Boss GT-8 Tear Down

Started by Elantric, February 24, 2015, 07:15:38 PM

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Elantric

http://guitar-dreamer.blogspot.com/2012/10/boss-gt-8-repair.html

Boss gt-8 repair!
Hello there! Here's a boss gt-8 pedal board that came in with a broken level control. The level potentiometer(pot) is located at the back of the board with all the in/out jacks. Unfortunately, the pot is a little plastic thing and it's not bolted to the metal chassis... So, it's not difficult to break this thing if you use it quite often.  In this case, the pot was cut in half and didn't work anymore. So I bought a metal mini-pot (dual 50kA) which could be mounted to the metal chassis of the gt-8. When disassembling your pedalboard, be careful NOT to remove the on-board mini ROM battery as it this the one that keeps the memory settings and gt-8 firmware stored...
The tricky part was to unsolder the old pot and solder the new one in. I decided to solder 5 small leads to the pcb and then connect them to the metal pot.  This way, I could place the pot on the same spot on the back of the gt-8, without having to match the location of its lugs with the pcb traces(that would be impossible).
Once the new heavy duty pot was soldered, I secured it in place and tested that everything works. And it did! Now the level knob is sturdy and secured in place. It's way less fragile than the old one and I don't think it will ever break again.
So, if you encounter problems with your pedalboard's volume control pot, try swapping it with a heavy duty one. And if it gets a bit noisy when you turn it, just spray some contact cleaner on its back and it will work just fine.
Here is the schematic diagram of the boss gt-8:
http://www.eserviceinfo.com/downloadsm/44816/Boss_gt8.html



UPDATE!

Sorry I couldn't get clear photos using my phone...
But I could help you change the pot on your gt-8!

The 1st step is to download the proper gt-8 service manual and schematic:http://www.eserviceinfo.com/downloadsm/44816/Boss_gt8.html


Based on the schematic diagram, you need to replace the old pot with a stereo 50Kohm "audio taper" one. You could easily find a stereo (dual) pot on ebay or something. A small metal housing pot will fit in there and will last long(longer than the plastic ones). These pots have 6 pins/lugs.


I used this one (you need the A50K model):http://www.ebay.com/itm/Stereo-Mixer-Pot-16mm-Audio-Taper-Splined-Dual-Log-A-/120743329648?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item1c1cdcff70


You 'll have to unsolder the old pot and wire 6 short cables in its place on the pcb. That way, you can easily mount/bolt the new pot on the metal chassis of your pedal board.
?he 6 pot pins are shown on the layout included in the schematic (circuit board main).
You could use your multimeter on the beeper/continuity mode to trace the capacitor and Ground pcb traces. These are the pcb traces where the 6 pot lugs (or the 6 short cables connected to the pot) should be soldered. See the attached image for a complete guide of "what goes where". Mind you, I used a A50k stereo pot and NOT a A500K one as pictured below!



If you don't feel confident about this, save this email and visit a tech or an amp repair shop that could help you out.
Cheers,Thanos



And here are some pictures taken during the repair:

























Andrea Zok Dell'Aria9 ?????????? 2013 - 8:50 ?.?.
Hi! What type of potenziometer have you used? Dual Unit? 50k Ohm? 9mm? I have the same problem with my gt-8

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gothico manhathan16 ?????????? 2015 - 1:24 ?.?.
i haven't opened the gt8 yet. my friend told me that he discover some boss multi effects repair shop? am bout to bring my gt8 there to see what happened, also for me to be able to know? can it still be repair?


Thanos S.16 ?????????? 2015 - 1:33 ?.?.
Hello! Yes, it's a 9mm dual pot, rated at 50k and it is audio-taper(log). The one I used is made by the "Alpha" company. I bolted the pot on the metal chassis of the gt-8.


Thanos S.16 ?????????? 2015 - 1:36 ?.?.
Hello Gothico,
the gt-8 can be repaired, especially by a boss dealer or authorized service. The Volume pot is easy to replace and they will not charge you a lot. Don't try to fix it yourself if you are not familiar with electronics or you may damage something else. The repair shop will have to estimate how much the repair would cost and notify you before proceeding to the actual repair.
Cheers,
Thanos

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Thanos Sottos10 ?????????? 2013 - 1:35 ?.?.
Hello! Yes, it's a 9mm dual pot, rated at 50k and it is audio-taper(log). The one I used is made by the "Alpha" company. I bolted the pot on the metal chassis of the gt-8.

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attilio mucciardi14 ?????????? 2013 - 1:52 ?.?.
hI THERE!! IT'S POSSIBLE TO HAVE THE ELECTRICAL SCHEME OF THAT REPARATION? OR ALSO A PHOTO MOST CLEAR OF THE CABLAGE? tHK U!!

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Thanos Sottos14 ?????????? 2013 - 2:21 ?.?.
Hello!
Sorry I couldn't get clear photos using my phone...
But I could help you change the pot on your gt-8!

The 1st step is to download the gt-6 service manual and schematic:
http://www.eserviceinfo.com/downloadsm/44816/Boss_gt8.html

Based on the schematic diagram, you need to replace the old pot with a stereo 50Kohm "audio taper" one. You could easily find a stereo (dual) pot on ebay or something. A small metal housing pot will fit in there and will last long(longer than the plastic ones). These pots have 6 pins/lugs.

I used this one (you need the A50K model):
http://www.ebay.com/itm/Stereo-Mixer-Pot-16mm-Audio-Taper-Splined-Dual-Log-A-/120743329648?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item1c1cdcff70

You 'll have to unsolder the old pot and wire 6 short cables in its place on the pcb. That way, you can easily mount/bolt the new pot on the metal chassis of your pedal board.

?he 6 pot pins are shown on the layout included in the schematic (circuit board main).

You could use your multimeter on the beeper/continuity mode to trace the capacitor and Ground pcb traces. These are the pcb traces where the 6 pot lugs (or the 6 short cables connected to the pot) should be soldered. See the attached image for a complete guide of "what goes where".






If you don't feel confident about this, save this email and visit a tech or an amp repair shop that could help you out.

Cheers,
Thanos

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Esma Ezhar20 ??????????? 2013 - 4:47 ?.?.
Hi, do u know, how to repair my gt8 is keep on changing the sound by itself?

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Thanos Sottos20 ??????????? 2013 - 11:22 ?.?.
Hello Esma,
I haven't encountered this problem yet.
Your gt-8 has probably as software issue but it could also be the onboard memory battery. I would suggest checking the onboard 3.1V battery that's inside the pedalboard.
There's a way to check its voltage on page 4 of the service manual:
http://www.eserviceinfo.com/downloadsm/44816/Boss_gt8.html

If that's not the case, it could be a faulty switch, a software/firmware issue or a chip that's gone bad...

Cheers,
Thanos

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Anonymous13 ??????? 2014 - 11:30 ?.?.
Hello!
Where can I find plastic replacement buttons (like write, or exit) for boss gt8?
Mugur

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Thanos Sottos13 ??????? 2014 - 12:16 ?.?.
Hello!
These buttons are custom made for Boss, so I couldn't find them on the market.
There are two ways to address this issue 1) The best solution would be to contact Boss and ask them who sells replacement buttons. Maybe they can even send you a free set. 2) The 2nd DIY solution would be to remove and old button and search for a new one that has the same size and shape. Then, you can replace the old ones very carefully.

Cheers


Anonymous20 ??????????? 2014 - 10:56 ?.?.
Hi Thanos, I have a problem with my Boss GT 8. The volume /expression pedal is crackly and sounds like theres a bad contact somewhere. Sometimes I cant get any sound at all, then all of a sudden it crackles in at full volume and frightens the life out of me. Is thus the same issue as you mention in the top post? Thanks
Garfield


Thanos Sottos21 ??????????? 2014 - 5:30 ?.?.
Hello!
Yes, it could be the same issue and this article is a guide on how to have it fixed (diy or by a technician). Your gt-8 should be checked for broken solder joints or anything suspicious before you replace anything. I would check the volume pedal circuit first, clean it with some oil-free contact cleaner spray and resolder any loose joints.

Cheers,
Thanos

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Anonymous22 ?????????? 2014 - 5:47 ?.?.
Hi
The foot pedal on my GT 8 has become way to sensitive. I have it set as a "Volume Wah". I can not fully press the pedal down without activating the WAH, no matter how lightly I press. It use to require some pressure to activate the WAH as it should.
Has anyone had this problem?
Thanks
Ray

Can someone tell me if there is a way to

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Elantric


TMAN

Here are some pics:

First is the schematic, pot wiring and the board holes id.