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NAD 402 New traffo compatible with power circuit?
NAD 402 New traffo compatible with power circuit?
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Old 1st February 2010, 12:34 PM   #1
berthej is offline berthej  Italy
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Location: The Alps
Default NAD 402 New traffo compatible with power circuit?

The mains transformer in my NAD 402 tuners died, and I would like to replace it. It is a standard IE48 type, but with a non-standard value of 2x23volts on secondaries.

I thought I could replace it with a 2x24volt one. But while most of the NADís power circuits are regulated by 7805 and 7812 type regulators, there is one power circuit that outputs 32v. The schematic is here below. The circuit is feed 23VAC, which is rectified by D11 and D12. Therefore, I believe, 23VAC translates into 32,5VDC, which is then regulated by the zener and transistor to 32VDC.

But what happens if I apply 24VAC, which is 34VDC after rectifying. Can the following regulators handle that, or will they be overloaded.

I greatly appreciate any input on this

berthej
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Old 3rd February 2010, 05:01 PM   #2
tvrgeek is offline tvrgeek  United States
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Considering the variance in power line input, you are probably OK. If you were really worried, just add another diode to the rectifier in line to take some of the drop.
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Old 3rd February 2010, 05:45 PM   #3
cliffforrest is offline cliffforrest  United Kingdom
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That series cap, parallel diode is a voltage doubler, to get enough voltage headroom for the cap-multiplier regulator to work. Presumably it is a low current supply (Tuning diodes ?? ;-)

The downside is that it has a high impedance and poor regulation, ie the voltage ( at Q17-C) will lower as current is drawn.

Given all that, I cannot see any problem at all with using a higher voltage. Measure across C58 when it is working. Make sure it is not wildly above 50V !
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Old 9th February 2010, 12:02 PM   #4
berthej is offline berthej  Italy
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Dear tvrgeek and Cliff

Thank you for your input. I will have a go at it, and hopefully not experience any smoke

The old trafo doesn't show the VA rating, but being a IE48-type, these normally have ratings between 8-12VA. Will try a 12VA one.

Thanks again

berthej
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Old 20th January 2012, 11:16 AM   #5
pdul is offline pdul  Denmark
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I know this is an old thread, but anyway here I go!
The trafo in Nad 402 gives out 16-0-16 vac, Which becomes 21,xx vdc in each rail..
So installing a 23-0-23v trafo will shurely give you much more action than you bargain for, First the smoothening caps rated 25v will pop with a bang.
Etc. etc
Just my ten cents.
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Old 20th January 2012, 12:08 PM   #6
pdul is offline pdul  Denmark
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I just salvaged one for myself from the dump. The only thing wrong was the bulb for the display light. I replaced it with four white Leds, And Voila, It works Then I replaced all the electrolytic caps with FMs and FCs and Silmics
in the audio signal line.
It is by the way the most simple and at the same time the most sweet sounding FM tuner I ever had
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Old 8th April 2012, 08:43 AM   #7
mjf is online now mjf  Austria
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had similar problems with my nad 402.
first the 12v bulb behind the display burnt through -now there are 4 yellow leds with a res in series- and after some time the transformer died. i built in a new one with 2x12Vac (the voltregs in my nad are 7812,7912,7805).
indeed it is a puristic good sounding fm tuner........
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Old 10th February 2013, 03:10 AM   #8
Commited is offline Commited  Ireland
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Hey guys,

Sorry to resurrect an old thread but unfortunately my NAD 402 has just died too I'm getting no voltage output from the traffo, and the positive and negative outputs are showing continuity so from my very limited knowledge of power supplies I reckon it's dead.

As I said, I have no knowledge on power supplies, one part of DIY audio I've never wanted to get involved with. Alas, I've owned the 402 for 12 years now and it's in mint condition so I have no intention of parting with it as it's fantastic so I need to fix it.

Stupid question time:
- Is it 2 x 23vac or 2 x 16vac? The top of my traffo has "T48-0023"?
- What power rating do I need? (power rating on the back states 15W so would 16VA be sufficient?)
- I'd like to keep it as factory as possible - are there any off the shelf replacements?
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Old 10th February 2013, 09:20 AM   #9
pdul is offline pdul  Denmark
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2*16vac and 16va is ok
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Old 10th February 2013, 10:36 AM   #10
Commited is offline Commited  Ireland
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Thanks for the confirmation. Would I get away with less than 16va?

Would this perhaps be suitable:
Buy PCB Transformers Transformer,PCB mount,flame retardant,thermal fuse,20VA pri,230Vac,50/60Hz,2x15V sec RS 10-6002 online from RS for next day delivery.

Hard to find 16vac!
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