Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chuggy on December 20, 2013, 08:40:56 am
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I am about to start on a Gibson GA 45 Maestro that has an eyelet board with resistors on the front and caps on the back. I am looking for tips and tricks for working on the back side-
Remove everything from the control panel and bend the board forward on the tube socket hookups?
How do you support the board to work?
Replace stuff on the back by mounting on the front?
Thanks
Jim
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Maybe post a picture. I myself have no idea what you're talking about.
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What kind of work will you be doing? Do you need to have power on while the board and control panel components are hanging in the air? If you're just gonna wholesale replace all caps on the back side of the board, just do it, then reinstall everything to check it out.
I use whatever nonconductive material is handy to prop up boards and components if power needs to be applied. Cardboard, paper file folders, wooden/plastic rulers, etc. If you need to keep control panel components in place while troubleshooting, consider making a front panel using cardboard.
Mostly just use common sense and work carefully.
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Does it look like these images: https://www.google.com/search?q=Gibson+GA+45+Maestro&rlz=1C1RNBN_enUS433US433&espv=210&es_sm=122&source=lnms&tbm=isch&sa=X&ei=bYy0UoD9NLOwsATxm4CICA&ved=0CAkQ_AUoAQ&biw=1280&bih=628 (https://www.google.com/search?q=Gibson+GA+45+Maestro&rlz=1C1RNBN_enUS433US433&espv=210&es_sm=122&source=lnms&tbm=isch&sa=X&ei=bYy0UoD9NLOwsATxm4CICA&ved=0CAkQ_AUoAQ&biw=1280&bih=628)
One option is to first make a careful layout drawing and then remove all leads from the board to pots and tube sockets, etc. Then entirely remove the board. Probably that will be less work in the long run; and be more convenient to work on both sides of the board without stressing connections.
If you want to power up the board to test it before replacing it in the amp, then do what sluckey said.
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It is the same amp has pictured here.
http://www.flickr.com/photos/99589251@N07/sets/72157634858993359/ (http://www.flickr.com/photos/99589251@N07/sets/72157634858993359/)
Note a paucity of caps on the board- most are on the back. Also, most of the hookup connections go to the back.
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It is the same amp has pictured here.
http://www.flickr.com/photos/99589251@N07/sets/72157634858993359/ (http://www.flickr.com/photos/99589251@N07/sets/72157634858993359/)
Note a paucity of caps on the board- most are on the back. Also, most of the hookup connections go to the back.
What are you doing to the amp>? Does it have original caps? I've had to overhaul quite a few old Gibsons and the best approach I've found is to replace all the tone caps (they are usually very leaky Astrons), though you can test for leakage first if you really must...but they will probably all leak a lot, replace all the electrolytics, replace plate load resistors, and check other resistors for value and replace as necessary. I draw a scratch layout as far as where the connections off board go so I know how to hook it up again when it goes back together. It can be figured out from a schematic but it is quicker and easier to make yourself up a scratch layout. The old Gibsons aren't fun to work on.....they should have put the caps on top....dummies!
Greg
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That was a pain.
I removed all of the controls and jacks from the panel, left the the socket connections untouched, and folded the board out. I could see the caps but had poor access to the far side board eyelets. I decided to focus on the cathode bypass caps, and caps for first gain stage of the non-trem channel. The trem circuit on this amp has more than 10 caps, all stacked up. I abandoned efforts at artisanal repair, cut the leads close to the cap bodies and soldered to the original leads. The power supply caps were off the board and routine. Now back together and everything works and sounds fine.
More extensive work would seem to require removing the board entirely. This would require wholesale disconnection of the socket side hookups or removal of tube sockets, some of which are riveted with shock mounts. Not for me.
Jim