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Bunyip
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 Posted: Thu Apr 23, 2009 7:28 am Post subject: Outputs Transistors keep blowing - NAD 7225PE |
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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??? |
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livewire
Joined: 23 Mar 2009
Posts: 19
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| Posted: Thu Apr 23, 2009 8:34 pm Post subject: Equipment Repairs |
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| 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. |
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vtech
Joined: 08 May 2006
Posts: 1264
Location: USA
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| Posted: Fri Apr 24, 2009 5:06 pm Post subject: Re: Outputs Transistors keep blowing - NAD 7225PE |
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| 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!
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Bunyip
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| Posted: Tue Apr 28, 2009 2:11 am Post subject: Post subject: Re: Outputs Transistors keep blowing - NAD 722 |
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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! |
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vtech
Joined: 08 May 2006
Posts: 1264
Location: USA
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| Posted: Tue Apr 28, 2009 4:31 am Post subject: Re: Post subject: Re: Outputs Transistors keep blowing - NAD |
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| 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. |
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livesteamfan
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| Posted: Sun Dec 13, 2009 12:22 am Post subject: |
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| 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. |
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Opera
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| Posted: Mon Dec 28, 2009 8:17 pm Post subject: |
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| 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. |
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Guest
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| Posted: Thu Dec 09, 2010 12:12 am Post subject: |
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| 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? |
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jts1957
Joined: 24 Nov 2008
Posts: 2476
Location: Far, Far Away
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