fender stage 100 head loud input hum

[sidman]

amp powers up fine everything quiet until you plug a cord in the input then get a loud hum, volume or any other controls has no effect and has no output sound when guitar plug in, any help would be great..thanks

[52 Bill]

Welcome to the place.

amp powers up fine everything quiet until you plug a cord in the input then get a loud hum, volume or any other controls has no effect and has no output sound when guitar plug in, any help would be great..thanks

That amp has an fet mute circuit that kills the signal to the power amp until there is plug inserted into either the input jack or the power amp in jack. Try plugging the guitar into the power amp in jack and see if the hum is still present and if the power amp will pass a signal.

[sidman]

Hi and thanks for getting back to me, im not sure which one is power amp jack, but i did plug the cord into the fx loop return and touch it had input with no hum, the only time it hums is when its plug in the front input jacks, with nothing plug in everything is guiet.

[52 Bill]

That would isolate the problem to the preamp circuit.

What is your skill set regarding doing electronics repairs?

[sidman]

im fairly skilled know how read diagram, iI know what all the components are,,

[52 Bill]

Do all of the switching modes work, clean, dirty, more dirty, dirtier, and do all of the leds light up?

First test would be of the low voltage power supplies. There are two 16 volt (plus and minus) located at TP8 and TP9 and one 5 volt (positive) at TP11. Pull the chassis and carefully read the voltage at these test points to ground with a voltmeter.

[sidman]

theres 2 red leds on the board that isnt lite up, tp8- 0 volts pt9- -5volts and tp 11 -2volts,,

[sidman]

ok I ck I do have voltage on u1 pin 4 and 8 is 15.6 v, all switches seem to work ok foot switch and drive lights ok, does the rev pan have to be hook up while im checking voltage cause i have it unhook so i could get chassic out?

[52 Bill]

ok I ck I do have voltage on u1 pin 4 and 8 is 15.6 v, all switches seem to work ok foot switch and drive lights ok, does the rev pan have to be hook up while im checking voltage cause i have it unhook so i could get chassic out?

I don't understand your readings please be more clear in your responses. Do you have positive 16 volts dc and negative 16 volts dc and positive 5 volts dc?

And no the reverb tank can be left disconnected for your tests.

[sidman]

Sorry about that. On u1 pins 1,2, and 3 is zero voltage. Pin 4 is neg 16.1. Pins 5,6, and 7 are zero voltage. Pin 8 is positive 15.8. It doesn't matter if input cord is plugged in or not. I appreciate all the help. Thanks, Don

[52 Bill]

Sorry about that. On u1 pins 1,2, and 3 is zero voltage. Pin 4 is neg 16.1. Pins 5,6, and 7 are zero voltage. Pin 8 is positive 15.8. It doesn't matter if input cord is plugged in or not. I appreciate all the help. Thanks, Don

Okay, you have the plus and minus 16 volt supplies, how about the plus 5 volts?

Now that you have tested U1 for voltage on the inputs and outputs, you need to do the rest of the signal ICs in the preamp section.

[Jazz P Bass]

[QUOTE=sidman;410587]theres 2 red leds on the board that isnt lite up, QUOTE]

If you are speaking of LEDs D9 & D10, they only illuminate when a signal is being passed through the amp.
(ie: they are clipping diodes)

Speaking of: do you have a setup for passing a signal through the amp?

All of the Test Points in an oval are for measuring a signal level (Volts Ac)

Oh, here is the schematic for those of you following along at home.

Stage 100-160 DSP.zip

[sidman]

Okay, you have the plus and minus 16 volt supplies, how about the plus 5 volts?

Now that you have tested U1 for voltage on the inputs and outputs, you need to do the rest of the signal ICs in the preamp section.

ok I ck rest of the other ic"s and they have tier plus and minus 16 volt, you mention the plus 5 volts? did not have it on ic 1

[sidman]

I have no setup for passing a signal through the amp, and thanks for the schematic that will be a big help also, many thanks

[Jazz P Bass]

The +5Vdc is to be measured over at the connector P19 (quadrant 2_A).
It powers the DSP board.
The DSP board must function to pass the signal out.

If you do not have a function generator to supply a signal, any signal source will do.
IPad, your phone, a CD player.
(Keep the signal amplitude at 25mv's as the schematic suggests.)

Or strum the guitar.

Your volt meter, set to read Vac, will work very well.
At the input jack the signal can be read to get an idea of how the voltages appear.

The fact that FX Return works would lead me to jump directly to P19.
Is the signal at L(-)/ L(+)?
(If no, then the problem is before P19)
If yes, is the signal at L_Out?

If No, then you have a DSP board problem.