need advice, repairing NAD C372

[silversweden]

Hello
I am repairing a nad c372 stereo amp.
The amp turns on but output relays dont click.

I noticed that the huge transformer gets very hot after a couple
of minutes and i decided to measure how many amps its drawing
at the primary side. (can barely touch it, same with/without load).

I removed the fuse and put my fluke meter here measuring amps.
When i switch the stereo on the current draw go as high as 3 amps
and after 1 minute there is "only" 2,5 amps drawn from the primary.

The readings are the same with / without any load connected to the
secondary side.

Does this mean that the transformer is shorted on primary/secondary side?
There is no short between primary and secondary side when i mesured it.

The fuse rating is 4A slow type. Need advice!

Service manual here: http://www.itxpress.se/c372.pdf

[nigelwright7557]

If the transformer is getting very hot with no load then it is broken.

You would need to know the original impedances to test the transformer.

You might get a clue from the secondary ac voltages which winding has gone.

[silversweden]

I can hold my hand on it but i dont want to do that for a long time.
Another thing i noticed is that after 2-3 minutes i get NO power from it.

I can se on the schematic that there is an autoreset fuse in it.
So probably it trips due to heat.

This is not a good sign because i bought the amp hoping to repair
it quite cheap. And the transformer is the most expensive part of them all.

Need to buy it from NAD i guess, will be hard to find a replacement

Someone with an idea what to do?

[nigelwright7557]

Originally Posted by silversweden

This is not a good sign because i bought the amp hoping to repair
it quite cheap. And the transformer is the most expensive part of them all.

Someone with an idea what to do?

It could be something else has caused the transformer to die.
So it might not just be a new transformer you need.

Its worth checking bridge rectifier and power supply capoacitors for a short.
I woudl also check the output transistors.

That would give you an diea if it is worth fixing or not.

[silversweden]

I have already checked the bridge rectifiers and they measure OK. (not expensive)
Supply caps are not shorted if i measure them with my DMM. (worth replacing in case)

Output transistors are not shorted (measured them i circuit).

But why would it die? In some cases i guess that the windings can short without
no reason?

I have voltage out from the transformer because i can measure ~57 volts on the
supply caps until the power goes of. A bit low if i look at the schematic.

Hard to tell from the schematic what voltage i should have from the secondarys.
4 pairs of them...

[nigelwright7557]

Originally Posted by silversweden

But why would it die? In some cases i guess that the windings can short without
no reason?

It could have overheated and melted the resin on transformer wire.

Perhaps just old age has cracked the resin coat and caused it to short out.

It would be useful to borrow a transformer and try it even if the transformer is low on VA, it would at least show the amp part is fine.

[silversweden]

the problem is that as i understand i have 4 different AC voltages out from this transformer.

if i only knew someone with the same amp....but i dont
and buying a working amp is quite expensive for testing purpose :P

is it possible to "open" the transformer and see if it could be repaired?
i know that you will say no

i live in sweden and NAD have parts in denmark.....and i guess since i am
not a business company buing from them they will charge me a big amount of money for buying this stuff

[nigelwright7557]

You could have a go at rewinding the transformer.
Either yourself or get a company to do it for you.

[silversweden]

feels like the only realistic option is to get a new one from NAD.
i dont have the knowledge and dont want to mess with something
like this. things can go wrong. but it is tempting to fix it if possible...

to let a company rewind it would for sure cost the same amounth of
money as a new. i spoke to a authorized NAD repair tech today and he tought somewhere between 1000-2500sek, around 140-340 dollars for a new one.

[silversweden]

This unit is repaired and i have tested it for about 2 hours after
adjusting the voltages, sounds great! :>

The amp is drawing about 0.385 amps with no input / output.
And in standby about 100mA. Are those readings OK or could
the "ON" state be drawing to much current?

I am still nervous about what killed the main transformer.

I have checked the bridge rectifiers with the diode-test and
reads OK. There is 2 KBU1504, and i could just find KBU1004
wich can handle less current. (the schematic says KBU1004)
so i dont know why i find KB1504 on the PCB.

Altough i dont want to change them to less spec. component
if i dont need to. Do i have to worry about the bridges?

The main power caps do not have a leakage if i measure in circuit
with DMM.

And the capacity test checked ok i guess, my fluke only reads up to
10 000uF so really dont know.

Is there anything else that i should check or should i just close the
cover and be satisfied that the amp now is working again? =)

Thanks so far!