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Peavey F-800G Super Festival Series Amp goes from bad to worse...

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  • Peavey F-800G Super Festival Series Amp goes from bad to worse...

    Hi,

    I have a F-800G for a customer that has been restoring it but it never had the fuzz and phase section working. I went through the preamp and found several of the SPS953 transistors bad as well as several polar caps. They were replaced and I still didn't get a response. Looking over the power section two of the pre drivers were reading bad and I switched them out as well. No I have no sound! Little bit of a pickle. The amp isn't worth the stress, but it means something to the customer and I'd like to make good on it for him. Any ideas? The amp powers up with lights n all. There's no sound now so I'm afraid I've shot a couple of the outputs on the back of the amp. I'm worried about testing while it's on because the damn thing is huge and the humongous filter caps are in a terrible position.

    Schematic below. Thanks in advance.
    Attached Files

  • #2
    Originally posted by SYSDOA View Post
    Looking over the power section two of the pre drivers were reading bad and I switched them out as well. No I have no sound!
    Exactly which transistors did you replace?
    Did the replacement part numbers match with what was installed?
    What test equipment do you have available.
    DMM, scope, signal generator?
    Attached Files
    Last edited by tboy; 12-03-2010, 05:02 AM. Reason: quote repair

    Comment


    • #3
      Thanks for the fast response JPB!

      I have all three, but still sitting on the longer end of the learning curve I'm affraid. I switched out the ep430's which have a comparable NTE match for nte291. Writing out loud now however, this is the moment that something changed. At least I've learned not to immidiately toss replaced parts, I'll reinstall, but what do you think might be keeping the fuzz and phase from working? I'm trying to look at the schematic as an explanation, but home grown hasn't really ever lead me to logic... minus the hard way...

      Comment


      • #4
        I cannot find any info on a EP430 transistor.
        The NTE291 is TO-220 package NPN, Base Collector Emitter (from the left) pin configuration.
        I am not an NTE fan, at all.
        If the pinout is the same, the NTE's should not have brought down the amp.
        Check the actual board & see if this is true. Base Collector Emitter leads must go to the components that the schematic shows.
        Also look for inferior soldering that may have occurred.
        Get the amp working, then the other stuff.

        Comment


        • #5
          We have posted this before, but someone made it a sticky.

          Go to the Schematics Request section, and up in the stickies is a copy of the Peavey semiconductor cross reference guide. VERY useful if you service PV gear.

          The EP430 is just a TIP 31C, and the EP431 is a TIP32C.

          Or just about any similarly spec'd transistors. Not very critical at all.

          I have never seen a TO220 transistor with a pinout other than the standard BCE.

          And stay away from that NTE junk.


          And I totally agree: get the thing working, THEN worry about details like the phaser.


          The power amp runs on +/-50VDC, the preamp runs of +24v. The 24v comes from the +50v. Any chance you killed the 24v to the preamp?

          When power is turned on, is there a speaker thump?

          And better get used to working on it while it is on, you can't accomplish anything with it off.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Thanks for the quick responses. Yes I don't like NTE but in a pinch... forgive me. This place is a mad house these days and getting amps back out if I have parts of any equivalence stocked within 48 hours is optimal right now. I replaced the ep430s and have revived the amp as hoped. So on to the original issue of no fuzz or phase. Thanks for the note, Enzo. The cross reference is great! I have a stockpile of the TO22 TIP X transistors that I recovered from a estate sale that suddenly have purpose. There is a slight thump when turning the amp on right now. There's lots of maintenance concerns with the amp as far as caps, most of them are leaking from what I can tell, but I apparently am still having issues with properly diagnosing transistors , doesn't seem to be stopping any other stage of the amp. I notice no reverb as well. I don't have a problem with the 400 series stuff and working on it while it's on, this thing is just so big, no counter space. Checking voltages compared to schematic now, but any advice as to where to focus on to help get this thing outta here would be much obliged!

            Comment


            • #7
              May I kindly advise working on one thing at a time.
              The amplifier is operating, correct?
              How about we start with the reverb.
              The first thing I would check is the resistance of the coils on the reverb tank itself.
              The input coil is typically a lower resistance than the output coil.
              The in & out should be indicated right on the reverb tank.
              After that (or during) check the reverb cables themselves.

              Comment


              • #8
                Amen, one thing at a time. Focus.

                I agree, check the revern pan itself for opens.

                And check the amp. There are the two cables to the pan, one is drive and the other return. Turn the amp on with reverb up partway. Now with the reverb cables unplugged from the pan, touch the tip of each plug with a finger. Does one HUM? That would be the return, and should plug into the OUTPUT jack on the pan. If neither hums, there is trouble in the recovery circuit.

                The remaining cable is the drive end. Check for signal at the tip. Well assuming there is a test signal at the amp input jack.
                Education is what you're left with after you have forgotten what you have learned.

                Comment


                • #9
                  Yes! I can appreciate the guidance. I need it. I've been able to recover the reverb section. I think I had just plugged it in backwards. I didn't know that little trick about identifying the return lead. Very cool. Won't to have to worry about making that mistake again.

                  Comment


                  • #10
                    So what's next.
                    The Phaser or the Fuzz.
                    I take it the Tremelo is operating properly.
                    (why did I ask?)

                    Comment


                    • #11
                      Funny you mention that... that doesn't work either... doh! I thought 'rate' was something to do with the fuzZ but i see it now on the schematic!

                      Comment


                      • #12
                        We usually think of the phaser as a swirling chorus or flange type effect. I may be wrong, but I don't think there is any time related effect in this phaser. In this amp, phasor is just more like a variable filter. So if the fuzz is not working, then the associated phasor probably won;t either.


                        I am a big believer in fixing problems before general overhauls, but this amp is 30 years old. See those little 2uf/35v e-caps strewn around? Get rid of them, replace them. When they dry out and get leaky, they no linger contribute to keeping DC levels where they belong, nor do they pass signal like they should. All the various signal path electrolytics.

                        Fix the fuzz. First, does the footswitch have to be on to even get fuzz? Kinda looks like it, I may be wrong.

                        The first group of three xstrs is the input stage, then another xstr and the fifth xstr is the fuzz stage. Looks like signal is through all those all the time, so is there signal atop the Fuzz pot? and out the wiper?

                        Now look at the two JFETs by the Fuzz FS pin. One is P type and one is N type, with a common control signal. So when one is on the other is off. Work the FS on and off. Does the gate voltage toggle off and on at each? Measure resistance across source-drain at each JFET while the circuit is running. Do they toggle between low and high resistance as the gates toggle? A bad JFET could easily kill the fuzz.

                        Look at those JFETs, one selects the clean input stage signal, the other selects the fuzz output. The line betwen them is the output to the rest of the amp. See the phasor? It is just a parallel gain stage to the other fuzz stage and it gets mixed it prior to the JFET. SO the phasor ONLY works when the fuzz is on since it is just a section of the fuzzz circuit.
                        Education is what you're left with after you have forgotten what you have learned.

                        Comment


                        • #13
                          Nice analysis Enzo.
                          Here is the Phasor part of the instruction manual.
                          Attached Files
                          Last edited by Jazz P Bass; 12-04-2010, 03:22 PM.

                          Comment


                          • #14
                            Oh, cool, I didn't know they had a theory page written up.
                            Education is what you're left with after you have forgotten what you have learned.

                            Comment


                            • #15
                              SYSDOA, a shop as close as you are to Anchor Electronics in Santa Clara should be able to avoid the NTE. ;-)
                              -Erik
                              Euthymia Electronics
                              Alameda, CA USA
                              Sanborn Farallon Amplifier

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