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Old 11th March 2007, 03:51 AM   #1
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Default nad 3020 fried resistors

Hi, All! i need your help!!!
just picked up the 3020 but ,unfortunately power amp section is not working (preamp works fine) there is tiny sound , more like noise coming out of speakers but, no full power
inside i noticed two fried resistors they are numbered as 643 and 644

any ideas what could cause this problem?


thanks!!!
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Old 11th March 2007, 05:14 AM   #2
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R643,644 are 68 ohm resistors on the prediver stage. I can pretty much guarantee that the transistors (Q607,608) associated with these resistors are blown. Also your output transistors are probably fried along with Q611,612,613,614. Possibly other damage as well. When these output stages go, they really go.
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Old 11th March 2007, 03:03 PM   #3
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thanks, Dan!
do you think i need to replace all the transistors you mentioned at the same time , or i can do it step by step trying to replace (Q607,608) first
im new at this and not equipped to check the parts if they work

it looks like the project for me but, i want to restore the classic
the preamplifier section of the 3020 sounds actually better than my separate nad1130

vadim
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Old 11th March 2007, 05:08 PM   #4
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Since you need to get to the bottom of the board anyway, and for the low cost of the parts, I would replace all the transistors from Q607/608 back to the outputs. If any one is burnt, all your work will be for nothing.
Testing transistors is quite easy if your meter has a diode check. Do a google search for transistor testing. You will find plenty of info.
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Old 15th March 2007, 11:50 PM   #5
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just replaced the transistors, no changes ,the same problem
is there anything else that could be the cause?

vadim
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Old 29th October 2007, 05:08 PM   #6
timbert is offline timbert  United States
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I also need help as I am having the same problem in one channel of a 7225 receiver which apparently uses a similar circuit. I tested or replaced all the transistors in both channels and used the newer outputs recommended elsewhere in place of the 2955 and 3055, and now the left channel keeps smoking that same 68 1/2w resistor at R643 when powered with a 60 watt bulb on the AC line. I can set the right channel properly to 0mv DC at the right speaker terminals, but the left reads about 29 volts with the blown open resistor.

I also replaced the 1000mf cap at C615 in the above schematic which was reading 150mf. I tested every resistor in both channels in circuit and they're all ok, but I did have to replace the 180R at C651.

Any ideas? Perhaps I put in a transistor backwards in that channel?
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Old 29th October 2007, 07:44 PM   #7
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There is probably a shorted diode (there are several to check) or an open resistor. I've found with these NAD amps, you have to pretty much check every part around the drivers and output transistors. Did you check the bias transistor? It mounts to heat sink as well.
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Old 4th November 2007, 09:29 PM   #8
timbert is offline timbert  United States
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Looks like I'm in good shape on the power amp side on this 7225, but the power supply section is giving me trouble. I've had a few of these units (7220 and 701) and the transistors run hot there, and now I've got some 1w and 2w resistors in the power supply that have smoked as well after I fixed the power amp section. I've checked all the resistors in the power supply and found that the values don't always match those in a schematic I have for the 701, which appears identical in most other ways. For example, two measure correctly as far as the markings on the resistors themselves, but where there are 1K2 parts on the schematic of a similar unit (a 701) there are 2K2 parts installed.

I am certain someone has done or at least tried repairs on this unit, especially in this part of the circuit as some of the caps obviously have been replaced.

I have compared the schematic of the NAD 3220 and the 701 to this 7225 and I was assuming they are the same in the power supply section. Is that my mistake?

I have the schematic of the 701, 3020, 3220, but not the actual one for this 7225. Can anyone help me with that?
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Old 5th November 2007, 02:40 AM   #9
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3020 service manual

http://p10hifi.net/planet10/manuals/...viceManual.pdf

dave
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Old 5th November 2007, 03:21 AM   #10
KP11520 is offline KP11520  United States
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Hello quiet fish,

If you only want the preamp, fix this , sell it and buy a 1020 and use any extra for a nice cartridge. The Phono stage in these is nice for what a used one costs. $75 to 150 on eBay.

I believe the 1020 is the preamp in the 3020. Maybe I am wrong and someone can correct me! Either way, you didn't do bad!

Regards//Keith
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