Output Transistors for 7250PE

[MinesJA]

Hi Guys,

So I'm relatively new to electronics and have been using a NAD 7250PE receiver I picked up at a thrift shop for $30 as a test project. The the speaker outputs and headphone jacks don't work but the tape out and inputs all work and from hunting around for awhile someone suggested that it's the output stage of the receiver that's not working at that I should test the output transistors to see if any of them have shorted. I've been looking at the schematic and have been comparing it to the receiver itself and am having a hard time figuring out where the output stage of the amp is and which transistors are the output transistors. Can anyone help with this? I've attached the schematics to this post.

Thanks!
Jonathan

[Mooly]

Can't open the link... just attach a jpg of the schematic to your post.

Output transistors will be the ones attached to a heatsink...

[WuYit]

Mooly,
copy and paste the link address into your browser. Hope you have good eyesight.

[Mooly]

Originally Posted by WuYit

Mooly,
copy and paste the link address into your browser. Hope you have good eyesight.

See what you mean

[MinesJA]

Yeah, sorry about that. I have a pdf of the schematic and was trying to upload it to photobucket so I could insert the image but had to take a screenshot of it to get it into a .png which let me upload it but it's horrible quality and really small. If there's a better way of doing it, let me know. B

But in regards to the ouput transistors. As far as I can tell, there is one long heatsink which runs from end to end of the receiver and has 14 transistors attached to it and then there are 8 mini heatsinks which each have 1 transistor attached to them. So that's a total of 22 transistors attached to heatsinks. Is it possible that they are all ouput transistors and that 11 are for the right channel and 11 are for the left channel? When I first started investigating this someone suggested I should check the output transistors and that there would only be 4, 2 for the right and 2 for the left. What do you think?

Thanks!

[MinesJA]

Here's a zoomed image of what I think is the ouput stage of the schamtic. I've been able to find most of these transistors featured here attached to the long heatsink and the individual ones.

[Mooly]

Have you been able to measure any as being short circuit or obviously faulty ?

The circuit shows paralleled outputs Q616,617,618 and 619. The driver transistors are Q614 and Q615. Those four 0.22 ohm resistors should also be checked.

This circuit uses two different voltages for the output stage the idea being that during high output the circuit switches too a higher supply rail. Thats done by Q703 and Q707 and associated circuitry. That could well be faulty to but even if it is the amp should still operate "normally" so concentrate on the power amp first.

Remember to check both channels transistors.

[MinesJA]

Awesome, I'll get right on that when I get home. Thanks so much for the help!! I'll let you know what I find.

[MinesJA]

So I did some measurements and they seemed a bit off which is why I wanted to double check with you. You can't measure transistors while they're still connected to a circuit if you just have a multimeter with a diode setting, can you? If not, this is gonna be a long night....

[Mooly]

If they are short circuit... which is a "usual" failure mode for power devices then they can be measured in circuit as a short is always a short. The same for low value resistors such as the 0.22 ohms. Other than that and they should be removed, or if there is any doubt over a reading.

Does the unit actually power up safely (with no speakers attached) ? as a few easy voltage measurements are much more revealing. It may be something else entirely... blown output stages usually leave a trail of destruction, fuses blown, resistors gone up in smole etc etc.