Guitar Input not working on amplifier - Audio in port works fine

Thread Starter

jameskeich

Joined Jan 20, 2021
4
Hi All,

This is my first attempt on any such repair so please explain like I am a 5 YO. I have a guitar amplifier where the audio line in (white plug on the board) works fine and as intended, whereas the guitar input does not work. Cable works fine, everything obvious is ruled out . It does not produce any sound whatsoever when plugged in into the guitar input (big black jack input on the board). IMG_20210120_143724.jpg

I have a multimeter and could solder anything if need by. I have taken pictures and attached the top and bottom of the board.
IMG_20210120_143724.jpgIMG_20210120_143628.jpg I have noticed some discolouration on the board (attached) where a couple of resistors sit.
IMG_20210120_143736.jpg

I have checked the with the ohm meter and for both resistors I get 100.0 (attached).
IMG_20210120_143920.jpg

Any ideas? Any help would be greatly appreciated.

Thank you and Regards,
James
 

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MrChips

Joined Oct 2, 2009
23,507
Without the circuit schematics you will have to reverse engineer the board.

Take photos of the green side of the board looking straight down at the board, and in three sections left, middle, and right close up.
 

Dodgydave

Joined Jun 22, 2012
9,951
Check the jack socket and the push switch marked on the photo below , A to B when the guitar is plugged in , and C to D on the switch on Ohms or Diode test.

Polish_20210120_193845341.jpg
 

MrChips

Joined Oct 2, 2009
23,507
Thanks for posting the photos. Unfortunately I strain my eyes looking at the photos. They are blurred or out of focus.

Try holding the camera further away from the board (about 2 ft) and use the zoom control to get a magnified view. And hold the camera very steady.
I need to be able to read the numbers on the black chips (or you can tell us the numbers).
 

abrsvc

Joined Jun 16, 2018
74
The small ICs are likely 4580 Op Amps and are very common. If there are no switches on the unit to change the input, I would first look to the IC closest to the socket. Pin 1 is the output for pins 2 and 3 (inputs). Pin 7 is the output for pins 5 and 6 (input). Pins 4 and 8 are the power supply and can be +/- when compared to ground. Since the guitar input is at a lower level, the Op Amp is the first stage to get it to the same level as the audio input. This is a common failure usually found when using active pickups.
 
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