From mschwa–(at)–as.com Tue Jun 23 21:12:44 CDT 1998
Article: 111555 of alt.guitar.amps
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From: mschwa–(at)–as.com (Mike Schway)
Newsgroups: alt.guitar.amps
Subject: Re: Change BF AC power cord to 3 prong [long]
Date: Tue, 23 Jun 1998 17:52:57 +0000
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In article <6mnlf4$oa--(at)--gtnsc03.worldnet.att.net>, n–(at)–pam.com (Rich ) wrote:
> What needs to be done to replace the power cord on a BF pro reverb to
> grounded 3 prong? I got zapped again. Outside on a concrete floor. It
> wasn’t real bad but it was annoying. Flipped the ground switch and all
> was OK, but wouldn’t a true ground be better (and safer)?
> Thanks.
Yes, under nearly any conceivable instance, the cordset should be upgraded
and the “death cap” removed.
First the standard disclaimer: If you have to ask, you probably shouldn’t
be working inside a tube amp. There is in excess of 500VDC in places on a
live chassis and if you don’t know EXACTLY what you’re doing, you can
DIE!!!! A good tech probably will charge about $40-50 for the job
(including parts), so if you’re a little unsure, have it done
professionally.
The following procedure may seem overly complicated, but it’s really
necessary since in addition to using an ungrounded 2 wire plug, the old
Fenders had the switch on one leg and the fuse on the other. If the
neutural (white) wire were fused, should the fuse blow, you would still
have lots of current, only it would travel through the green wire, not the
white. Best have both fuse and switch on the hot (black-wire) leg.
That said: Here’s the best procedure (I assume you know how to solder).
Also, these instructions assume the chassis is set up with the knobs
toward you and the power stuff is on the right hand side, away from you.
1) purchase a 18g/3 wire cord set (type SJO or SJT) about 8-9 ft long.
16/3 is overkill and probably won’t fit too easily in the old strain
relief (or the convenience outlet lugs for that matter.)
2) Solder a ring-type terminal onto the green wire of the cord set. Get
a couple of toothed lock washers of a size which can fit over one of the
studs holding the power transformer in place. Tin the exposed ends of the
black and white wires.
3) Clip out the old cordset from the “convenience outlet”. Remove all
traces of solder and cordset wire from the outlet terminals…you’ll need
the entire original insde diameter of the lug free for the rewiring.
Don’t try to unscrew the lugs…not necessary nor easy. Use a solder
sucker and/or desoldering braid to remove the solder.
4) Remove the old cordset and strain relief by using pliers to compress
the longer diameter of the strain relief and pull it out. Save the strain
relief ’cause you’ll need it later.
5) Unsolder the white/yellowish jumper from the power switch (leaving one
black power transformer primary lead attached to the other terminal).
Remove the fuse from the holder and unsolder both leads of the
fuseholder. Unsolder all 3 terminals of the ground switch. Keep the
wire…especially if it’s the nice cotton-wrapped stuff…the longer
pieces are good to use for the rewiring coming up.
6) Remove the “death cap” (.047uF/600V) attached to the wiper of the
ground switch…lower right side lug. Clip the other end of the cap where
it goes through the hole in the chassis. You probably would have a hard
time actually desoldering it from the chassis, neatly clipping is ok.
7) Now it’s time to rewire. Starting from the power switch and working
toward the AC cordset: Using 20-22g unstranded wire, neatly run a jumper
>from the unused lug of the power switch (the other lug has one power
transformer primary attached) to the “ring” lug of the fuse holder. Keep
wire runs neatly tucked inside the lip of the chassis.
8) Run another wire from the tip (innermost) lug of the fuseholder to one
upper terminal of the ground switch (probably should be the right-hand
side in order to make sure the other black PT primary will be long enough
for step 10.)
9) Run still another lead from the ground sw right side lug from step 8
to the lug on the convenience outlet with a *brass-colored* screw.
10) Attach the other power transformer primary (black) to the left side
lug of the grounding switch. Run a jumper from that (lefthand) lug to the
convenience outlet lug with the *silver-colored* screw.
11) Strip about 4 inches of the power cordset outer insulation and neatly
remove any paper or fiber wrapping. Insert the cordset through its
chassis hole and attach the green wire ring terminal over the rear right
stud holding the power transformer. Use a lockwasher above and below the
ring terminal and top it all off with the original nut. Tighten securely.
12) If possible, twist the black and white cordset leads and solder the
black cordset lead to the convenience outlet terminal with the brass
screw, attach the white wire to the silver-screwed terminal. Make sure
you don’t have too much exposed wire between the end of the insulation and
the solder terminal here…it’s really pretty easy to short across to the
other 120V leg.
13) Secure the cordset to the chassis with the strain relief. If the new
wire is much thicker than the old, you may have to hog out the inside of
the strain relief with a rattail file.
14) Inspect all work, make sure wire ends are neatly clipped and all
residue removed from the chassis (it’s all too easy to inadvertently
damage the insulation on the existing wiring by slipping with the
iron…you’ll have to repair *any* insulation burns with heat-shrink
tubing), replace the fuse and speaker, hold your breath and power up.
Note: if you follow these instructions, you’ll still have the appearance
of the ground switch but it won’t be functioning. No problem as long as
the venue you’re playing is wired to code. Always try to use a 3-prong
outlet, and if it ABSOLUTELY HAS to be a 2 prong outlet, use a “cheater”
adaptor on your cordset and DON’T under ANY circumstance defeat the
polarity of the cheater by filing down a prong (note that one prong is
wider than the other).
Good luck and work safely!
–Mike Schway
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Mike Schway | [visualize your favorite quote here]
mschwa–(at)–as.com |
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