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Old 24th March 2012, 02:34 PM   #1
p10ness is offline p10ness  United Kingdom
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Smile NAD 7225PE

Hi Guys

I have a NAD 7225 on my bench for repairs, and have been having a look around all of the useful contributions on this site about the amp.
It was not working and showed the signs of burn out with R453 smoked,R431 and 447 had been hot.
So, being a new boy with amps (I'm an engineer although I'm an amp Virgin!)..I followed the advice:
415 and Q417 are the outputs, Q411 and Q413 are the drivers, Q409 is the biasing transistor, also called a Vbe multiplier. (wikipedia has a good explanation.)

Basically you need to isolate the emitters of Q415 and Q417 and insert a 0.22 or 0.33 ohm 3W resistor between the emitters and the common connection point that goes to E-401 which is an over current protection device. Also insert 4.7 ohm resistors between the drivers and the bases of Q415 and Q417.


As I said before, you'll probably find that the drivers and bias transistors are also blown, and R453 (the 180 ohm between the emitters of the drivers) gets really hot when that happens so check it.

2N2955/3055 and BD139 are easy to find, 2SB669A and 2SB649A will be harder, I found them at B&D Enterprises [/I] B&D Enterprises - electronics, projection lamps for hitachi, sony, mitsubishi, panasonic, samsung and they have the other parts as well. Buy a set for the other channel as well and do the same resistor modifications. Don't forget mica insulators for the transistors and some thermal grease.

Changed the lot. Powered up via a 60watt tungstem lamp and could hear a mains-hum from both channels.
Then I applied the mains directly and in a few seconds R 431 1/2 watt 1k8 started o smoke, so I switched off.....
This amp is really for me to play with and so it dosen't matter if I can't fix it or not but it would be rewarding to do so.!
Any advice for more experienced hobbists would be gratefully received. thanks very much.

Paul

Last edited by Pano; 24th March 2012 at 02:58 PM. Reason: fomatting
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Old 24th March 2012, 03:32 PM   #2
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The service manual for this unit can be obtained here.

Have you done a basic diode test on the remaining original transistors? That doesn't show all kinds of defects but hard shorts or opens are detected quite reliably without having to unsolder anything.

Otherwise you'll have some poking around to do with the bulb tester back in place. The schematic gives typical voltages; note that some of them are dependent on mains voltage and thus have to be interpreted relative to whatever the supply rails are.

If R431 gets hot, the amplifier is running open loop. Basically a defect in any single transistor can cause this.
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Old 24th March 2012, 04:08 PM   #3
p10ness is offline p10ness  United Kingdom
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Thanks for the reply. I have the tech manual, but really lack experience with interpretation!

Would you please clarify the term open loop for me?

I have had a poke around and have not discovered any shorted transistors but obviously I'm missing something.
Paul
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Old 27th March 2012, 09:44 PM   #4
p10ness is offline p10ness  United Kingdom
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Guys
R437 between q405 and q407 smokes even running through a 60 watt lamp, in just a few seconds.
R447 and 449 have also been hot in the past and electro C427 was also blown to bits,now replaced.
I can't find anything shorted and the resistance values seem ok....
If anyone would like to comment I would be grateful.
regards
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Old 31st March 2012, 04:59 PM   #5
p10ness is offline p10ness  United Kingdom
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Could really do with some input with this problem if anyone is out there knows about this amp please;

Symptoms now are the same as before in spite of a lot of poking around. I can't find any obvious shorted or faulty semicondutors. I've changed several caps that I thought may be suspicious but the thing just sits there taking the mick...

R437 68 ohm smokes in a few seconds after turn on (via a 60 watt lamp.) R447 1k2 1/2 watt is very discoloured,it must be getting hot and R431 1k8 also 1/2 watt is a little discoloured but not so much.
Thanks to anyone that may be able to help, please be gentle I've never worked on an amp before
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Old 31st March 2012, 06:23 PM   #6
jaycee is offline jaycee  United Kingdom
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Looking at the service manual the amp part is similar to the old NAD 3020 design. They used 2955/3055's that were hometaxial instead of epitaxial, and as such they omitted the emitter resistors from the design.

If you install modern 2955/3055 devices, you need to add at least 0.33R 2W emitter resistors into the design or the amp will go into thermal runaway.

R453 smoking means both driver transistors have turned on hard. I'd check all of the transistors at this point, including the small ones like Q401, Q403, Q405.

You may find Fairchild KSA1220/KSC2690 easier to get than the 649/669 devices.
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Old 31st March 2012, 09:36 PM   #7
p10ness is offline p10ness  United Kingdom
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Thanks Jaycee
I have replaced the 2955 and 3055 on both sides and included 2 watt emitter resistors as you suggest too. Also did this Basically you need to isolate the emitters of Q415 and Q417 and insert a 0.22 or 0.33 ohm 3W resistor between the emitters and the common connection point that goes to E-401 which is an over current protection device. Also insert 4.7 ohm resistors between the drivers and the bases of Q415 and Q417.Took out Q401, Q403, Q405.etc and on a meter they seem to be ok? Replaced the drivers and bias as well..There must be something staring me in the face that I can't see for the trees!
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Old 31st March 2012, 10:03 PM   #8
p10ness is offline p10ness  United Kingdom
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68 ohms R 437 is the smoker..
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Old 31st March 2012, 10:49 PM   #9
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I think that was one of my old posts that you quoted in #1...

going by a redrawn version of the schematic that I did a long time back for playing around in LTSpice (see attached pdf) because I find it much more easy to read than the factory schematic, if R437 is smoking that means Q407 is turned on hard. Possibly because Q407 is bad, or Q405 is bad, or there's a short somewhere. I'd start by pulling Q407 and checking from there.
Attached Files
File Type: pdf leftAmp.pdf (20.4 KB, 103 views)
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Old 1st April 2012, 01:14 AM   #10
sesebe is offline sesebe  Romania
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Hi,

Check Q401 and maybe Q405.

In schematic there are some points to check the voltage. Do you manage to measure this voltages?

Last edited by sesebe; 1st April 2012 at 01:16 AM.
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