains and remove the two fuses which will prevent power getting to the circuits. Plug the amp back in and switch it on. If it still blows the fuse then it could be that the mains transformer is shot (unlikely but you never know). According to the diagram I have there is nothing on the input side except a capacitor across the switch so if it still blows the fuse then the transformer is the likely candidate.

If the plug fuse stays intact then I would replace one of the on-board fuses, turn the amp on and see what happens. If it still doesn't blow the fuse then I would take out that fuse and replace the other and then see what happens. This will tell us whether the fault is on one side of the amp or not.

Depending on what happens with this testing determines where to go next.

That's what I would do as a first stage.

It would be wrong of me not to point out that there are hazardous voltages in the amp, especially with the part of it we are talking about (the on-board fuses are 3.15 amp @250V) so take great care and make sure that before each test the amp is unplugged from the mains supply prior to making changes.

Obviously, if you are not comfortable with this then get the amp checked out by someone competent.

Hope this helps

Regards

Steve
 
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Anonymous

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Hi everyone, thanks for all the replies! :)

To answer eggontoast firstly - Lancs, and yes I do have a multimeter

To Steve (stevieg330) - thanks for the detailed instructions! :) I did everything you asked and used my last 3A fuse to test it (must go out and get some more) and it blew the plug fuse. I continuity tested the two internal fuses to be 100% sure they were okay when I removed them, and they were both fine. So from your post, it may be indicative of a main transformer problem (presumably the same as the PSU problem that Overdose suggested)

So I guess the question now is: a) can I test the PSU to be 100% certain or is it best left to an expert; and/or b) is it replaceable (and if so, a DIY repair or a repair shop repair)?

Many thanks for all the help, really appreciate it. Am really missing my music right now :-(
 

stevieg330

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Hi Andrew,

so are you saying that you removed both fuses, turned the amp on and it again blew the mains fuse. If so, interesting. If the diagram I have is correct, with the fuses removed there is effectively nothing on the output of the mains transformer so it does suggest that it is faulty.

Now comes the fun part, trying to track down a replacement as I dont currently have a parts list. I'll do some digging around and see if I can find one. The schematic I have doesn't show a part no. or give values unfortunately.

Cheers

Steve
 

stevieg330

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Hi Andrew,

As luck would have it, I found a picture of the internals of a NAD 310 and it looks as though the transformer is a toroidal. On the one in the pic there was a sticker which seemed to have details such as a part number and input voltage. If yours is the same then maybe we can figure out what the spec of the tranny is. Then we can get a price for one and see if it's worth repairing the amp.

So could you get all the details from the sticker on the transformer (assuming it has one) and post them on here

Have you got a soldering iron? From the look of it thats what you are going to need (at least) if you want to attempt to repair it.

I have no idea how much one of these is worth so don't know how much you would want to spend to fix it.

Cheers

Steve
 
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stevieg330 said:
Hi Andrew,

so are you saying that you removed both fuses, turned the amp on and it again blew the mains fuse.

Hi Steve,

That's exactly right. It blew the plug fuse without sending power any further than the power switch and Toroid transformer. I did take the info off the transformer while I had the top off - it reads as follows:

115/230V, 50/60Hz

29-2152

048-00301-001

TOROID International Limited

AA-62133 SL 9527

Don't know if that is of any help, but in my experience, the more info offered at the outset, the more helpful it is.

I've also emailed a company called Nuvotem Talema (http://www.talema.net/) and Noratel (www.noratel.co.uk) with that information in case they are able to shed any light on the part number and price/availability etc. Incidentally, apparently the Noratel company used to be Toroid International (Noratel is its new name). If I hear anything back, I'll shout up. :)
 

chebby

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stevieg330 said:
I have no idea how much one of these is worth so don't know how much you would want to spend to fix it.

In excellent (and fully working) conditon and with box + instructions, about £50 - £60. (Completed sales on Ebay UK.)
 
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Anonymous

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chebby said:
In excellent (and fully working) conditon and with box + instructions, about £50 - £60 + postage. (Completed listings on Ebay UK.)

I saw similar - quite pleased that my little old amp is still worth that much considering its age. Always heard/read that it was quite a good performer back in the day (and no reason that one would think that differently now I guess) :)
 

stevieg330

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Hi Andrew,

The main thing we need to know about the transformer is the output voltage of it. The rest we can make educated guesses about and anyway will be down to the physical size of it. I read somewhere that the output voltage is 24V but until we are certain, we can't work out how much the part will be.

I don't suppose it has a VA rating on the transformer or even better tell you what the output is.

In terms of proving that the tranny is the culprit, if it was me, I would disconnect the transformer outputs from the circuit board, make them safe and then turn on the power. If the fuse blows then it really can't be anything else.

Based on what Chebby found about how much these things go for, I would have thought that it would be worth repairing provided we can get a tranny at a reasonable price.

Regards

Steve
 
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stevieg330 said:
Hi Andrew,

The main thing we need to know about the transformer is the output voltage of it. The rest we can make educated guesses about and anyway will be down to the physical size of it. I read somewhere that the output voltage is 24V but until we are certain, we can't work out how much the part will be.

I don't suppose it has a VA rating on the transformer or even better tell you what the output is.

That's the one thing it doesn't say on it! I've had an email back from Toroid though and they are looking into it and will let me know as soon as, so hopefully that will turn up a bit more info :)

Thanks again for all the help - let's hope we get this thing sorted!
 

stevieg330

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Hi Andrew,

I managed to get hold of a service manual for your amp but it doesn't give the rating of the transformer (typical). However it gives the voltage out of each bridge rectifier as 54.3v so I reckon the actual voltage of the tranny could be 55-0-55 (55v less .7 voltage drop across rectifier) but I'm not sure of the VA rating. It can't be that high though as the rectifiers are only rated at 2 amp each.

As an example I could get a 160VA 55-0-55 toroid for around £22 +vat +carriage. Thats just a quick look, may be able to do better . But you are in this sort of price range I guess depending on the VA rating.

Hopefully they wil come back with the specifics for the transformer but that's my take on what the figures are from the auick look I've had today.

Cheers

Steve
 

eggontoast

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stevieg330 said:
I managed to get hold of a service manual for your amp but it doesn't give the rating of the transformer (typical). However it gives the voltage out of each bridge rectifier as 54.3v so I reckon the actual voltage of the tranny could be 55-0-55 (55v less .7 voltage drop across rectifier)
This is not correct, 55VAC into a full wave bridge rectifier does not give 54.4VDC.

The transformer is more likely to be 30V - 0 - 30V something like this Transformer if it fits, but wait for the response from toriod.
 
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Anonymous

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Right folks, I've had a reply from Noratel (Toroid as was) and it reads like one of those 'boo, hooray' stories.

It's a very old part that is out of stock (boo!) but they can get the factory to manufacture one for £35 (hooray!). But as the factory is in Sri Lanka, the carriage charge would be £41 (BOO!). So I guess I'm back to looking for an alternative.

That said, the very helpful chap that emailed me included a schematic of the transformer so I could try to source on locally (let's call within the UK local!) - it's as follows:

image001.png
 

stevieg330

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Hi Andrew,

Yes I have to admit that I wasn't too sure about the calculation on output voltage but anyway we now know what we need.

Can you measure the physical size of the transformer in your amp. Believe it or not I have a 40-0-40 volt toroidal transformer here (been up in the loft for years!) but I suspect it is a higher current rating than the one in your amp and it may not fit.

This one I have is 140mm x 50mm and may be too big. If it fits then you can have it for the cost of postage.

However I have found a couple on-line that with vat and carriage comes to around £22 or £24 depending on VA rating.

Let me know

Cheers

Steve
 

Mooly

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I skimmed through this thread the other day and you have received some good advice.

If the transformer is faulty, and to be 100% sure I would now go as far as isolating the primary side and making some basic measurements just to be sure it's nothing really really daft like a damaged mains lead with live/neutral short etc.

A replacement transformer needs to be around 35-0, 35-0 with separate isolated windings (most are). I advise 35 volt because the voltage quoted for a transformer is at its nominal VA rating (full load) and at light loading the voltage will rise (the regulation figure). The caps in the Nad appear to be 63 volt rating so a 40 volt winding leaves no safety margin. You have to factor in the mains tolerance too. The transformer is 230 volt primary, the mains could be over 250 at times again increasing the output voltage.

I would advise something like this which has a regulation of 6.4% meaning it will read closer to 37.5 volts giving around 52 volts DC after rectification,

http://cpc.farnell.com/multicomp/mcta250-35/250va-toroidal-2x35v/dp/FF01571?in_merch=Products

Make sure it really is the transformer that's short though. I have never had a primary go short on a toroid... and although there is always a first time... make sure :)
 

stevieg330

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Hi Mooly,

Nice one, thanks for the info. You're right, the caps are rated at 63 volts according to the schematic. I agree that he now needs to prove that the transformer is faulty.

Cheers

Steve
 

Mooly

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Hi stevieg330,

I feel it has to proved conclusively.

It's an amp worth saving though, for it has absolutely unique sonic qualities due to the circuit design used.
 
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Okay, firstly I have to say thanks so much for all the help - you are giving me the confidence to press on and find out what's wrong with the old girl! :)

Further to your comments about making sure that the transformer is actually the problem, I thought I would eliminate the mains lead by continuity testing it from the plug to the switch - that checked out okay. The switch would seem to be working correctly too as borne out by a continuity test on the live circuit across the switch.

Not being too sure how I can test the output/function of the transformer itself (if it keeps blowing fuses, how can I check it's outputting the correct voltage etc?), but thought I'd have a good poke around the wires coming out of the transformer and into the small circuit board alongside.

I've got a healthy respect for electricity (and capacitors - my dad used to tell me that the capacitors in the back of a telly can kill you, hence the respect!), so I carefully poked around a bit further on the output wiring from the transformer. Lo and behold, what did I find nestled down in the depths but the following:

P1010766.jpg


P1010762.jpg


P1010760.jpg


Looks like the wire is completely burnt out - to the extent where there is no connection/continuity between the melted part of the wire and the connection to the circuit board. (Bit dusty in there too, isn't it? Makes me feel a bit embarrassed...)

So what do you reckon - symptom or cause?
 

eggontoast

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Looks like you have just found the fault as it is shorting out the secondary windings.

Rewire it an fire it up I think you may be OK the TX should have survived OK.

There shouldn't be any voltage on them caps now but you can do a quick check with your DVM to be certain, just measure between GND and the + terminal on the bridge.
 

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