Fender M-80 SS combo amp channel switching LED problem

[Slobrain]

Hey fellas,

I have a Fender model M-80 1x12 combo from the late 80's and the channel switching light does not work. The channel will switch and the footswitch works too but I cannot get any voltage to the LED that will bring it on. I think the issue might be a switching chip Motorola MC14053BCP causing the issue but wanted to see if anyone has ever dealt with that type issue?

Thanks

[Enzo]

Why would you think that 4053 over there is affecting the LED over here? If I read correctly, you have all the switching function, just the LED won't light. Look at the schematic. Everything from CR4 on over to the right is working. All that is the switching circuits. Now the LED. The channel LED is LD3. The circuit is not complex. Current from the +16v supply flows through R57, 1.5k resistor, then through LD3 and on to the panel switch S1. If S1 is open, the circuit is broken and no current flows, so no LED light. If S1 is closed, then that completes the circuit, leaving the resistor and LED in series across the 16v supply.

So since the switch works the switching, we know it is OK. SO that leaves the LED or resistor being open, the solder to either being cracked, a trace connecting them all together is cracked, or the 16v is not getting to R57 to start with.

My guts tells me you have a bad LED or broken solder to it. If you are like me, every time you put the board back into the amp you catch the LED on the panel hole, which pushes it back, stressing the solder.

With the LED dark, which in your case means always, power on, measure for +16v on both ends of R57. Is it there? If so, fine. Then look for it at the anode of LD3. Present? Now the cathode end of LD3? If you have voltage at the cathode end of LD3, and no light, then either the LED is shorted, or the connection to the switch is open. LEDs very rarely short. Open LEDs all the time. LEDs test like any other diode, but they will have a higher junction voltage than a regular rectifier.

That is really all there is to this, a resistor and LED. Ground the bottom end of the LED, and it should light.

[Slobrain]

Thanks for the info Enzo,

I checked the voltages and then to my surprise my bad eyesight missed the bad solder joint on the board for the LD3, soldered it back and working now.

The Fender M-80 is a pretty good amp for being late 80's but the distortion lacks some low end the the channel switching pops some when switched. Other than that its got a nice sounding clean channel that I run a distortion pedal thru to get some good dirt sounds. Only paid about $105.00 total used at a pawn. Can't beat that deal...

Got any mods to beef up the low end on the distortion channel???