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Old 15th January 2013, 09:10 PM   #1
mcd99uk is offline mcd99uk  United Kingdom
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Default NAD 319 Repair

It's like an addiction. I've found I love repairing amps.

Anyway, I have obtained a blown NAD 319. I have done some checking and found that it has lost its +67V rail. Found that the fusible R203 has gone open circuit. The question is why?

Figure that the best thing to do is check every component. Resistor check and transistor check. So far so good. Part way through checking the amplifier boards and all fusibles and output transistors are ok.

Seems strange that there are so many fusibles. Are they really necessary and do they degrade sound quality? I have doubts over fusibles after my sansui repair.

At some point I think the whole amp needs to be dismantled to get access to the psu pcb. So I really want to find the reason for this failure before powering up after repair.

Does any one know of any inherent weaknesses in this amp?

Some people may think I'm mad spending time on this amp but the basic design appeals to me, for example, big power supply and four pairs of output transistors. And at the end of the day this is DIY audio

I do have the service manual...


Cheers

Paul
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Old 15th January 2013, 09:39 PM   #2
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Sometimes they just wear out with heat and age, make sure it gets replaced with like for like, and use a dim bulb tester for first time switch on.

Chris
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Old 15th January 2013, 09:55 PM   #3
mcd99uk is offline mcd99uk  United Kingdom
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There is definite evidence of heat on the pcb and the track has lifted where this component terminates. It's electrically sound but not physically.

When replacing the fusibles is the power rating essential to match as 1W 100R fusibles don't seem to be readily available from RS or farnell? 2W 100R are available.
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Old 15th January 2013, 10:17 PM   #4
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The caps in the power of theese amps are of a peculiar type.
Seen relatively many of them blown. Very near their max voltage often, and 85-specified.
Not good enough in often a bit crowded amps.
Also they have a tendency to blow the "whole amp" when an output transistor breaks down.
If You love DIY, this is a good amp to practise it on.
Lots and lots of potential to improve things.

Good luck anyway.
Check and doublecheck every component before you start it up as Reddish75 recommends. Do you have a variac? Continously variable transformer?
VERY useful when starting up amps after repair. Will make it possible to se breakdowns even before they happen.
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Sooner or later you end up with TANDBERG
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Old 15th January 2013, 10:27 PM   #5
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by TANDBERGEREN View Post
The caps in the power of theese amps are of a peculiar type.
Seen relatively many of them blown. Very near their max voltage often, and 85-specified.
Not good enough in often a bit crowded amps.
Also they have a tendency to blow the "whole amp" when an output transistor breaks down.
If You love DIY, this is a good amp to practise it on.
Lots and lots of potential to improve things.

Good luck anyway.
Check and doublecheck every component before you start it up as Reddish75 recommends. Do you have a variac? Continously variable transformer?
VERY useful when starting up amps after repair. Will make it possible to se breakdowns even before they happen.

Sounds interesting about the caps blowing. There isn't any visible evidence of blown caps. As I have never experienced blown electrolytics, do they go short circuit or open circuit?

I don't think there is too much damage to this amp as the fuses don't blow and the output transistors look to be ok.

As for improvements to this amp. This was the idea when I bought it. The basic design looked good to my relatively inexperienced eyes. What sort of things can be done? I have read in many places that the treble is lacking.

Should be able to borrow a variac from work one weekend.
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Old 18th January 2013, 07:20 PM   #6
mcd99uk is offline mcd99uk  United Kingdom
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A few quick questions. I have been closely inspecting the caps on the PSU board. Have removed one (C212) it is 160V 220uF 85 degree. I have found a small brown spot on the top along with some dark brown crusty residue underneath. This residue is also present on the side of the cap. Is this likely to be adhesive or electrolyte?

How would I know if the cap is damaged or not? I have applied 130V DC to it and no excessive current has been drawn from the PSU. Does this result mean anything at all?

All I can really make out from research is that whatever this residue is, it is a sign of over heating. Is this true?

Thank you for your time.

Paul
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Old 19th January 2013, 01:21 PM   #7
RS232 is offline RS232  United Kingdom
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If in doubt just replace them 220uF x 200V are cheap enough.
You say that the fuse is/was blown, have you checked the output devices ?
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Old 19th January 2013, 03:24 PM   #8
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The brown stuff is most likely glue that was used to hold the capacitors down.

The capacitors in Nads of this vintage are usually poor quality, if i was to replace them i would be using the panasonic fc/fm/fr range and for the big powersupply caps the panasonic tsha range.

A good tool to have to check the health of capacitors is the Peak Esr meter.

Chris
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