Episode IV: A New Hope For A Pair Of GDEC Jr. Amplifiers

By: Brian Lojeck, brian@lojeck.com, 12-17-08

1: Who am I?

My name is Brian Lojeck, I'm an Electrical Engineering student at Cal State, Long Beach, and I'm trying to teach myself to play guitar. I don't have a lot of audio knowledge, but I'm good with Google. ;-)

2: WTF, dude?

I recently purchased a pair of damaged Fender G-Dec Jr. guitar amps on Ebay. The G-Dec series is modeling amps, with an audio processing chip to simulate classic guitar amps in a small package. Similar to the Roland Micro Cube I repaired in the previous article, the G-Dec is more feature rich, with more amp models, a drum machine, and I beleive a recording option, so you can play guitar with yourself. It's also larger then the Micro Cube, which is bad in some ways (can't toss it into a backpack), but better in others (it comes with an 8" speaker, and with some work I think I could fit a 9" or 10" in there.

I noticed, while trolling ebay, that there are a lot of Fender amps available with holes punched in the front cloth and speaker. Most of these are listed as "does not power up", and have dents near where the power cord plugs into the amp. Fender does a lot of assembly overseas (like Indonesia), so I imagine when they get a boatload of amps delivered they test them, and when one doesn't power up in their test they damage the speaker to discourage dumpster divers. I have no idea where all these people get these broken amps to sell on Ebay, but they do all seem to be in California, near Fender's dumpster in Corona, CA.

The thing is, when a brand new piece of electronics doesn't power up, the issue is usually a loose wire or blown fuse. That was my hope when I placed my order. I'm not a gambler, but I'll gamble $15 on a busted guitar amp. (The seller is local to me, and kind enough to let me pick it up to save shipping.)

 

3: The Fender G-Dec Jr. Disassembly and Investigation

A. Uncrating
 

 

The two amps were listed as "new, never used", and that appears to be true. Apparently these amps arrive with the tags already hung. I'll take measurements at a later date.


Figure 1: The two amps
 

 

The top of the amp.


Figure 2: The top
 

 

A closeup of the controls. You can see the dent they make near the power socket near the top right of the image. Perhaps they dent the sheet metal like this so somebody like me can't turn it in for warranty repair.


Figure 3: Closeup of Controls
 

 

Speaking of which, I wonder if they'll honor the 5 year transferable warranty.


Figure 4: 5 years, huh?
 

 

Damage to the power socket of one amp


Figure 5a: Power Socket Damage
 

The other amp's power socket, also damaged.

Oddly enough, the power input is listed as 50 Watts for a 15 Watt amp. I realize they overengineer these things, but that's pretty heavy...


Figure 5b: Power Socket Damage
 

I tested the amps by applying power to the socket, and turning them on. No LED's lit up, but when I turned them off the speaker made a "wuff" noise. This makes me VERY happy, because it means the power distribution system could be in good shape. Most likely, a bad fuse is preventing power from reaching the control panel.

(that's foreshadowing, by the way)

 

 
  The tolex is mostly in good shape, but the top panel is covered in a funny way that lets an edge peel up. The corners are done rather badly as well, making me wish they made this amp with metal corners, or, like the Micro Cube, plastic or metal covers over the ENTIRE corner from front to back.

Figure 6a: Tolex Damage
   

Figure 6b: Tolex Damage
B: Opening The Case
 

 

The back panel opened with 8 screws. The case is made of chipboard and plywood. Overall the quality of construction was not up to par with the Micro Cube, but the differences are not huge.

The speaker is 8", 8 ohms, and has a hole in it. No car audio gear this time, but they do make full range speakers for home audio at 8 ohms...


Figure 7: Back Panel Removed
 

 

The metal control board is held in place by 4 bolts in the top. Two bolts are the black ones near the bottom of the image, and the two bolts holding the handle on also attach to the metal plate.


Figure 8: 4 Bolts To Remove The Control Panel
 

 

Plenty of room for a bigger speaker...


Figure 9: Interior of case
 

 

Turn the metal control panel over, and this is what you see. The front of the amp is at the bottom of the photo. That metal plate sticking up in the lower left...


Figure 10: The circuit board
 

 

... is a heat sink for what appears to be power regulators. The way this is positioned in the amp, it gets in the way of a larger speaker being placed in the amp. Perhaps it can be bent or cut, I'll have to whip out the IR thermometer to run some tests later.


Figure 11: Heat Sink
 

 

Plenty of adhesive used on the board to keep everything in place.


Figure 12: Circuit Board Epoxy
 

 

The power transformer is lead-free. That's good if you like to lick transformers, you won't get all retarded and slow.


Figure 13: Lead Free Transformer
 

 

Apparently the guy who tests boards wasn't there that day, but the guy who puts the "tested" sticker on was, and still working hard.


Figure 14: Tested
 

 

To remove the circuit board from the metal chassis you have to pull all the knobs off the controls, and remove all the retention nuts. Watch out, there are a lot of washers both above and below the metal chassis.

There are also several small screws holding the circuitboard in from the underside, including one that's a bit tough to get to under the transformer. Each of these screws has a star washer as well, don't lose them.


Figure 15: Knobs and retention nuts removed
 

 

I call this the "underside" of the board, even though it's really on top when the amp is assembled. This is the side hidden nearest to the sheet metal. I had to check to make sure there was nothing important hidden on this side, there wasn't.


Figure 16: hidden side of board
 

 

Do you see how some solder joints are smooth and shiny, and others are rough and dull? That's REALLY bad soldering. It shows a lack of care on the part of the technician, and a lack of quality control on Fender's part. These joints should be redone. I'll have to debate if it's worth the effort. If I hear any odd noise or static this will be my first place to try and fix it.

Dull solder joints are made by not letting the iron get hot enough, or not heating the joint enough.They are brittle, and often less conductive. If I were Fender I'd be pretty ashamed by this photo.


Figure 17: Poor solder job
 

 

Now that I had the board out, I could look for likely points of failure. I didn't see any lose wires, or blown capacitors, or toasted resistors. I didn't smell any burning IC's, and it didn't look like anyone had let the "magic smoke" out of any of the chips.

The board had one of those little tubular glass fuses on it, but it was whole, and tested at 3ohms on my multimeter, a reasonable value. (if it was popped off it would read as an open circuit)

Then, after much looking around, I found a pico-fuse. This is a tiny fuse, about the size of an axial 1/4 watt resistor. It's soldered onto the board (I'll show you where in a second.) It looked fine, but these don't always burst into flames when they fail. You have to test them.

The natural instinct is to jumper across the pico-fuse's leads with a wire and see if the system comes online.

THIS IS WRONG! NEVER EVER EVER EVER EVER EVER SHORT ACROSS A FUSE!!!!!!!! This is dangerous, and may very well subject you to dangerous current levels that the fuse is meant to prevent. It might start a fire, or you might even die!

 

 

 

... so, I take a bit of wire, put tiny hooks on the ends, and use it to short across the two leads of the pico-fuse

The red wire is my jumper, and the little grey-green cylinder just behind the two blue cable links is the pico-fuse.

The black plastic object on the left is the back of the power socket, and the transformer is just offscreen to the left. With that, and the Fender logo on the board, finding this in your own amp should be easy.


Figure 18: jumpering the fuse
 

 

With that jumper in place, the system powered right up.

 


Figure 19: Power On!
 

 

So, after a week of waiting, the pico-fuses I ordered arrived. I was lazy, so I decided to solder the new one in place without removing the whole board from the chassis. This is normally not a good idea, but it's not in a bad place, so we should be ok.

 

 

 

 

Here you see where I removed the two cables that would get in the way. I don't know if polarity matters here, so I marked them. The next photo I took is too blurry, so I'll just tell you that I took a sharp pair of diagonal cutters and cut the old fuse's leads as close to the fuse as I could. Then, I used the two posts that remained in the board as solder points for the new fuse, and trimmed any extra leads left after the soldering was done.


Figure 20: Cables moved to access pico fuse
 

 

 

Here's the new pico fuse, soldered in place and ready to go.


Figure 21: The new fuse in place
 

 

Now comes the time to fix the speaker cone. Here's the hole punched in it.


Figure 22: Hole in speaker
 

 

I removed the loose material


Figure 23: The hole, cleaned
 

 

...and to prevent stress tears from forming in the corners, I punched round holes at the corners. This may be overkill, but I have some training in aircraft maintenance, and they were big on overkill.

The edges of the hole are a bit rough, next time I think I'll use an xacto blade instead of scissors.


Figure 24: Stress Relief Holes
 

 

Then, with some bits of paper towel and a 2:1 mixture of White PVA glue and water I patched the speaker front and back.

I blotted away excess glue after this photo was taken, although I think the glue that soaked into the paper may help to keep the patch strong.

The glue took 2 days to dry fully (it was very cold here in So. Cal.), at which point I laid on a couple layers of spray paint (front and back)


Figure 25: Hole Patched
 

 

Here's the completed patch. I tested this by playing the speaker at high volume for about 12 hours, it showed no signs of degrading.

Oddly enough, the paper towel/glue/paint patch feels VERY similar to the paper material the cone is made of...


Figure 26: The completed patch
 

 

Here is the amp, reassembled for the torture test I mention above.


Figure 27: amp ready for testing
 

 

Some new grill cloth, and the amp is ready for Ebay. In fact, the auction is going on as I write this.


Figure 28: The amp, ready to sell
   
  Questions or comments? Email me.