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:
http://www.diyaudio.com/forums/pass-labs/95067-repair-question-re-threshold-400a.html
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.
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:
http://www.diyaudio.com/forums/pass-labs/95067-repair-question-re-threshold-400a.html
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:
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....
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....
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!
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!
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/
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/
Last edited:
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...
You very rarely see this form of protection in modern amp though...
Last edited:
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.
http://www.diyaudio.com/forums/pass-labs/161730-threshold-400-help.html
http://www.hifi-museum.com/lib/threshold_lib/800a_bro.pdf
Those look like amp PS rail protection - see this thread (post # 8) - plus look on the 'net for 400a info and schematics.
http://www.diyaudio.com/forums/pass-labs/161730-threshold-400-help.html
http://www.hifi-museum.com/lib/threshold_lib/800a_bro.pdf
Attachments
Last edited:
No, the older ones do not... and FWIW, I tried to retrofit my older 400a (had both) and I could not seem to stop them from blowing so I gave up (Im a novice at electronics).
Disconnect any inputs and see if you have the same problem.. presently my older 400a is not playing with my 1950's tube preamp well.. the preamp is probably feeding it DC or something it can't handle (works with other preamps).. so try powering it up with no input... see if the meters peg or you get any noise..
Beyond that, from some experience, Id do a rebuild of it if it hasnt been done... its not hard.. just get all new caps and consider ON MJ21193/MJ21194 power transistors.. when they are all soldered in and you have used synthetic heat sync compound with new pads then follow the bias instructions you'll find with a search.
I bought one with a channel out.. there were about three power transistors bad when I checked them with a multimeter... its best to just replace them all and be done with it...
Do the caps on the meter also.. trust me on this.. you dont want to go back in there if you dont have to.. and mine were bad... use a razor blade carefully to get into the meter panel...
Contact Jon at Vintage Amp Repair and get yourself a rebuild kit. That wway, when you are done, you will have an amp that will last many more years.
The group here is great about helping when or if you get stuck. I had a little trouble with the bias circuit, but Nelson sorted me out.
Good Luck!
The group here is great about helping when or if you get stuck. I had a little trouble with the bias circuit, but Nelson sorted me out.
Good Luck!
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.