Nad 2240pe

[Gagarin]

Hi! I am new here at DIYAudio.com, and it looks like a great place to hang out!

As for now, I am struggling hard to bring my NAD 2240PE power amp back to life.
It was dead on arrival (bought used, excellent outer condition), and burns all four fuses immidieatley when turned on.
I downloaded the service manual, and discovered to my big surprise that five (yes 5!) semiconductors in the right channel section was replaced with totally wrong sorts.
Three of them were shorted.

My questions:

1. Is there any way to make sure if its just the faulty semiconductor replacements that blows the fuses?

2. Is there any risk involved in removing the faulty semiconductors and try to power up the amp w/o them?

3. How can you tell if a rectifier is blown/shorted? I have a measuring instrument.


There are so few components in this amp, so I really think that a happy amateur like me should be able to get it up and going!

I uploaded the service manual on my website if anyone feels like taking a closer look:

Nad 2240PE Service Manual

The faulty semicon´s were in locations: Q424, Q426, Q432, Q434 and Q438.

Any help at all would be very appreciated!!

 

[analog_sa]

1. No. You have to check all transistors, starting from outputs and drivers. Being lazy i would just check for shorted pn junctions before unsoldering.
2. No risk. You shouldn't randomly apply full power though. If you have a variac use it, otherwise connect something like light bulbs in series with the power line to limit the current.

3. Draw a bridge rectifier and see which junctions you can measure. Obviously measuring from the AC side it will behave as an open circuit which is usually enough to apply power and see if the PS is ok (Provided you can disconnect the output from the blown amp).

Once you are pretty confident that all faulty semiconductors have been replaced apply power, but again, very carefully. Use either low power 30-100ohm resistors after the PS (it may not be easy) or a variac/light bulb in series with the mains transformer primary.

Once it shows signs of life you'll have to adjust bias and offset

 

[Lou914]

I just bought a 2240PE which I hope to use as the tweeter amp on my Orion speaker system. I'd like to ask Gagarin to post that service manual once again so we can check out the procedure for setting bias and offset on this amp. We never know what time and use has altered or the previous owners have tweaked...

 

[Ronj]

Reviving an old thread. Today while trying re-bias my amplifier, I guess, my filmsy bias pot opened producing magic smoke.

I haven't assessed my damage yet.. But the magic smoke seemed to have come from one of the output transistors for that channel. Later re-powering the Amp through series bulb protector, shows the bulb glowing brightly. Bias also is staying very high and I am unable to control it... So guess Bias pot is indeed permanently open now...

Any idea how deep the damage could be? Other than transistors, what else shall I check? Any specific resistors? When changing transistors should I change for both channels considering what kind of transistors I would get this time??

VR403 is the Bias Adjustment pot. R471 is the 1Ohm resistor, which temporary "unshorted" for bias adjustment

Below is the Bias adjustment process
1) Remove short across R471 and R472.

2) Connect a DC millivolt meter across R471 (R472) (1 ohm output
transistor collector resistor.) and adjust VR403 (VR404) for 26-30mV
reading on the meter. (No loads

3) leave power on for 5 minutes and recheck both offset and bias.

4) DON'T FORGET to re-short resistors 471 and 472.

5) Repeat step 4

6) Repeat step 5

 

[Ronj]

Ok Output transistors are shot. 2SD1047 and 2SB817. Others seem to be okay...

 

[Ronj]

Just wondering, can I replace 2SB817/2SD1047 pair with 2SC3281/2SA1302 pair? The latter looks a beefier alternative and I have hFE Matched pair... I will replace both channels then...
The Datasheets
http://pdf.datasheetcatalog.com/datasheet/sanyo/ds_pdf_e/2SB817.pdf
http://datasheet.octopart.com/2SC3281-Toshiba-datasheet-101323.pdf

 

[Ian Finch]

The substitute transistors should be fine, if genuine, since they were obsolete 15 years ago. Drivers 3423/1361 possibly, may have been taken out with the output devices. Are you certain all the other transistors are good? A DC amplifier is like dominos - when the outputs blow, the load immediately falls back on the previous device and so on, unless a limiting resistance or protection device can prevent that. Check and replace all semis as necessary, before power up with much reduced voltage, or a bulb limiter or resistors in place of fuses as advised earlier in the thread. Clear the problem completely before you try testing at full supply voltages.

DONT just hit it with full power and hope for the best or you'll likely have to go through the whole process again.

 

[Ronj]

Nah.. After the magic smoke. I did not power the amp directly.. I always used bulb-limiter. I was not using one earlier when it blew because, I was bias adjusting... I did check for shorts on other driver devices and did not find any issues. also I did not really powered up much with blown output devices. As the device was on bench I switched off immediately...

Anyway I have 2SC3281/2SA1302 pair in hand, and from Datasheets device power rating hFE, IC etc look almost identical. So is it okay to use 2SC3281/2SA1302 pair instead of 2SB817/2SD1047 pair? If it works, I will change in both.. As I have 2 matched pairs in hand anyway...

 

[Ronj]

The substitute transistors should be fine, if genuine, since they were obsolete 15 years ago. Drivers 3423/1361 possibly, may have been taken out with the output devices. Are you certain all the other transistors are good? A DC amplifier is like dominos - when the outputs blow, the load immediately falls back on the previous device and so on, unless a limiting resistance or protection device can prevent that. Check and replace all semis as necessary, before power up with much reduced voltage, or a bulb limiter or resistors in place of fuses as advised earlier in the thread. Clear the problem completely before you try testing at full supply voltages.

DONT just hit it with full power and hope for the best or you'll likely have to go through the whole process again.

I have original drivers and output transistors salvaged from a Pioneer A400... So they have to be original. I will check again other transistors. On the board, I couldn't find any short. Now I will de-solder and test them off the board... to make sure everything is good.

Any other possible causalities? Like resistors?

 

[Ronj]

The substitute transistors should be fine, if genuine, since they were obsolete 15 years ago. Drivers 3423/1361 possibly, may have been taken out with the output devices. Are you certain all the other transistors are good? A DC amplifier is like dominos - when the outputs blow, the load immediately falls back on the previous device and so on, unless a limiting resistance or protection device can prevent that. Check and replace all semis as necessary, before power up with much reduced voltage, or a bulb limiter or resistors in place of fuses as advised earlier in the thread. Clear the problem completely before you try testing at full supply voltages.

DONT just hit it with full power and hope for the best or you'll likely have to go through the whole process again.

Ok you are right! Two more transistors are short. Q432(2SB826) is one such transistor that is short. And I think the driver behind that also is short... Will check tomorrow.

 

[Ronj]

Hi,
Any idea whether there is a modern equivalent of 2SA1360/2SC3423 pair?

 

[Ronj]

I guess there are no equivalents for 2SA1360/2SC3423 pair.

 

[Ian Finch]

These are the VAS transistors and all types are near extinct since they were HV types also used in old CRT Televisions. One TO126 package type is still available and very good - Fairchild KSA1381/C3503 from the usual authorized component sellers.