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Author Topic: 1989 Fender Power Chorus  (Read 2841 times)

BobS

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1989 Fender Power Chorus
« on: November 23, 2016, 09:55:58 AM »
Hi!  I bought this amp back in the late 80's and played it loud--in a garage setting--frequently back then.  I put the guitar down for many years (wife, kid, house payment, etc) and recently got back into it.  When I dug the ole amp out, I--of course--had to clean all of the pots.  I also ended up having to replace 2 of the plastic input jacks.  While I was in there, I noticed that the 2 power diodes looked a little crispy around the edges (photos below).  Flipping the main board over, you can see where the traces are pretty burned up.

The amp still operates (a few of the pots are a little scratchy still...probably need another blast of cleaner).  I know this is probably an odd question to ask but in your collective opinion, what performance issues can I expect with the diodes and board looking the way they do?  Aside from the couple scratchy pots, there is an annoying hum thats new from when I played back in the day.  The hum volume increases and decreases with the volume control and is present on both channels even when the guitar isn't plugged in.  I was assuming it was a bad ground but with those traces burned up, I didn't know if that could be the cause of my trouble.  (I know heat can mess up conductors.)


« Last Edit: November 23, 2016, 09:58:44 AM by BobS »

g1

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Re: 1989 Fender Power Chorus
« Reply #1 on: November 23, 2016, 02:20:31 PM »
  The diodes and nearby power resistors are nothing to really worry about.  I would clean off the old solder and re-solder them with fresh solder.  Their job is to regulate voltage by converting excess voltage to heat.  If you really want to replace them, you can raise the new parts off the board a little so less heat gets transferred to the board.
  As far as the hum, did you use the exact same replacement jacks?  Did you triple check your work & check for any cracked traces or pads going to the jacks?

BobS

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Re: 1989 Fender Power Chorus
« Reply #2 on: November 23, 2016, 04:43:17 PM »
  The diodes and nearby power resistors are nothing to really worry about.  I would clean off the old solder and re-solder them with fresh solder.  Their job is to regulate voltage by converting excess voltage to heat.  If you really want to replace them, you can raise the new parts off the board a little so less heat gets transferred to the board.
  As far as the hum, did you use the exact same replacement jacks?  Did you triple check your work & check for any cracked traces or pads going to the jacks?

Thanks for the info on the diodes!  As for the jacks, I ordered them from a company called Studio Sound Electronics.  They don't look exactly like the old jacks I pulled out--the old ones had a clear plastic top--but the pin configurations match and that was the part their site matched to mine.  I'm fairly new at soldering but I thought the jacks came out well enough.  Pic below shows the connections.  Excuse the flux splatter--I'm still getting the hang of soldering.

I'd actually replaced 3 jacks.  Inputs 1, and 2, and the foot switch jack.  Input 1 had loose solder connections from years of use and 2 basically shattered when I was putting the front facing nut back on after blowing out the pots (combination of using a nut driver, over tightening, and old, brittle plastic).  As far as potential problems, I had 1 issue with Input 2.  In the photo, it'll be the lower left pin of the left jack.  There was a metal o-ring conductor that sits on the board that the pin goes through.  It came off when I heated it up, though I was able to get it placed and soldered back in.  The traces seem to be okay.  I tried not to leave the heat on in any one spot for too long.

I'd love to figure out the hum issue.  It's a nice old US made solid state amp...and I'm the only one who has ever owned it.



Enzo

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Re: 1989 Fender Power Chorus
« Reply #3 on: November 23, 2016, 08:11:44 PM »
Careful.  It is not enough to match the solder pattern, the jack has to sit up off the board the proper amount so it lines up with the holes in the panel.

The correct jacks are readily available.

g1

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Re: 1989 Fender Power Chorus
« Reply #4 on: November 24, 2016, 02:54:13 PM »
  I'm not sure about your terminology, but it sounds like one of the pads got disconnected from it's trace.  The pads are what I think you mean by 'o-ring'.  Each pin is connected by solder to a pad.  The pad is just a widening out of the trace.
So you will need to restore the broken trace to pad connection.
This is usually done by scraping a bit of the green coating off the trace, then soldering a tiny bit of wire to each side of the break (in your case, one side of the break is the pad).
Also have a look at all the solder connections on the 'wings' of the pots.  They are sometimes used to carry ground connections and can cause hum issues (as can loose nuts on the pots).