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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: 20
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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: 866
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: 866
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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