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Old 18th August 2010, 02:31 AM   #1
nickate is offline nickate  United States
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Default New guy here... My trusty 400a died...

I turned on my trusty Threshold 400a for the first time since Christmas (At Christmas I got bummed because one of my speakers started rattling inside... Took it apart... Rotted surround...Took the mate apart and found one stuck woofer)... after hooking up some interim (Radio Shack Optimus 7 bookshelf) speakers.

I swear you could hook up garbage can lids to this thing and they would have a soundstage...

I played it for about 5 minutes until I lost a channel.... The left meter was pegged... Tonight I sent off an email to Jon Soderberg about repairing it and then I found this great site... I have replaced speaker terminals and re- attached the LED meter but I am by no means an electronics repair guy... Should I attempt to repair this or leave it to the expert?
Thanks!

I hooked up my spare 400a (everybody has a spare, right?) and it plays the Radio Shack speakers splendidly!

EDIT: I just pulled up this thread:
Repair question re: threshold 400a

I'm in over my head here....
But I'm gonna pull the cover tomorrow and have a look... (How long those caps. carry a charge you figgr???) Not gonna touch anything... just look.

Last edited by nickate; 18th August 2010 at 02:55 AM.
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Old 18th August 2010, 03:27 AM   #2
nickate is offline nickate  United States
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Just pulled off the top cover (had to use an impact as not to mess up the screw heads) and I don't believe anybody has ever been in here.

The internal fuse ON THE RIGHT SIDE is blown (real thin guy) I don't know the size of it but I'm not touching anything in here for a few days....

I know the Left meter pegged when I hooked up a speaker.... Both sides were one light until I hooked up the left channel then no sound just a "pop" and the meter was pegged even after I removed the speaker wires....
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Old 18th August 2010, 04:23 AM   #3
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1st suggestion - it's not the best practice to hook up speakers while the amp is powered up 'cause if you short the wires to something you are begging for trouble. The sudden load of a speaker on an unloaded amp can cause some of them to trip out the speaker protection circuit or ....... blow a protection fuse.
Additionally - fuses "age" and when they get old they simply blow due to mechanical fatigue of the metal that makes up the fuse. Replacing really old fuses is a good thing.
All fuses should have the voltage and amp rating on them somewhere and a replacement should not be hard to find (in most cases).
Power down the amp - disconnect it from the AC power, discharge the caps and replace the fuse. If it keeps blowing ya gotta problem that should be handled by a capable repair person.
Hope this helps!
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Old 18th August 2010, 08:58 PM   #4
nickate is offline nickate  United States
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Quote:
Originally Posted by nickate View Post
I turned on my trusty Threshold 400a for the first time since Christmas (At Christmas I got bummed because one of my speakers started rattling inside... Took it apart... Rotted surround...Took the mate apart and found one stuck woofer)... after hooking up some interim (Radio Shack Optimus 7 bookshelf) speakers.

I swear you could hook up garbage can lids to this thing and they would have a soundstage...

I played it for about 5 minutes until I lost a channel.... The left meter was pegged... Tonight I sent off an email to Jon Soderberg about repairing it and then I found this great site...IN THE PAST I have replaced speaker terminals and re- attached an LED meter but I am by no means an electronics repair guy... Should I attempt to repair this or leave it to the expert?
Thanks!

I hooked up my spare 400a (everybody has a spare, right?) and it plays the Radio Shack speakers splendidly!

EDIT: I just pulled up this thread:
Repair question re: threshold 400a

I'm in over my head here....
But I'm gonna pull the cover tomorrow and have a look... (How long those caps. carry a charge you figgr???) Not gonna touch anything... just look.

sorry.. I meant to say I had done a basic repair or two in the past.
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Old 18th August 2010, 09:14 PM   #5
nickate is offline nickate  United States
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I Just pulled the blown fuse....

It is a 4A 32v 311 glass fuse.


Strange that the fuse is on the right side of the amp but the channel the meter was pegged on was the left.

Anybody know what the fuse does and what makes it blow?
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Old 20th August 2010, 03:51 AM   #6
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Sorta like this one? Digi-Key - F2235-ND (Manufacturer - 0311004.H)

Do you have a schematic or service manual??? Is there a fuse # on the PWB somewhere?

You might query Jon Soderberg of Vintage Amp Repair http://www.vintageamprepair.net/
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Last edited by c2cthomas; 20th August 2010 at 04:01 AM.
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Old 21st August 2010, 05:11 PM   #7
nickate is offline nickate  United States
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God I love these 400a's....

I replaced the fuse and IT WORKS again!

Can anybody please tell me what the 4a glass fuses INSIDE the amp do?

I've had it running for about 20 minutes now..... Time will tell...
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Old 21st August 2010, 05:44 PM   #8
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Well, if you f.e. take a look at the power supply of the A75, you'll see that the fuses on the secondary side right after the rectifier bridge are FAST blowing fuses (rated 6A). The mains fuses usually are SLOW blowing fuses, otherwise the inrush current, no matter how limited, would evaporate the fuse every time you power on the amp. The fast acting fuses are like a hundred times faster than the slow acting ones, so fusing the secondary can help avoid damage to the circuit better than with mains fusing alone, given the fuse is properly rated. AFAIK fast acting fuses take around 20ms to fail open with a current ten times their rating. I'm just guessing that to determine the rating, the sum of all secondary fuses times ten should probably be around the peak output current the amplifier can supply. When you produce a short the fuse will probably blow before the output-stage does. But I'm just guessing.
You very rarely see this form of protection in modern amp though...
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Last edited by Rodeodave; 21st August 2010 at 05:50 PM. Reason: typos
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Old 21st August 2010, 07:21 PM   #9
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Ahhhh - the thrill of saying "it's alive - IT'S ALIVEEEEEE!!!!!"
Those look like amp PS rail protection - see this thread (post # 8) - plus look on the 'net for 400a info and schematics.

Threshold 400 help

http://www.hifi-museum.com/lib/thres...b/800a_bro.pdf
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Last edited by c2cthomas; 21st August 2010 at 07:32 PM.
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Old 17th December 2011, 10:34 AM   #10
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Reopening this thread because my 400A is doing the same peaking thing. Where are the internal fuses? I took the lid off and cannot see any.
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