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Adastra 6ch 120W PA Amplifier - blows fuses

 
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David Webb
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PostPosted: Fri Aug 26, 2005 12:42 pm    Post subject: Adastra 6ch 120W PA Amplifier - blows fuses Reply with quote

The power amp seems to be the problem - if I isolate it, the pre-amps work fine.
Its a 4 x TIP35C unit.
I have a full set of circuit diagrams, but I am very rusty on servicing. I would appreciate some hints on where to start looking.
Thanks
David
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absorbentgnome



Joined: 30 May 2005
Posts: 51
Location: Cardiff, UK

PostPosted: Sun Aug 28, 2005 9:46 pm    Post subject: Reply with quote

You need to check the output transistors for shorts. Test the forward junction voltage drop with a multimeter on the diode setting between BC and BE. Test both transistors in each output pair, remember that PNP transistiors need to be tested in reverse. The voltage drop should been ~0.5V for any good junction, out of circuit. Any shorts while the transistors are in circuit should be double checked with the transistor removed, and any duff transistor should be replaced along with its complimentary transistor. Make sure that all faults have been cleared before switching on, and any fuses blown are replaced with the same type to avoid killing more components should you switch the amp on with faults still present. Smile
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PostPosted: Tue Jun 20, 2006 5:28 am    Post subject: Adastra 120watt blowing fuses Reply with quote

Hi David,
Could do with a copy of the schematic. I have the same problem with this amp. This is my second time to repair it. There is more than the output transistors faulty. Something is possibly wrong with the bias!
Regards,
Martin. Smile
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PostPosted: Tue Jun 20, 2006 7:14 pm    Post subject: Reply with quote

Never replace the fuses until the fault is repaired, otherwise you will create more damage.
Remove all output transistors and test. Test the output resistors (the big ones near the outputs). Test the previous stage transistors and any resistor between those stages (they would have been feeding into a dead short when the cold fuse was inserted) Test each leg of the bridge rectifier.
An amp should start with fuses less than half (about a third) of the amperage rating. This MAY limit further damage if you are unsure.
Can you post the schematic here? then we can give you more specific hints.
I guess you have already tested as per absorbentgnome's post?
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PostPosted: Thu Aug 09, 2018 7:51 pm    Post subject: Re: Adastra 6ch 120W PA Amplifier - blows fuses Reply with quote

David Webb wrote:
The power amp seems to be the problem - if I isolate it, the pre-amps work fine.
Its a 4 x TIP35C unit.
I have a full set of circuit diagrams, but I am very rusty on servicing. I would appreciate some hints on where to start looking.
Thanks
David


Hi all.

I have 2 of these 952 996 amps, the first started sounding distorted a couple of years ago and has been on the shelf since, the second seems to be doing the same, I have done dead tests on o/p stage (without removing components) and all appears to be as expected.

Any experience with these or a schematic would be greatly appreciated. So far google has failed me.

Here's hoping.
Ray
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