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May 28, 2024, 01:01:16 PM

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Repairing a Gibson G-105 Guitar Amp

Started by Timko, May 15, 2024, 09:46:49 PM

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Timko

Hello!  I've been a pedal builder for the past 10 years, building both from PCBs in the DIY community as well as designing some of my own.  Last winter, I began to dabble in amps.  I first took on this Gibson G-105 from the 70s. I first worked at re-capping the thing.  In a poor decision by me, I used some cheap caps to replace the two 3000uF caps located in the rectifier section of the amp.  It sounded great for a while.  It was super clean unless you pushed it, and was phenomenal hooked up to an electric piano or guitar.

One night, the amp began to totally fuzz out, producing this gated, starved sound.  Sort of like when you start screwing with the voltage going into old Germanium fuzz pedals. Luckily, the amp has a signal line out right before the long tail pair phase inverter, and I validated that it sounded fine.  I did some sanity checking around the speakers to ensure that wasn't the issue. I changed the crappy caps in the rectifier to some nice Mallory screw in terminal caps.  I even changed out the 4 power transistors. 

I've gotten a little over my head in terms of what step to take next.  I'm pretty capable of testing, but I'm unsure of where to look next.  I'm hoping the people in this forum could help point me in a good direction to begin to look.  Thanks!

Schematic for reference:  https://el34world.com/charts/Schematics/files/Gibson/Gibson_g_105.pdf

DrGonz78

#1
List voltage reading for Q8 & Q9 at  all pin's connection points. Measure all power supplies. What are your -/+40v rails? -/+12v rails? Can you scope a signal through the unit and figure out where the signal goes bezerk?

Edit: If you scope the output is the fuzz actually crossover distortion?
"A person who never made a mistake never tried anything new." -Albert Einstein

Timko

Quote from: DrGonz78 on May 15, 2024, 11:01:19 PMList voltage reading for Q8 & Q9 at  all pin's connection points. Measure all power supplies. What are your -/+40v rails? -/+12v rails? Can you scope a signal through the unit and figure out where the signal goes bezerk?

Edit: If you scope the output is the fuzz actually crossover distortion?

Thanks for the reply, and sorry for my belated response.  I had someone at my company walk out, which pretty much threw my last two weeks into a tailspin. I checked the power rails, and both have -/+40v and -/+12v I expect.  As for the long tail pair, here are the readings I got from them with a 1kHz signal at 5v:

Q8
C -0.704
B -0.127
E 38.83

Q9
C -0.713
B -0.137
E 39.35

I'll admit my oscilloscope skills are lacking in terms to how to best use it to figure out  where the signal goes off of the rails.  I did put a scope at the end of the chain as suggested, and I can see that the bottom part of the sine wave appears to be clipping as it appears to be pretty flat. I'm currently looking into how to measure crossover distortion via watching some youtube videos, but any suggestions you can provide to an amp noob would be greatly appreciated.