View Full Version : Fender stage 100 ? very weak dist sound
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
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.
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
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
04-17-2008, 11:53 PM
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.
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.
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
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.
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 ?
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.
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?
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
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
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.
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
There are two main caps, one filters the +45v and the other filters the -45v. Both caps go to ground, one has its + side to ground and the - side to -45v, and the other has its - side to ground and the + side to +45v. COnsequently each one has one leg to ground - so they have a common connection. There SHOULD be 45v on the non-ground ens of each cap, one of each polarity of course. You don't have the -45v.
I mention D56, 57. D54,55 are the +45 rectifiers, and your +45 is OK. I need to know that there is sone negative voltage on the non-striped end they are the -45v rectifiers.
Those two diodes come together on that end and connect to the filter cap C64.
R158 is connected vrom the hot pin of one cap to the hot pin of the other. It SHOULD have 45v and -45v on its ends. Your reading verifies there is no -45v.
Your circuits are on printed circuit boards. There are stripes of copper on this board, and they take the place of wires. The parts on the board are soldered to these stripes of copper. The stripes TRACE a pattern around the board, so they are called "traces." Just like a wire, a trace goes from point A to point B. If there is a crack or break in that copper trace, then the trace is said to be open - in other words the path is no longer complete. This is the same sense as we say a switch is open when it is off.
If D56,57 have some -30v or something like that on the end I specified, but there is no -45v at the cap and R158, then I have to think the circuit path from the diodes to the cap is open.
Note, if this is the case, it might just be a mechanical thingand something caused a crack. However, if we find the trace burnt open - the copper stripe got hot and vaporized away - then after we repair it, it might just burn up again from something we cannot yet detect that has ben wrong all along. We won't know until we get there. Hazards of the repair game.
Ok I checked C63 one side was -0V and +45.2V and C64 was -0. and -.2V and I also check I dont know if it helps I checked C66 had +22.8 V and -22.8V Iam not sue what they do
How about the two diodes D56,57. You won't find -45 therer, but if you have about -30 or so, then I really think there is an open between them and the cap. Since that -2 is pretty consistent all over, I think once teh voltage gets to the cap, it will get everywhere.
I was looking on the green side and between D56 and D57 and the C63 I dont if this is the right wy to do it but I used cotinuity on meter that makes the beep noise and from put A to B I get noting so I desolder the - side on the C63 and looks like the little eye loop deal on the green side looks like its half way off and making contact to the other side is their any way to fix this ? and do you think this could be my problem? Thanks
On the circuit board, the copper traces are coated with a green laquer. if you sand away the green coating, the copper will be exposed. You cannot solder to the green coating, it must be scraped off to expose the copper for soldering. FIne sandpaper would work, as would a razor blade or Xacto knife used as a scraper. I use a stiff fiberglass brush eraser myself.
C63? Didn't we have the 45v on it? Wasn't it the C64 with only 2v?
Follow the trace from the point where D56,57 come together over to the filter cap leg. If there is no continuity along that trace and you cannot find a break in it, then tack on a wire from A to B. If you do see a break - for example if the came came loose and tore off a piece of the trace - sand away the coating and place a pieve of bare wire across the gap and solder to the remaining trace on either side of the break.
You were right it was C64 that had -2V so I scraped the green stuff and I and solder to the copper trace. I tested and now I got -45V on Q15,17,19 is their any where else to check? is their start point and the out point? were you should have a certain volts?
You have restored the power voltage to the circuit, so does it work now?
If you can determine exactly where the copper trace was broken, I recommend scraping off the green stuff on either side of the break, then solder a piece of bare wire across the break, soldering on both sides of it. This way we are not relying on the solder alone to bridge the gap - it might crack. Soldering a wire across the gap makes it reliable. The wire takes hte place of the trace in that area, so to speak.
If it was just a matter of failed solder connections, then redoing them is enough.
When I got the amp put back together it worked. but I had two more questions Is it bad to turn the amp on with the speaker not hooked up ? and the reverb dont work I think its the reverb tank where is cheap place to get a reverb tank? I want to thank you for your time and all my dumb questions I have learned so much from you and hope some day to know as much as you your the man Thanks
SOlid state amps do not care if they are loaded. You can crank them all the way up without a speaker. But tube amps MUST have a load.
Disconnect the reverb pan and measure resistance across the little jack at each end. One end will show about 100-200 ohms or something. The other end somewhere from half an ohm to, I don't know, maybe 100. In any case, it will either be open or correct. it won't be wrong. SO any indication of resistance means it is OK. That is a simple measure of the thing. FLip it over and look at the little black and green wires inside. Any broken off?
If the pan is bad, look for a number on it in the format:4EB2C1B. Might even be exactly that. That number is the pan type. A reasonable place to buy them is www.tubesandmore.com
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