Salvaged transformer Measurments help!

serj.t.s

Member
2007-07-09 3:28 pm
I had an old NAD audio amp in the garage and since i don't have ANY amp, and don't have the money to buy one at the moment, I decided to try and fix it. Well that went wrong.. now I gave up and decided to use it's inners by creating an amp from scratch. I'm a newb to this i have to admit, but i have some electronics background.

The first step I decided to take was to remove the transformer and start building a power supply.
My first step also came with a first problem, how to figure out where to connect this correctly!

I'm afraid of damaging it, but I measured the resistance between the various outputs, and the red and black seem to be the main winding since they have the higher resistance between them (14.6Ohm). But is it safe to connect this to main power line and measure the voltage of the other outputs without a load?

Thanks
 

Attachments

  • dsc00580_001.jpg
    dsc00580_001.jpg
    37.2 KB · Views: 313
Serj,

I would put the xfmr back into the amp it came out of, and re-connect all the primary-side wires to their original locations, and then conduct a no-load test with the unit plugged in. From there, you can determine the desired voltage(s) and current(s) available.

What was the original output power rating per channel? This will give you a good idea what power level the transformer is good for. Be advised, however, that a common NAD practice is to over-design their power supplies for plenty of headroom, so don't let high voltage readings mis-lead you.

Also, look at the values of the power supply-related components (i.e. caps, diodes, coils, etc.) to get an idea of what you'll need.

Steve
 

serj.t.s

Member
2007-07-09 3:28 pm
N-Channel said:
Serj,

I would put the xfmr back into the amp it came out of, and re-connect all the primary-side wires to their original locations, and then conduct a no-load test with the unit plugged in. From there, you can determine the desired voltage(s) and current(s) available.

What was the original output power rating per channel? This will give you a good idea what power level the transformer is good for. Be advised, however, that a common NAD practice is to over-design their power supplies for plenty of headroom, so don't let high voltage readings mis-lead you.

Also, look at the values of the power supply-related components (i.e. caps, diodes, coils, etc.) to get an idea of what you'll need.

Steve

I've managed to get the service manual, it says the amplifier did minimum 40W [email protected] and max 160W dynamic power 8 Ohm also. I also browsed threw the schematics and is seems the transformer has 3 separate center tapped windings ( :xeye: ).
I'm thinking if using this power supply in a chipamp featured here in the forums, but really haven't investigated much about it.

This amp had 2x6800uF 80V and 2x6800uF 50V ELNA capacitors, do you think I can still use these, or are they probably affected by their age? They seem to be in good condition.
 
The Elnas are probably still good. Elna is a pretty well respected name in Japan, but not so common over here among the DIY crowd like Panasonic is. They mostly do OEM stuff.

Three secondaries. Two power the amps in a tiered arrangement: +/-V(hi) & +/-V(low), to minimize heat generated by the power amp section for varying power levels. I believe this is called either Class G, H or T, not too sure of the terminology. The third is probably for the pre-amp, tuner, phono pre-amp and other low-level sections, providing, most likely, +/-15V.

BTW, What model NAD is this? I have a 7225PE Receiver (25WPC, 80W Peak), 1155 Pre-Amp, and a 2200PE Amp (100Wx 2 into 8W, 200W x 2 into 4W), and 750W bridged into 8W so I have a little experience with their stuff.
 
The 3240PE is the Pre-Amp/Amp version of the 7240PE, minus the tuner section, which is essentially the 40W version of my 7225PE. PE, BTW stands for Power Envelope. A piece of NAD tech jargon. The 3240PE is piece of equipment! Don't let it get away. I'm suprised you don't want to fix the existing amp circuit. With their rather high headroom levels (~6.6dB at rated output), and high S/N ratio (~114db), and low THD ratings, you might want to take another look things.

Not to dampen the DIY spirit here, but if this can be fixed, why not? On the other hand, if the amp section is so FUBAR'ed, then DIYing a new section, especially a chip-amp, might be worth taking a look at.
 

serj.t.s

Member
2007-07-09 3:28 pm
Thanks for you input N-Channel!

I have all the other parts as well, that is, tuner, cd player and cassette player, but the cd player(haven't even opened it yet) and amp don't work.

This already happened before to the amp, my father sent it to be fixed and it cost about 100€, seeing it now he was certainly ripped off (a costume here in Portugal:mad: ) but at that time I didn't know even what a transistor was.

I've already seen the 3240PE on ebay for about 100Dollars so I thought I might as well use what I could from the NAD to save some money and at the same time learn a thing or two.

But by taking your advice I'll keep it in the garage, when the summer holiday comes I'll try and fix it.

Meanwhile I'll build a chipamp from scratch:D

Once again, thanks for the expert advice N-Channel!

EDIT: I now remember that when i tried to fix it I couldn't get equivalent spare parts, and the result was smoke=P. But I'll keep it stored until further more.