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Post subject: G-DEC Squeals
Posted: Sat Nov 08, 2014 5:42 pm
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My friend asked me to look at his GDec. He said it just recently started doing this. When you turn the volume up, it squeals like it's getting feedback. No inputs connected. I reset to factory defaults, no difference. If I play with the effects, the squeal changes tone and amplitude. So it really sounds like "feedback" (or maybe analog delay in the DSP), but there are no inputs connected.
Also, it sounds too bright to me. I don't know if it's normal, since it's not my amp, but with tone centered, there is a lot of hiss as the volume comes up. Is that normal?
I have owned a few guitar amps in my time, Peavey, Fender Princeton, small Marshall practice amp, and a long time ago I worked in a large music store repairing guitar amps, so I know what the noise floor is supposed to sound like in guitar amps. And this one seems too hissy.
I have the schematic (found it online) and it's all digital inside except for some opamps at the analog connections. So not much I can do to troubleshoot.
Anyone have any ideas about this?
Thanks!
Dan


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Post subject: Re: G-DEC Squeals
Posted: Mon Nov 10, 2014 8:48 pm
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I figure this one out. I'll post what I found in hopes that somebody else may benefit.
I popped the amplifier unit out, and found that I could flex the chassis very slightly and cause the squealing to start and stop.
Looking at the PCB side facing me (up), and poking around with a plastic poker, I couldn't find anything on the board that was mechanically sensitive.
I did find that one end of C43 was NOT SOLDERED. The solder had balled up on the part but the pad on the PCB was still bare copper. Turns out this cap is just a high frequency filter, so you would have a hard time hearing anything. It's a good thing it wasn't a more important cap. But jeez, where's the QC at Fender?
You can imagine how disappointed I was when that did not fix the squealing issue.
Some of the solder joints looked questionable, so I resoldered most parts on that end of the board (the end with the instrument input and the headphone output). Didn't help.
So I removed the entire main board assembly, and resoldered most stuff on the bottom side, again just on that end of the PCB. The solder on the ICs all looked pretty bad, the chip parts looked okay. But I hit everything I could. Cleaned it off with some flux cleaner, cleaned out the pots while I was in there, and the squeal was gone. So it was definitely a bad solder connection on the back side of the PCB on the input end of the board.

By the way, to remove the electronics:
5 screws on the wooden backing board behind the speaker, then remove the wood cover;
3 wood screws along the top edge of the back of the unit;
4 screws with the cup washers on the top of the unit. Note that these have the flat heads and pointy ends. The ones on the back of the unit are pan heads and are flat on the threaded end.
Pull the wires off the speaker, and slide the electronics out through the back.

To remove the main PCB:
Remove the four knobs, remove the nuts and washers;
remove the two plastic nuts on the input connector and the headphone connector.
Disconnect the two ribbon cables that connect to the front control assy (which will stay in the unit). One of these I removed after I had the PCB assy loose, as it was hard to get to. Also, that one, it was a brownish color, uses a locking FPC connector, so be careful to release the latch before you pull the cable out. If unfamiliar, look up FPC connectors and see if you can find instructions.
Two screws + lockwashers + nuts holding the heat sink to the main chassis. The screw access is on the bottom of the chassis.
Two screws holding the PCB to the chassis. These are on the corners of the PCB.
Unplug all of the other connectors on the back of the board, and the two speaker outputs.
The board assembly lifts out.

I hope this helps somebody else. I have to think there must be a lot of intermittent problems out there, judging by the poor soldering in this unit. Really bad.

Dan


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Post subject: Re: G-DEC Squeals
Posted: Mon Nov 10, 2014 9:40 pm
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Great to hear that you solved the problem with your G-DEC.

Could you give me the link where you found the schematic ?

Thanks.

Jerry

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'62 Jazzmaster/'78 Precision Bass/'88 Fender F250 acoustic/'07 Gibson Less Paul Classic Antique/'12 Squier Strat/'14 Squier VM Jaguar/'15 Classic 60s Telecaster/'68 Showman Reverb/'70 Bandmaster Cabinet JBL D140F speakers/'69 Super Reverb/'12 G-DEC 3-30


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Post subject: Re: G-DEC Squeals
Posted: Tue Nov 11, 2014 1:06 pm
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Schematic:
http://support.fender.com/schematics/gu ... ematic.pdf


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Post subject: Re: G-DEC Squeals
Posted: Tue Nov 11, 2014 2:34 pm
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dnmeeks,

Thanks for the link.

Jerry

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'62 Jazzmaster/'78 Precision Bass/'88 Fender F250 acoustic/'07 Gibson Less Paul Classic Antique/'12 Squier Strat/'14 Squier VM Jaguar/'15 Classic 60s Telecaster/'68 Showman Reverb/'70 Bandmaster Cabinet JBL D140F speakers/'69 Super Reverb/'12 G-DEC 3-30


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