NAD 3020e fried R451 R437 resistor

[optilab]

Hello friends,
I have a problem with my NAD 3020e.
The resistor marked r451 and r437 are fried.
The fuse are still in good condition.
I suspect Q409 or Q407 ?

What do you think ?
Also do you think it is a good thing to change all the capacitors to start a new life with this amplifier?
Thank you for any help.
Pablo

[IMGDEAD]http://www.pdnb.fr/img/diagram3020e.jpg[/IMGDEAD][IMGDEAD]http://www.pdnb.fr/img/image3020e.jpg[/IMGDEAD]

 

[Ian Finch]

Hi Optilab and welcome, if this is your first post.
Why not check the voltages marked on the schematic in the area from Q405 to the right side first?
You can mark up the schematic or type as text with a clear indication of the point referred to.
That will help others understand what is wrong much easier.

 

[gmphadte]

First of all, do not mix repairs with a mod. Repair it first.

You should check the power transistors and the drivers Q 411,413,415,417.

Gajanan Phadte

 

[optilab]

Hello guys,
thank you for helping.
I can power up the amp to check voltage or should I desolder the transistor and check them with my multimeter?
I heard about putting a light bulb between the amp and the socket?
Anyone could tell me more?
Thank you.
Regards.

 

[Mooly]

A bulb is a great safety measure to avoid mishaps. Use a 100watt filament bulb.

Both voltage checks and static checks should be used to try and determine what has failed. The two driver transistors (the 2SB649 and 2SD669 would be prime suspects for failing intermitently and could be the source of the original failure).

Its always nice to find the fault but sometimes its more cost effective and certain to replace any suspect parts rather then find the amp appears fixed only for the same problem to surface again later. The cost of replacement is minimal and there are more up to date and easily available replacements such as the MJE340 and MJE350 for the drivers.

Good to learn though... get it powered up and get measuring

 

[Mooly]

Nearly forgot... for initial working on and testing of the amplifier you can apply a short (solder a piece of wire) between collector and emitter of the vbe multiplier (is it Q409 ? can't make it out clearly... the one in the middle with the square around it). The amp will work and function normally like this (apart from slight crossover distortion ) and it will force the output stage to run at zero bias current (assuming no other problems exist).

And don't connect any speakers to the amp until its fixed. A DC offset could destroy them.

 

[jaycee]

If you replace the transistors, you will need to make a modification. The original parts work OK without emitter resistors, and so they are not in the design. However, modern parts will not, and you will need to add 0.22R emitter resistors for stability.

I'd say the damage has been caused by failure of at least one output device, and it's driver transistor