Help NAD 7225PE (3020) receiver output transistors replacement - diyAudio
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Old 19th October 2009, 06:16 PM   #1
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Default Help NAD 7225PE (3020) receiver output transistors replacement

Hi Everyone,

I usualy post and consult this very fine forum mainly for tube matters, but I recently was given a NAD 7225PE receiever for repair by my sister.

This receicer is supposed to be a fine sounding one built upon the NAD 3020 amplifier for it's output stage. My sister is unable to give me informations as per the condition in wich it broke down. But from what I can see all the fuses (4 X 4A.125 V) related to the linked to the 4 & 8 homs circuitry keeps blowing out. I have very little understanding on this POWER ENVELOPPE principle, and I can only assume for the time being that one output transistor (or more) is shorting. Upon reading of various informations available on the internet it seems these unit are plagued with this sorts of problem. It could be initially caused by the break-down of capacitors or bias resistors or possibly both.

The receiver works as a preamplifier and or tuner on it's own using the preamp out rca plugs , and as such, it could possibly rule-out a major power supply problem.

I would like to know if anyone has experience with them, and if anyone knows of any electronic source that could provide matched output transistors. I can not possibly buy a hundred units of these in order to match two pairs of 2N3055 / 2N2955. I do not have the tool to match transistors.

Thank in advance.
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Old 20th October 2009, 06:14 PM   #2
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The 7220/25 PE design was a real pain, they blew up all the time.

It will probably need a complete overhaul, outputs, drivers, and probably other misc. bits.

Use MJ15015/15016 for the outputs (or MJ15003/15004), the originals are inadequate.

Cut the traces going to the base of the outputs and tack in a 4R7 1/2W resistor, this will reduce the chance of the thing blowing up again.

Good luck.

PS

When replacing the small TO92 parts, mind the pin-outs. Many of the replacements are different.
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Old 20th October 2009, 08:53 PM   #3
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The 7220/25 PE design was a real pain, they blew up all the time.

It will probably need a complete overhaul, outputs, drivers, and probably other misc. bits.

Use MJ15015/15016 for the outputs (or MJ15003/15004), the originals are inadequate.

Cut the traces going to the base of the outputs and tack in a 4R7 1/2W resistor, this will reduce the chance of the thing blowing up again.

Good luck.

PS

When replacing the small TO92 parts, mind the pin-outs. Many of the replacements are different.
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Report Post Reply With QuoteI am no candidate to the Darwin Award (I hope ), and I have no other option than to read this author for the time being...

Thank you for your reply,

I certainly hope I am not getting into a complete overhaul project. I will most certainly consider putting the 4.7 R resistors . I have took the transistors out (output) to ***** their condition : one of them is definitly shorting, I have no clue as why the 4 fuses (both chanel) have blown all together however. I understand this thing was based (quite possibly lously) on the NAD 3020 amplifier, and left to beleive it might still be worthy to save as such.

You do mention a replacement reference number for a suitable replacement.
Do you know of any company that could provide me a complete set (4 of them including their complimentary counterpart) The drivers, attached close on the heatsink seems to be of the regular flavour, i.e. BD139.

Should I expect to need more of these transistors, or could I just go on and replace some of the capacitors which looks...well... like crapp

Thank you again for the time taken to reply to this post.
Judging by the attention given to my initial inquiery. I may tell my sister to get rid of it and buy herself something new.
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Old 21st October 2009, 12:43 AM   #4
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I had a similar experience with a NAD 701. If you have the time and a little bit of electronics knowledge, you can easily repair it.

Do you have the schematic? If your amp stage is similar to mine, it was prone to thermal runaway due to no emitter resistors on the outputs. I had to replace the output transistors, drivers, and bias transistor. While I was at it I added 0.33 ohm emitter resistors on the outputs and a resistor to the base of the drivers as described above, and also replaced all the electrolytics.

You can check the transistors with a multimeter, a PNP will look like two diodes with the cathodes tied together at the base, for an NPN it will look like the anodes tied together at the base. If one or more of the outputs are gone then it is likely that the drivers and bias are gone too.

No need to match transistors. You can use 2N2955/3055 or as suggested by djk, but you will most certainly need emitter resistors to get it thermally stable. Apparently the NAD designs relied on the internal base-emitter resistance of the original 2N2955/3055, newer parts have different construction and do not have that resistance. In any case it seems like a dangerous way to have designed the amp, based on the number of times I have seen things like "the NADxxx blew up all the time."

Also look for any resistors that might have overheated as the outputs failed, and any electrolytics that are bulging or leaking.

Again, a little time, patience, and willingness to learn and you can fix it. If you get stuck, diyAudio is a great resource - it worked for me!
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Old 21st October 2009, 02:43 PM   #5
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Thank you for your encouragement mightydub,

I was lucky enough to find a service manual distributed freely on the web.
From what I can see (the manual is of poor quality but still readable) there seems to be emiter resistors tough . I will possibly go for the output transistors as suggested by djk. I assume I can saffely use the same driver type (BD139) ?

You do mention matching is not requiered , probably because on pair with complementary type. I did mentioned I had more knowledge of tube type amplifier, for some reason I was beleiveing that matching of these device was a prerequisite for proper operation.

Thank you again
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Old 21st October 2009, 03:44 PM   #6
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post a pdf of the schematic and let's have a look.
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Old 21st October 2009, 06:39 PM   #7
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Thank you mightydub,

Here is the schematic (PDF file attached)page off from the service manual. I may have been wrong with regard to the emitter resistor, there seems to be one common to both output device of the same channel, my little knowledge suggest me it might be for NFB ?? Is the biasing transistor BD139 located, or framed in doted square : Q409 just before Q411 & Q413 ? it's potential would adjusted trough R443 variable potentiometer ?

Your help is much apreciated.
Attached Files
File Type: pdf NAD 7225PE Schematic.pdf (114.3 KB, 332 views)
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Old 21st October 2009, 10:25 PM   #8
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This is identical to the circuit in my 701, even the same reference designators.

Q415 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 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.

Somewhere I have the schematic in LTspice form, I'll dig that up and post it.

You can absolutely fix this and you'll have a nice little receiver. When you do, play some dub.

mightydub
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Old 21st October 2009, 10:59 PM   #9
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Here's the schematic showing the modifications. (assuming you use Windows, remove the .txt extension so the file name is nadAmp.wmf, then double click.)

The over current protection and output network are not shown for simplicity.

You will be adding R2, R3, R4, R5 to each channel.
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File Type: txt nadAmp.wmf.txt (35.4 KB, 294 views)
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Old 22nd October 2009, 01:36 AM   #10
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...You have been more helpfull then I could have hoped-for...Thank you.
I did not know of B&D entreprises for wich you left me with a link. As they do deliver to Canada ,I will order from them.

I will get down to it : cut the copper traces in order to install these resistors, hopefully there should be enough room for them on the pcb, change the most obvious electrolytics and then carefully rebias the thing.

Many thanks again.
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