Fender stage 100 ? very weak dist sound

[Jam]

I have a Fender stage 100 and all it does is if you turn it all the up and hit the strings you get a bad dist crackle sound comes and goes away .but if the volume is lower you get nothing I dont know where to start

[Enzo]

Plug the guitar into the effects return and play. Is it clear now?

If it is clear there, then the preamp has issues. If not clear, we need to look farther.

Look inside at the main board. Towards the rear, and not too far from the fuse, there are two red wires connected to the board right next to each other. In front of them are a couple caps a couple resistors and a couple diodes. The diodes are D62 and D63 - should be printed on the board next to them.

COnnect your volt meter to chassis, then probe the voltage at each end of those two diodes. The far end should show zero volts, or darn close to it. The front end of the diodes should each show about 16 volts - one positive and one negative. If one is missing, there is our problem.

If the voltages are persent, then we start looking for DC on the output pin of the op amp ICs.

[Jam]

stage 100

I pluged in the back and nothing so I checked the diodes D62 and D63 and I get about 15.9 + and - volts so the voltages are persent

[Enzo]

Then ground your meter, look at every 8-leg dual op amp on the board for DC on the output pins, pins 1 and 7. Unless ut drives an LED or something, it won't have a DC offset.

[52 Bill]

Sound more like a bias problem in the power amp. The outputs are not conducting until the're hit with a big signal.

I'd check the output and driver transistors, as well as the entire output section for any loose or broken solder joints.

[Enzo]

I dunno, maybe. The least bias you can have is shorting the bases together. And the signal only has to overcome a half a volt to turn the output devices on. Crossover distortion for sure, but you only lose a half volt out of the 30-40 or whatever the rail is.

[Jam]

stage 160

When you say 8 leg dual op amp are those the little black things do they call them IC chip ? I not real good with this stuff when you said DC is that still voltage? on the leg 4 and 8 I get a -16v and +16v and their are 7 of them

[Enzo]

Yes, those. Pins 4 and 8 should have +/-16v, so yours have the correct power. However, the output pins of those ICs are pins 1 and 7 on each one. CHeck those pins for DC voltage. Yes, when I say look for DC, I mean look for DC voltage. There is one exception, U7 will have large DC voltages on the output pins since it is in the footswitch circuit.

I notice you now say Stage 160 instead of Stage 100. WHich one do you have? The circuits are the same except the power amp.

[Jam]

Stage 100

Sorry its a stage 100 I check all the Ic chip outputs pins1and 7 they all read nothing but U6 reads 4.4 V on pin 1 and U7 reads 15.2 V on pin 1 and 14V on pin7. When you say power amps or pre amps are those the ic chips ?

[Enzo]

Power amp and preamp are portions of the whole circuit. Just like on your stereo system at home, the preamp is hte part with al the controls - volume, tone, etc - and the power amop is the part that boosts the signal for the speakers.

U7 is supposed to have those voltages, as I pointed out.

U6 should not. Either U6 is bad or the power amp stage has problems. Before replacing U6 check the main power voltages. Look at the six larger transistors on the neat sink. Note each one has a metal tab coming out the top, and a mounting screw through the tab to mount it to the heatsink. Ther screw is insulated from the tab by a plastic washer. CAREFULLY measure the voltage on the tab. One of the left three and one of the right three should be enough. Just don't let your meter probe touch the tab AND screw at the same time, and don't let it touch the tab and the chassis at the same time.

The tabs should have about 45 volts on them, three positive and three negative. The voltage might be higher or lower, but both positive and negative should be about the same voltage. If they are way off from one another, we need to have the power circuits serviced.

[Jam]

Stage 100

Ok I checked the transistors Q14 and Q 16 I have on tab one 3.3v tab two +45.3V and tab three 0v and then Q15,Q17,Q18 have on tab one 1.9 V tab two -2.2 V and tab three 0V . Does that mean I should have -45V on the Q15,Q17 and Q19 ? Which ones are the power amp? and which ones are the preamp?

[Enzo]

By tab, I mean the metal part sticking out the top, the part with the screw going through the hole to mount it. I am not referring to the three legs that go to the circuit board. The center leg on each transistor is in fact connected internally to the mounting tab though.

Q14,16,18 should all have +45 on their mounting tab.

Q15,17,19 should have -45v on their mounting tabs.

-2.2 doesn't sound right. Measure again. If you don't get -45, then that is your problem, a missing power rail. Rail means voltage, not a physical thing.

All those large transistors are part of the power amp. The ICs are in the preamp

[Jam]

Stage 100

Ok I check the middle leg on Q14-16-18 I have +45.3 V and on Q15,17,19 I have -.2 V I do not have -45V

[Enzo]

OK, there is a problem to solve.

With power off, are legs 2 and 3 shorted together on any of the Q15 group? Probably not, but if so that means bad transistor.

Ober by the main filter caps, C63,64, there should be a resistor R158 47k. SHould be 45v and -45v on its two ends. The two caps each have one lead common, then the other lead will be one of the 45vs. Alternatively the anode end - non-stripe end - of diodes D56,D57 should have the -45v. If we have -45 there but not at the transistors, then there is an open copper trace somewhere on the board. If it is not there either, then the diodes would seem open. Doesn't seem likely, but anything is possible.

[Jam]

Stage 100

I dont see any legs shorted together it would look like? On resistor 158 one end I have +45.3V and the the other I have -.2 V. I not sure if I understand by The two caps each have one lead common, then the other lead will be one of the 45vs. ? I checked D53 and D54 on the non striped side and I get -30.1V on both But why does D53 and D54 have + 45on the strip side? What is open copper trace ? Sorry for all the questions Thanks