hum problem on jc120 and suspect input jack

[Amapola]

Hi there,

I have a jc 120 which come with this schematics...



... and i find the ground wired to positive on "input" jack very strange ^^ !! and the schematics show the same thing ^^ !!




I've always had a problem of hum in this jc120... Many years ago, i even pulled the transformer out the amp several meters away...but always hum.
I'm not the first hand but I've always seen this amp wired as well...and always had a hum problem !

One day i would like to change all the input jack because they were too old, i changed for jack like this... ( i didnt know at this time but i took insulated closed jack instead of non insulated)

http://www.banzaimusic.com/image.php?id=7208&type=D

In fact with the insulated closed jack, it's like if the input jack were removed ...and i remember there wasnt hum at this time...


For me It's seem a ground loop , no ? Maybe there's a mistake on the input in the schematics ^^??
Am sure it's come from these wrong input... For me it's very strange to see the ground wired to positive ^^i never seen it in any audio unit or hardwear ...I also checked with a multimeter by a test of continuity, always "bip" between lug !!


So what do you think about it ?

Thanx in Advance for reply

Greetings

[J M Fahey]

Ground is not always connected to input jack "hot" tip.
That is known as a switching jack.
When nothing is plugged the leaf contact is down and touches a ground tab, what you measured (the beep), to *avoid* Hum.
When you plug a Guitar the leaf contact is pushed up, away from ground, and guitar signal goes to preamp.