Help NAD 7225PE (3020) receiver output transistors replacement

[flavviuss]

Hello,

hope that there is is still someone watching this thread. So let me tell you my Nad 701 story:
I bought it from the free market a few weeks ago and everything went quite fine until i wanned to try it on some 8 Ohm speakers and moved the switch from behind to 8 ohms. After a couple of seconds one channel was lower in volume and finally blown up.
Took the Nad to some electrician and he replaced all the blown transistors including the 180 resistor. After 20 seconds of playing, the channel blown up again.
Than i found this excellent thread and decided to do the patch myself. So i replaced all the stuff again and made the patch on the channel. This was done by switching the impedance to 8 ohms as described in the service manual...
After replacing i started the Amp again and this time, after couple of seconds a transistor blown on the other channel. Than i decided to make the patch also on the other channel after removing the bad transistor. This time i also made the voltage adjustments by removing the solder short as described in the manual. Every thing went fine but from accident i was doing the settings on 4 ohm and decided to make the calibration on 8 ohm as in the manual.
After switching to 8 ohms in the back, right after start i measure correct voltage on the test points on one channel and 48 Volts!! on a test point in the other channel....
This amp is totally messed up but now it became my personal war that i want to win. Does it haHello,

hope that there is is still someone watching this thread. So let me tell you my Nad 701 story:
I bought it from the free market a few weeks ago and everything went quite fine until i wanned to try it on some 8 Ohm speakers and moved the switch from behind to 8 ohms. After a couple of seconds one channel was lower in volume and finally blown up.
Took the Nad to some electrician and he replaced all the blown transistors including the 180 resistor. After 20 seconds of playing, the channel blown up again.
Than i found this excellent thread and decided to do the patch myself. So i replaced all the stuff again and made the patch on the channel. This was done by switching the impedance to 8 ohms as described in the service manual...
After replacing i started the Amp again and this time, after couple of seconds a transistor blown on the other channel. Than i decided to make the patch also on the other channel after removing the bad transistor. This time i also made the voltage adjustments by removing the solder short as described in the manual. Every thing went fine but from accident i was doing the settings on 4 ohm and decided to make the calibration on 8 ohm as in the manual.
After switching to 8 ohms in the back, right after start i measure correct voltage on the test points on one channel and 48 Volts!! on a test point in the other channel....
This amp is totally messed up but now it became my personal war that i want to win. Does it have anything to do with the impedance changing? Now i measured all the transistors again and they seem fine. What else can be broken now?
So as a summary: i am with the soldering short disconnected near the test points and i measure 26 mv on one part and 48V! on the other part

I need to win this war!! any help will be apreciated.

Thankd

 

[djk]

Start a new thread please.

 

[sirius1967]

701 mod

Here's the schematic showing the modifications. (assuming you use Windows, remove the .txt extension so the file name is nadAmp.wmf, then double click.)

The over current protection and output network are not shown for simplicity.

You will be adding R2, R3, R4, R5 to each channel.

hi do i have to add the resistors to 3055 and mj transistors ? thanks

 

[Regular Steve]

Newbie

Hello,

I am a newbie to all of this. I bought a NAD 7225PE and have no output as well. I have NEVER fixed anything electronic. Does somebody have a step by step for this process, and a list of components and tools that I will require? (Short of the Phillips screwdriver I need to open up the case.)

Thank you in advance!

 

[Ian Finch]

As said earlier, this is a nasty design to tackle and I would not attempt anything mains powered, let alone this model power amplifier, as your first repair. Be prepared to repeat your attempts and spend quite a bit of cash on more components in the event of mistakes and misunderstandings. Another obstacle is that original parts for the output and driver transistors (those parts that bear the abuse of shorted speakers, leads and overloads) are now unobtanium (see#2) and this means a careful choice of substitutes and equally important, who you buy from.

DIYs flock to Chinese Ebay suppliers since they are they are cheap, easy to buy online and plentiful but the confusion and trouble caused by the poor copies and fake parts is not worth the risk. You have no way of knowing whether the chip inside is even close in performance to the original either. Never buy just because the type number is not available elsewhere. It's not hard to deduce how such obsolete parts are now offered brand new, in high volume and from so many small suppliers in the one region, though.

There are no step-by-step repair instructions because you need to diagnose the fault(s) properly before planning any work and this will vary from one set of repairs to another. There may be common separate faults though, and these are already covered to some extent by the repair descriptions here and in many other repair threads about these 3020 type power amplifier models. Search more widely for NAD 3020 models - the 7225 power amplifier section is similar.

Given that you can locate a professional repairer who is familiar with NAD models, ask for an indication of the repair cost. Quotations can approach the total repair cost because most time is spent in diagnosis and this isn't the way to go about it unless you were already committed. I think though, pro. is the best way to proceed. By all means, learn from some DIY building experience before you try your own repairs because this will aid your understanding of circuits, troubleshooting skill and use of of test equipment greatly but it takes time - probably more than you are are prepared to wait.

 

[northpaw]

I second Ian's great advice and his cautions. But before you do anything further, it may be worth a shot at cleaning the several switches, contacts, and potentiometers. It seems that these components in quite a few of the lower end NAD devices (includes the PE series, of which I have the 7225 and 7240) develop oxidation on contacts over time, especially if not used for a while, which will make the output intermittent or cut it out entirely. Since you have no tools or experience, making measurements to figure out if this is the issue or if it is blown parts isn't really an option for you at present, meaning it might be worth a try cleaning the contacts, which is an easy and cheap fix if it works.

You can get some electronic contact cleaner, such as the CRC brand (Walmart carries it), or DeOxit, and with the unit off and unplugged, start working working the Volume and Balance controls while spraying their pots (these are located inside the unit, right behind the faceplate). The volume pot will be a little hard to get to because a tuner board is located over it, partially blocking access to it, but you should be able to get the red straw in there. Then do the same for the bass and treble controls, and the input selector switches (CD through FM). Just spray those switches (the parts inside the unit) and keep working them back and forth. There is also a 4-8 ohm switch mounted on the back panel, and you should clean it the same way. I'd do a total 2-3 minutes of exercising for each switch/pot, spraying a few short bursts a few times during that. Then let the thing dry overnight, so the cleaner can fully evaporate off.

While you have the top cover off, take a look at the 4 internal fuses located towards the back of the main board. If those are blown, that could be your problem, or your proximal problem, meaning that some underlying problem might need to be resolved before it will work properly (not blow the fuses again). It is also helpful to look around inside for potential problems such as bulging or leaking capacitors (ignore the brownish glue around the bases of the larger ones), or resistors or other components that show heat distress (discolored body or board under them).

Other things to check are that the jumpers between the Pre and Main jacks on the back panel are installed, and that the Tape Monitor switch on the front panel is not engaged. Either of these will give you no output.

For a little more context on some of the cleaning issue, you might search out a recent thread on audiokarma concerning a 7240PE that had no output, but came back to life after a persistent cleaning.