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July 17, 2024, 02:54:11 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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A   1.141
B   1.10
C   0.838
D   0.564
E   0.558
F   0.364
G -37.63

I've done some additional testing around the amp to see what I can find.  I noticed that when the amp is powered on, both of the terminals on C52 (one of the two 3000uF capacitors in the rectifier section) both connect to ground per my continuity tester.  When the amp is off and the caps are discharged, there is no connection.  But the moment the amp powers on, the resistance reads 0 and they have continuity connection.  So such thing happens on C53.  When I remove C52 from the circuit, it reads as expected.

C and D for the power transformer function how I think they should.  When the amp is off, the two terminals have continuity.  When the amp is on, the have a resistance of around 128E. I'm not sure if that tells me anything but it was something I observed. 


As far as the additional testing, you should never try to do resistance measurements when unit is powered up.  You could damage the meter, and the readings are meaningless anyway.  Also applies to batteries or capacitors that are charged.  Or anywhere voltage is present.  Resistance measurements are only done when power turned off, and power supplies given time to discharge.

The voltage readings on the bias string indicate something is wrong with R47 or it's connections.  36V across it (points F to G) would mean 13mA through that string, That in turn would cause approx. 20V across R46, which you do not have.  What does R47 resistance measure?


Thank you for the testing tip!

Also, I think this may be the culprit.  I tried the resistor against 2 different multimeters after removing it from the circuit.  Both registered 140k.  The resistor color (red, green, purple, brown, gold) aligns to the 2.7k that I'm expecting.  So I'll look to replace that. 

I generally use 1/4 watt resistors in all of my guitar pedal builds. I don't see anything on the schematic that denotes the wattage on these resistors.  Is there a general standard I should be adhering to?  The resistor I remove looks about 25% bigger.  Would that be a 1/2 watt?


Yes, they would likely be 1/2 watt resistors.
It's a bit odd they are using 5 band color code (usually precision) resistors.  With a green band in there it would be 2570 ohms.  I would have expected regular 4 band color code with red-violet-red and gold for tolerance.


I ended up running to my electronics store (yay big city living), picking up a pack of 2.7k 1/2 watt resistors, and getting them into the amp.  It works like a champ now!  Thank you so much for the help and direction.


Great to hear you fixed it. Good work G1  8)