Announcement

Collapse
No announcement yet.

Peavey 400 amp missing part.

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

  • #16
    How is that different than jst monitoring the current as you brg the voltage up?

    Comment


    • #17
      Originally posted by capndenny1 View Post
      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.
      Who says so?

      In fact here we are against hit and miss, go figure.

      We call it shotgunning and if you search the forum for that word you´ll find it´s consistently un-recommended, it being the slowest, longest and most wasteful repair system.

      In fact you can replace *all* parts in an amp and NOT fix it, because a lot of the time a problem is NOT caused by a "bad part".

      How is that different than jst monitoring the current as you brg the voltage up?
      Just monitoring current can tell you amp does not work, you have a gross fault ... which you already know , and not much else.

      A lightbulb limiter can tell you the same, but also allows you to make voltage measurements ... lots more data which lets you suspect certain problems and test there.
      Juan Manuel Fahey

      Comment


      • #18
        Originally posted by J M Fahey View Post
        A lightbulb limiter can tell you the same, but also allows you to make voltage measurements ... lots more data which lets you suspect certain problems and test there.
        In some circumstances you can also monitor if something that is shorted is heating up, helping narrow down the problem.

        Comment


        • #19
          Yep, I get it. Maybe as you increase the voltage something happens and poof. I will add a light bulb limiter.

          I have fixed amps by just replacing all the transistors. When it works it's cheaper than troubleshooting. But you are right, it doesn't always work. And the possibility of breaking a trace or something is pretty good, especially if the board has been worked already!

          Thanks again. Don't take my questions as being obstinate. I like to know why, not just because somebody said so.

          Comment


          • #20
            Ok.
            *One* suggested repair path is this, and sometimes order may vary, but basically:

            * disconnect loads, normally speaker but in this particular amplifier the output transformar too, because it´s practically a DC short to ground.

            * build a lamp bulb limiter.

            For an SS amp which has nil idle consumption, a few Watts (compared to a tube amplifier, which may eat, say, 30 or 40W idle, just feeding filaments, screens, preamp, etc.) , a 40W to 60W bulb is fine.

            Since old style filament lamps are practically unavailable (although you might still have a couple still screwed to some old desk lamp , in the attic, etc.) , modern "high efficiency" halogen/quartz bulb inside a larger envelope bulbs are fine, we need a red hot wire inside a glass bottle:



            read the *actual* Watt rating,for example these are "60W equivalent" but actually 43W.
            No big deal, they are well within range.

            * turn amplifier ON

            * with a not-that-dead amplifier, bulb will blink while capacitors charge, and then cool down to red or orange; with a small amplifier, say 25W RMS it may become invisible.

            Measure voltages, basically main supply +/-V rails, check you do not have DC at output, also preamp voltages (say +/- 15V or whatever it needs).

            Warning: most/all? voltages will be low, because the bulb limiter is chopping some of mains voltage, so having, say, 60/70% of normal is perfectly fine.

            * if amp is in that "reasonable" condition, connect a speaker,play some music, whatever, it does not have a *gross* problem, test whatever you need, scope it, tweak pots, etc.

            -------------

            * if lightbulb shines *BRIGHT* then you have a gross short.

            There you turn it off, unplug it, start checking for shorted transistors, diode bridges, shorted main capacitors, etc.

            Replace what you find bad, and turn on again, always through bulb limiter.

            It might already work (say, 50% of the time), or you will need further troubleshooting.

            Ok, will take care of Lunch, continue later.

            But rest assured that amplifier IS repairable.

            Doubly so being a Peavey, some of the best built amplifiers out there. Serious.
            Reliable workhorses if I ever saw one.
            Juan Manuel Fahey

            Comment


            • #21
              Thank you sir. What you say makes sense.

              I started to build it. I have all the stuff I need already. Just have to put it together.

              I took all the transistors out and ran them thru the transistor tester. They all checked out.

              So I started to check out the other semiconductors. The amp has +/- 15V, or it is supposed to. It wasn’t working. The voltage regulators were loading down the power supply. The outputs on both were -0.6v. Once I removed both regulators the dc voltage that feeds them seems ok.

              I also found one of the connectors that connect the +/-15V from the transformer. The femal contact is split and missing half the metal. So I will remove the connectors and solder the wires in directly. I may extend them a bit so there is room to lay the back panel down. I also bought some bullet crimp on connectors to use, maybe.

              So I have regulator Ic’s on the way. In the meantime I will get the lightbulb thingy together.

              Thanks again!

              Comment


              • #22
                Here is the complete schematic including the preamp with phaser, in case you or someone should ever need it:
                https://music-electronics-forum.com/...4&d=1391478054

                Did you verify that the regulators themselves are bad, or is something is loading them down, which in turn loads down the supply?
                Do resistance checks from the pads that connect to their outputs to ground. (across C40 and C41 with regulators out of circuit)
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                Comment


                • #23
                  Yes, I didn’t find anything loading it down. I lifted the output legs of both regulators, and they both had weird voltages on them.

                  Thanks for the schematic.

                  Comment


                  • #24
                    The problem with regulators is that the owner attempted to repair tge amp and swapped the 7915 and 7815 regs. I hate working amps where people have been working that should not have been.

                    The amp seems to go between working and loading down everything. Probably because I have a lightbulb is series now. But I did have it power up slowly without shorting out. I am questioning the insulatirs, or maybe a cap is shorting out.

                    But at least I made some progress.

                    Comment


                    • #25
                      Originally posted by capndenny1 View Post
                      The problem with regulators is that the owner attempted to repair tge amp and swapped the 7915 and 7815 regs. I hate working amps where people have been working that should not have been.

                      The amp seems to go between working and loading down everything. Probably because I have a lightbulb is series now. But I did have it power up slowly without shorting out. I am questioning the insulatirs, or maybe a cap is shorting out.

                      But at least I made some progress.
                      If your questioning the insulators and ohm meter should be able to tell you or not. And as far as caps my understanding for electrolytics it will be either shorted or not again analog ohm meter should tell you.


                      nosaj
                      soldering stuff that's broken, breaking stuff that works, Yeah!

                      Comment


                      • #26
                        I have seen both come and go. Sometimes caps break down at high voktage, and then recover for a while. Likewise with insulatirs.

                        Comment


                        • #27
                          Originally posted by capndenny1 View Post
                          I have seen both come and go. Sometimes caps break down at high voktage, and then recover for a while. Likewise with insulatirs.
                          well 7915 and 7815 are not high voltage and cheap as are the insulators. Replace them and rule it out as are the caps in this amp nothing expensive.
                          Though with some insulators I have found the hole is wallowed out a bit and can allow continuity through there sometimes.
                          nosaj
                          soldering stuff that's broken, breaking stuff that works, Yeah!

                          Comment


                          • #28
                            Yes, probably a good plan. The big caps are pretty big,

                            The power transistor insulators are some foamy kind of things. I wonder if they are oem or something the owne tried?

                            I will try caps and insulators. I may try running each side with a single of the 3 or 4 in parallel. Maybe one of those is iffy. It should work fine with just one.

                            Comment


                            • #29
                              The gray foamy things are Bergcuist Sil-Pads or similar. Perfectly normal to find from the factory.

                              If the insulators short through, you blow fuses.
                              Education is what you're left with after you have forgotten what you have learned.

                              Comment


                              • #30
                                But they probably fail at elevated voltage? Have you seen them fail? These have been on and off several times. I know the owner has replaced the power transistors several times.

                                There is no adhesive left on these. I noticed the new ones say they are adhesive on one side.

                                They are $0.80 each, so I may just order 10 of them.

                                Comment

                                Working...
                                X