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5150 problem

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  • #1

    5150 problem

    Just got a 5150 with no output.

    all fuses are good inside
    all tubes lit up
    there is 500v to the output tubes , but no high voltage to the preamp tube board. Not even at the other end of the 4 ribbon cables

    also the standby light is out...yet i have 500v to the power tubes.

    running another amp in / out of the fx jacks = no sound either way .

    cant find anything burnt or loose

    the schematic i have is somewhat complicated (to me)--im trying to find a board layout to help

    where would i start looking for the missing pre-tubes voltage ? There are 3 large electrolytics under the ribbon cables to the pre pcb, one being 47uf/500v....none of them have voltage

    and shouldnt the amp work going into the return jack if i have power at my power tubes ?
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  • #2
    Nope. You need to have B+ to the power tube plates - pin 3 - yes, but you ALSO must have it at the screen grids, pin 4 of each. And you don;t. The green standby light is telling you there is no high voltage in the circuits. When the screens have power, that light comes on.

    Please be careful, this amp can KILL YOU.


    It won;t be your capacitors. Caps can fail open, and that is as if they were not even there. That would result in loud hum, not silence. Caps can also fail shorted, in which case, your high voltage fuse would blow instantly.

    Look on your schematic, the page with the four power tubes. The four power tubes are not the power amp, they are only part of it. The power amp also includes V3a, the FX return buffer, and also both halves of V4, the phase inverter. Essentially everything after the FX loop is power amp. All those tubes need high voltage on their plates. As convenient as it is to call all the small tubes preamp tubes, they are not. Some are power amp tubes, as I just described. SO when your screen supply goes dead, and all the "preamp" tubes go dark, your power amp will not amplify.

    Why do you have no screen voltage? I'll bet my lunch money on that 10 watt 400 ohm power resistor on the power tube board. That or the standby switch itself. If that resistor is open, and you cannot find 400 ohms (that is a non-standard value), then 390 ohms is perfectly fine, and IS a standard value.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      if anyone has a layout for this it would help greatly

      Comment

      • #4
        Peavey does, call customer service in the morning and ask for the schematics. The factory schematics include a parts list and a board layout.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          enzo....did i ever tell ya i love ya .. ??


          that was it--- resistor is open .

          THANKS !!!!

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          • #6
            It's almost like I been doing this for a while or something...
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              well..crap.

              fired the amp up and got a very loud low freq hum then the main 5A fuse blew.

              Now , ive got all the tubes out (wasnt a tube).....and if i unhook the ribbon going from the main pcb to the output tube pcb.... my problem goes away.

              so, ill pull that board. the components are underneath...and as some of you know, ill have to drill out the damn rivets to get it out.

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              • #8
                Check the flyback diodes.

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                • #9
                  Bingo-- 1 burnt diode

                  and 2 open 1/4w , 100 ohms

                  Heres the deal... the scem has the diodes as SR2573 ........but the diodes are labeled GP 1250 .

                  So..what do I need here? and , can a 1N4007 be used here ?

                  Comment

                  • #10
                    No, you really want the 2000-3000v diodes. You COULD use three 1N4007 in series in each spot, but that is clumsy. Order the right part from Peavey or Mouser or someplace.


                    I don't consider it all that large a chore to drill out 8 rivets, fix the board, then install 8 new rivets. But I do prefer not to have to bother. usually it is the screen resistors, but most any component on that board is removable from the solder side. Solder side is up, and with the layout, I know where each part is, so i can unsolder it, and poke its leads on out the hole so it drops into the chassis where I fish it out. Now nothing says the replacement part has to be on that underside. So I mount a new part on the solder side. Sometimes I need to offset the part a little to clear the impedance switch or jacks or something. Don't forget to trim the leads first.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      Ok--good points enzo. Thanks .

                      ill try to find those on Mouser first --- i need some other parts anyway.

                      So anything specific besides 2-3kv ? or maybe they have one of those part numbers



                      how about :

                      http://www.mouser.com/Search/Product...-GP02-30-E3-73

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                      • #12
                        Yeah, that ought to work. Is that the one Fender uses? No, they use R3000, well someone uses the GP02.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #13
                          This thing is still fighting me.

                          It will power on , but when in run mode 1 tube red plates in 5 seconds.
                          I checked bias, 1 side of the ot is fine at 60 ma, but the other goes way up to 190 ma .

                          Before this -- i changed the 4 screen resistors , the diodes , and a 2.2k resistor on the power tube board .

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                          • #14
                            Move that tube to a different socket and see if the problem follows the tube or the socket. You might just have a bad tube.
                            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                            • #15
                              not the tube. tried 3 .

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