Announcement

Collapse
No announcement yet.

Peavey 400 amp missing part.

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Peavey 400 amp missing part.

    I hav a 70’s model peavey 400 amp. Supposedly it sat in my customer’s Dad’s closet for years. It was working. He took it out and it tarted blowing fuses. So he tried to fix it himself. He bought power transistors, and replaced the burned resistors. But there is sill too much current. I put about 30 vac in and the curret was up to 2A already. I have checked every transistor nd they all check good with the dmm.

    I measured bout 2 or 3 volts on the output with 16vac going in. So something is wong.

    But this evening I was looking at the IC’s for damage, ad Icouldn’t find U3? I kept looking and discovred it’s not there at all? It’snot like it is an optional circuit because all tgeassociated resistors are there. It is a 4558 opamp that measures the voltage across soe 0.1 ohm resistors at the center rail. I guess it is an idle curren adjustmnt circuit? Not he kind of thing that the amp wouldwork without?

    Anybbdy seen these amps with missing parts on purpose? I think my customer’s story is a bit flawed.

  • #2
    What is the actual model of the amp?
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      Is it a Renown 400? That has a 4558 as U3. According to this thread (https://www.diyaudio.com/forums/inst...-issues-2.html) U2 and U3 are not in certain revisions of this amp, but since it isn't MEF they probably don't know what they are talking about.

      Comment


      • #4
        It is not a Renown.
        Attached Files
        Last edited by capndenny1; 04-05-2019, 09:26 AM.

        Comment


        • #5
          Will you look at that. The schematic I just posted doesn’t have U3? It also doesn’t show the output impedance matching transformer the amp I have does have. The other schematic I have shows U3 and the transformer.

          Comment


          • #6
            Here is the correct, or at least closer schematic. This one the pc board layout matches. It’s U2 which is not there. U3 is there.

            http://schems.com/bmampscom/peavey/P...ss_with_pa.pdf

            Ignore the preamp, I just have this power amp.

            Comment


            • #7
              Ok, now I taped the map fragments together and can solve the mystery.

              Most manufacturers have a certain basic power amp design, which they use all over vthe nplace , maybe with slight twists where needed.

              1) The basic amplifier does not need U2 and U3 to work at all.

              U2 is a "Compressor/Limiter" IC, if not mistaken a house branded CA3080 , used so amplifier never ever clips, so it´s *added* to PA/Bass/Keyboard/Steel Guitar amplifiers, and not at all to plain Guitar amps.
              So if this belongs to a plain Guitar amp, U2 is not fitted and U3 which detects distortion and triggers it is not needed.

              2) output transformers are not used in 99.999% of transistor amplifiers, period, but there is still 0.001% out there which do.
              From the Peavey brand there´s a grand total of two: a standard guitar amp (so no compressor/limiter so no U2) and a Pedal Steel which does.

              So from 1 and 2 I guess you have a pedal steel power amp module.

              U2 CA3080 has been out of production for decades so quite hard to find , and even less under a house number so I guess somebody pulled it and couldn´t replace it, or even pulled it from a "dead amplifier" to repair another.

              If everything else works, you can use amplifier as-is, where it will work like any regular amp (will clip when overdriven).

              If important to you, you may try to find one, but never ever from EBay/Alibaba China/Taiwan/etc. where they will sell you a cockroach sprayed with transparent varnish so it meets the conditions:
              a) it´s black
              b) it has legs
              so they can claim it´s any IC you need to purchase.

              I *think* Mouser is selling a version, made by a "second line supplier" ; if they sell it, it can be trusted. I guess.

              As a side note, you should have mentioned the Output Transformer (a most unusual thing) in the first post, not in the 5th, after sending us on a wild goose chase.
              With that info we would have identified the amp on the first try.
              Juan Manuel Fahey

              Comment


              • #8
                I want to thank you for your input. The transformer is there to keep the amp able to deliver full power with either 8 or 4 ohms of speaker impedance.

                The owner said it was used for guitar, and the special thing about it was that it had a phaser effect built in.

                So I have verified all the transistors check out with the diode check. Diodes as well.

                Not sure how best t proceed. I have an isolated variable AC source with current and voltage. It was pulling 2A with 15 or 30 vac in, so the low voltage DC currents were probably much higher.

                I’ve fixed a few of these high powered SS amps. Most of them the bad part can be found with the dmm. The last one I did I was worried about the power transformer, so I kept the ac voltage low. But aftera lot of wast time deugging a fixed circuit I discoveredthe circuit needs about 10 or 15 volts DC to actually start working. Below that it had a dc offset on the output I could not fix. Bt once the voltage rose high enough the circuit started t work, and it was already fixed, and had been for a while.

                Comment


                • #9
                  Huh, never saw a SS Peavey with a output transformer, but haven't had a 400G. Regarding the high current, I think you need to pull output transistors and driver transistors and see if you are still drawing a lot of current. Diode check with a DMM doesn't necessarily prove a transistor will be good at high voltage. A light bulb limiter is pretty helpful for this.

                  Comment


                  • #10
                    So 400B/G power amp. That IC is indeed not needed.
                    Education is what you're left with after you have forgotten what you have learned.

                    Comment


                    • #11
                      So would I be able to leave the power transistor boards disconnected in order to troubleshoot the driving circuit. Would Q2 and Q12 go ahead and balance themselves out, with no power transistors connected?

                      Or do I at least need Q3 and Q13 in there?

                      Comment


                      • #12
                        Originally posted by capndenny1 View Post
                        So would I be able to leave the power transistor boards disconnected in order to troubleshoot the driving circuit. Would Q2 and Q12 go ahead and balance themselves out, with no power transistors connected?

                        Or do I at least need Q3 and Q13 in there?
                        Last old Mark III I worked on was shorting even with the power transitor board disconnected.

                        Comment


                        • #13
                          **Disconnect Output Transformer** until you repair the amplifier.

                          The transformer is a DC *short* and any DC unbalance will make amp blow fuses.
                          Juan Manuel Fahey

                          Comment


                          • #14
                            Yeah, I figured that out. I just disconnected it. It just plugs in parallel to the speaker jacks. I don't have anything plugged in to the output, no speakers,, no transformer. But it the positive rail is fighting the negative rail, they don't need a load.

                            It doesn't sound like anybody has any sort of systematic method to troubleshoot these power amps, just hit or miss and guess till you find it. I mean, it does help to understand the circuit. But when everything is connected and things go poof in a second or less, it's not easy.

                            I will put some time in tomorrow. I hope to make some headway.

                            Comment


                            • #15
                              Originally posted by capndenny1 View Post
                              But when everything is connected and things go poof in a second or less, it's not easy.
                              Build yourself a light bulb limiter and use it during start up tests. It might save you a bunch of new parts if you don't catch all of the shorted ones.

                              Comment

                              Working...
                              X