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Marshall plexi prob


 
12/17/2005 2:08 PM
km Marshall plexi prob
im trying to figure out my buddies brand new Marshall 1987x re-issue head. It keeps blowing the 500 ma fuse when standby is turned on. This is Without the power tubes in it. Any ideas where to start looking?
 
12/18/2005 2:29 AM
km
yes---- i looked at the other fuse posts, but they are different, my 500 ma HT fuse is popping the instant i flip on the standby(amp ON). the main fuse is fine. again, this is with no power tubes.
 
 
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12/18/2005 3:00 AM
Rick Erickson
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quote:
"my 500 ma HT fuse is popping the instant i flip on the standby(amp ON)."
 
 
Then you must have a short in the HV (B+) supply.  
This is where you need to break out your ohm meter and look at (test) the power supply components. If you don't have the schematic - search for it online or contact Korg USA and they will usually send one out.  
 
RE
 
12/18/2005 1:08 PM
km
thanks rick--i assume the scem in the Marshall book for the old amp is fairly close?ill start there.  
 
i did notice the re-issue has 2 adjustable trim pots on the board--would that be one bias adjust for each tube ?
 
12/19/2005 12:22 PM
Rick Erickson
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You can find some 1987x schematics at:  
http://www.marshallschematics.com/  
Not sure about the re-issues. Might want to email Korg USA for those - I've found them to be very responsive.  
If your amp has two bias trimmers then my guess would be one for each tube. I would do a visual check of your power tube sockets between pins 2 & 3 for any sign of arcing. This is a common failure mode that eats fuses. Beyond that - check the filter caps, rectifiers etc. for shorts. Marshall is not terribly consistant in their wiring so your rectifiers may be after the standby switch, even though the schematic shows before. Be sure the caps are discharged before you start poking around in there or you could get a nasty shock. We don't want you to be late...  
 
RE
 
12/19/2005 4:53 PM
km
I have the 1987x scem now.  
ok, ive checked the 2 -50/50 caps and everything i can see in the power supply, diodes, resistors....eveything looks good. Now im really stumped !!
 
12/20/2005 4:55 PM
km
i clipped in my meter today, where the HT fuse goes and powered up the amp. Without power tubes i get 17ma . with power tubes i get 107ma . I was out of 500ma fuses so i tried a 750ma. Ive been rocking the amp for a good hour, now its idling for about 2 hours, ive shut it on and off,.....so far it will not blow the 750ma fuse ! ???
 
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