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Thread: 1990 marshall reissue jtm45 mkii blown ht fuse

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    1990 marshall reissue jtm45 mkii blown ht fuse

    (3)12ax7 (2)6L6 GZ34. original condition. head only
    the owner is an old friend.got this from a guitar player that used power soak devices. 6L6 version. first visit- shorted section of 50uf 600v B+ cap before T3 and shorted rectifier tube. replaced both dual 50uf caps with F&T 500vdc and JJ GZ34 rectifier tube. owner comes in a month later.got to a gig with crappy line power.amp began distorting badly.fuse blew after he tried to jumper ch1 into ch2. Now as soon as you turn on standby switch 500ma HT fuse goes...with only the rectifier tube installed. replaced with known good rectifier tube...same issue. with only rectifier tube installed, disconnected yellow wire from 50uf cap(it carries B+ to plates of preamp tubes)...HT fuse doesn't blow. normal heater voltage, 417vdc to plates.I also see 417vdc on plates of empty preamp tube sockets but thats coming in from T3 which is still tied to B+ of other section of 50uf cap.. this device labeled T3 on schematic is not a conventional choke like what is seen on the outside chassis of most jtm's mounted near the output tranny. this is inside and looks like a miniature dummy resistance load.gold appearance. 100R stamped on it.ohms out at 100 ohms.inserted between the positive terminals of first 50uf / 500volt cap. output tranny ohms out at 75 and 73 ohms to center tap.looks like bias circuit gets power from mains switch before standby switch. I did measure both dual 50uf caps just to be sure and they checked out.sorry to be so long winded about this thing.thanks for your opinions.

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by ogeecheeman View Post
    ......sorry to be so long winded......
    Yep. Everything, but a schematic.

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    Click image for larger version. 

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    Senior Member Sowhat's Avatar
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    I'm sorry but I'm a bit confused, don't know exactly what you mean by a yellow wire. You say you disconnect it from one of the dual 50's and you get good power all the way up to the front end so I do imagine you are not breaking the B+ line then. The output transformer primary is good and the choke, regardless of it's unusual appearance is probably good because they either fail open which would cutoff the B+ or they short which is what they virtually are anyway so you would still pass B+ but maybe a little noisier. I dunno, something don't add up here, from the sounds of it the dual 50 is bad but somehow but you measure that as being good... mmmm?

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    thanks for responding.looking at the schematic- T3 is across the positive terminals of the 50uf cap.the circuit leaves T3 then ties into the 470ohm screens, the 50uf cap and a 10k resistor that feeds the plates of the preamp tubes.physically, there is a yellow wire that runs from that 50uf cap to a circuit board junction tying in all of these circuits.if I lift that wire from the cap, the short is no longer there.my first thought was the same.bad cap.when I originally replaced these 2 50uf caps I researched to be sure the 500vdc rating was enough. it was unanimous.I used the black F&T replacement.at first the problem was intermittent. just an occasional thing. and as long as the output tubes were not in the fuse held.I will say this...I measured the empty + terminal of the 50uf cap after lifting the yellow wire a couple of times and got 97vdc.it did not drop. then just as suddenly as it appeared..the voltage was gone. there should be no bleed or leakage here.that much I'm certain of.I'd like to know what the specs should be on that T3.there is no reason why I can't test with T3 out of the circuit is there?

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    Last edited by ogeecheeman; 05-03-2018 at 05:07 PM.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by ogeecheeman View Post
    I'd like to know what the specs should be on that T3.there is no reason why I can't test with T3 out of the circuit is there?
    If it says 100 ohms on it, then you can test with a 100 ohm power resistor instead of the choke.
    It may well be that it is really a power resistor like you mentioned in your first post. There is a * marked beside T3 on the schematic, there may be a note somewhere that says something about that.

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