Question to Change 2-3 prong plug on 68-69 Gibson Duo Medalist

[Lost_Cause]

Hi guys, pretty new here but I have one of these recently purchased and looking at another Gibson with a similar set-up (one chassis vs 2) so I thought I would ask about this since I am pretty sure I can do this myself if I have a little direction. I am hoping to get some help and or the correct direction here please. I just purchased a Gibson Duo Medalist with a 2-prong in it. It definitely has an Input or Power Transformer. I am looking to put a 3-prong cord into it and I have never had to do it on a vintage piece of equipment. I have replaced cords but it was a direct replacement. Here is the schematic and what I was thinking is that all I need to do is remove the .022 capacitor between the bottom leg of the polarity switch (that goes to ground) and connect the ground from the plug to the chassis and then connect the ground to that connection on the switch. This particular amp has two separate chassis they call on the preamp (mounted on top but contains the power switch) and the other is the power (bottom mounted). I will obviously need to connect the two chassis with a grounding wire as well.

http://i99.photobucket.com/albums/l310/RONAUER/Gibson%20Duo%20Medalist/KT9thBirthday060.jpg

http://www.gibson.com/Files/schematics/duo-medalist.jpg

Thanks in advance for all of the help.

[Lost_Cause]

Or if anyone is in the Central Illinois area (Mattoon is where I live) and might be able to lend a hand, it would be greatly appreciated. I can pay but if I have to do it twice I would like to learn what I am doing and what to look for so I can do it myself int he on the second one and in the future.

[tipetu]

In my champ head I just grounded the "ground" to the chassis.. Believe this will work, and I find that cap you mentioned rather unessecary?

[WaltC]

the work you'll be doing will be all in the "bottom" chassis, but lots of high voltage DC lives there in the filter caps, so you could definitely get hurt or killed if you're not careful. You'll basically be removing the old power cord, installing the new, 3-prong cord w/ a strain relief, connecting the green (ground wire) on the new cord to the chassis either by soldering it to an existing ground connection or w/ a nut to one of the PT bolts, connecting the white wire of the new cord to one of the PT primaries, connecting the black wire of the new cord to the fuse holder (back) and connecting the other PT primary to the side terminal of the fuse holder.

I wouldn't worry about running a ground up to the pre-amp chassis, that should have already been taken care of.

If you're the slightest bit uncertain, please take it to your handy amp tech and let him do it.

[WaltC]

that's what I get for typing too fast:

" connecting the other PT primary to the side terminal of the fuse holder"

what I meant to say was :
connecting the side terminal of the fuse holder to one side of the on/off switch wire from the pre-amp chassis and then connecting the other PT primary to the other on/off wire from the pre-amp chassis.

now that you're sufficiently confused...<G>

[oldhousescott]

Remove the .022 cap from the power switch. Connect the green ground wire in the replacement cable with a ring lug to a bolt/screw on the chassis close to the cord inlet.

[Lost_Cause]

I believe that understand. A little variation here but I think I can manage it. Safest to discharge the filer caps correct?
Believe that I was over thinking it a bit before. I have modded tons of circuits but nothing this complex.
Thanks I appreciate the help.

[Fauxkinbacker]

It's always best to discharge the caps.

And you should definitely get rid of that polarity switch!

[tipetu]

Just one question regarding discharging caps.. I have this tiny preamp I`ve been working on.. When I discharge the caps I just short the + on the caps :-P.. Put a isolated screwdriver on the + and touch the chassis.. some sparks and it`s "drained"..

Is this a NONO? :-P

[capnjuan]

Put a isolated screwdriver on the + and touch the chassis.. some sparks and it`s "drained"..
Is this a NONO? :-P It isn't good practice; get a 1 watt or higher / low value resistor ... 100 ohms or so. Hold one lead of the resistor to a + cap section, the other to the chassis; use needlenose pliers, hemostats, whatever works for 30 seconds or so. If you're concerned about them being fully drained, check w/ meter. Shorting the caps stresses them and shortens their useful life. CJ