electronics, repairs, service manuals and datasheets discussion community for the repairs professionals and technicians
FAQSearchMembersGroupsRegisterProfilePM'sLogin/Out
Outputs Transistors keep blowing - NAD 7225PE

 
Post new topic   Reply to topic    Electronic equipment repair and service manual search discussion Forum Index -> Equipment Repairs
View previous topic :: View next topic  
Author Message
Bunyip
Guest





PostPosted: Thu Apr 23, 2009 7:28 am    Post subject: Outputs Transistors keep blowing - NAD 7225PE Reply with quote

Faulty NAD 7225PE => Output transistors, Driver Transistors and a fusible resistor 180Ohm R453 all blew again after replacing. Cannot find any other fusible resistors that have gone high nor have any pre driver transistors blown. I can't seem to work out what else would cause this 'catastrophic' failure. Replaced all parts, powered up unit with no load and bang! ...well sizzle actually.

Had a similar problem with a Sherwood amp AI-1110, once again replaced all faulty parts in the output section, powered up and pop they all blow again.

I guess the question is after taking all the 'bad' parts out of a faulty amplifier, apart from powering up to see if any fuses blow, what tests can you do for further analysis???
Back to top
livewire



Joined: 23 Mar 2009
Posts: 19

PostPosted: Thu Apr 23, 2009 8:34 pm    Post subject: Equipment Repairs Reply with quote

I guess you just have to test it that way...and bad parts are always a headache you sometimes can not even replace them and throw away gadgets.
Back to top
View user's profile Send private message Visit poster's website
vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Fri Apr 24, 2009 5:06 pm    Post subject: Re: Outputs Transistors keep blowing - NAD 7225PE Reply with quote

Bunyip wrote:
Faulty NAD 7225PE => Output transistors, Driver Transistors and a fusible resistor 180Ohm R453 all blew again after replacing. Cannot find any other fusible resistors that have gone high nor have any pre driver transistors blown. I can't seem to work out what else would cause this 'catastrophic' failure. Replaced all parts, powered up unit with no load and bang! ...well sizzle actually.

Had a similar problem with a Sherwood amp AI-1110, once again replaced all faulty parts in the output section, powered up and pop they all blow again.

I guess the question is after taking all the 'bad' parts out of a faulty amplifier, apart from powering up to see if any fuses blow, what tests can you do for further analysis???


This is more aimed for someone wanting to do this as a profession;
The best way to handle this is to invest in a Variac (adjustable isolation Transformer). This way you can slowly bring up the mains voltage as you monitor the current draw on the unit's meter. At the same time it will give you headroom to be able to do spot voltage checks and a cushion to back down if you need to.
I used to run across this all the time. Depending on the circuit design, you may still miss one or two components and end up wasting new parts which can get expensive quick!
_________________
Back to top
View user's profile Send private message
Bunyip
Guest





PostPosted: Tue Apr 28, 2009 2:11 am    Post subject: Post subject: Re: Outputs Transistors keep blowing - NAD 722 Reply with quote

Thanks vtech, I had heard of this once before but it was not explained to me in the concise way as you have just done. I was told though that if you slowly increase the level on the Variac and the current more or less follows it linearly, then the amp is drawing as much current as it can and thus still has problems.

I'm starting to repair amps professionally now and its beginning to look like a Variac is a must. I'm sure after a few amp repairs I'll get to know what's normal start up/no load current, compared to the current drawn for outputs going hard on in over drive (bad).

This is the first time I've posted a forum topic anywhere (because most of them seem fruitless) but I must say I'm impressed with this one. Nice Work!
Back to top
vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Tue Apr 28, 2009 4:31 am    Post subject: Re: Post subject: Re: Outputs Transistors keep blowing - NAD Reply with quote

Bunyip wrote:
Thanks vtech, I had heard of this once before but it was not explained to me in the concise way as you have just done. I was told though that if you slowly increase the level on the Variac and the current more or less follows it linearly, then the amp is drawing as much current as it can and thus still has problems...


Generally correct... If still has problem(s), you will indeed see a rather steady peg on the meter and can easily back off. On the other hand on a working circuit, there will always be an initial draw coupled with the "dip" in the meter as you would hit the biasing threshold. A typical indicator of a successful repair.
The more you use the Variac, the easier it gets to spot a working circuit. A valuable observation inherent in majority of the amps especially for the final stages.
Back to top
View user's profile Send private message
livesteamfan
Guest





PostPosted: Sun Dec 13, 2009 12:22 am    Post subject: Reply with quote

Bunyip, could you tell what rating R454 is? I have the same unit and due to a stupid mistake R454 blew out and I can't tell what the color bands are on it. It is located under the first/second fin on the heatsink going out to the right when looking from the front. Thank you very much for any help you can give me.
Back to top
Opera
Guest





PostPosted: Mon Dec 28, 2009 8:17 pm    Post subject: Reply with quote

I've repaired amps for years and you need to put a 100W lamp in series with the AC power when first turned on. A lamp has about a 7 to 1 increase in resistance from cold to hot. This will generally protect any new components that were installed and allow voltage readings to be taken. Lamp will be very faint if everything is ok. I saw this in an EICO ad 40 years ago and has worked for me.
Back to top
Guest






PostPosted: Thu Dec 09, 2010 12:12 am    Post subject: Reply with quote

livesteamfan wrote:
Bunyip, could you tell what rating R454 is? I have the same unit and due to a stupid mistake R454 blew out and I can't tell what the color bands are on it. It is located under the first/second fin on the heatsink going out to the right when looking from the front. Thank you very much for any help you can give me.


I looked on my unit and R454 is a 180 Ohm resistor

anyone know what value R453 is?
Back to top
jts1957



Joined: 24 Nov 2008
Posts: 2472
Location: Far, Far Away

PostPosted: Thu Dec 09, 2010 12:27 am    Post subject: Reply with quote

Find out for yourself:

http://elektrotanya.com/?q=showresult&megnev=&megnev2=7225PE%20&kategoria=&kat2=all
_________________
You're over there, I'm over here.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    Electronic equipment repair and service manual search discussion Forum Index -> Equipment Repairs All times are GMT
Page 1 of 1

 
Jump to:  
You can post new topics in this forum
You can reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Електроника София - Service Manuals Free Download - Prikachi - Service Diagrams

Free TV Schematics Download - HARDW.net - Pinout Master - Jabse Search - Jabse.InK  - Pulsator

Connector Pinout Forum - Новини - SnimkiOt.com - безплатен DNS - Whois разузнаване - Poblizo.com : пътешествия