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Blues Deville switching stuff....oh boy....

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  • #1

    Blues Deville switching stuff....oh boy....

    Got one of these where if you switch from the dirty channel to clean, it lingers in between the channels and has no volume control. You can tap the board and it will settle in on the clean channel like normal.

    16V +/- supply is screwed up for sure. I've got 10+ and -5 or so on those rails....

    I've:

    -gone over trace continuity and the pads

    -replaced the 5 watters (330 up to 470)

    -changed that 10k to 100k as recommended in the service bulletins

    -changed the relays

    -inspected the zeners for resistance both ways and diode checked them- they seem to be ok

    Also- prior to the 5 watters I do have the 32 +/- that the schematic says should be there...

    Is it possible for these zeners to be screwed even if they seem to test ok? I can't figure out why the hell I'm lacking voltage here.

    thank you for your time!
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  • #2
    Originally posted by Wes View Post
    Is it possible for these zeners to be screwed even if they seem to test ok? I can't figure out why the hell I'm lacking voltage here.
    If you mean standard diode test, then yes they could still be bad.
    To really test them you would have to build a circuit and verify they are maintaining their specified voltage.
    But your statement that tapping on the board corrects the problem implies that there is still a bad connection somewhere.
    My guess is a cracked trace near the resistors that measures continuity but will not pass current.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​

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    • #3
      If your 15v rails are 10v and 5v, there is absolutely no point in throwing resistors and relays at the amp. Nothing works right with bad power supply. Get solid 15v rails, THEN start fixing any leftover problems.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Ok, so the rails have 10+ and the other 5-.

        As far as the tapping it to get it to settle in- I've found several bad solder points and touched those up. Now there is hardly any volume coming from either channel, but the volume control of the clean channel works- and you can tell it is switching properly between the two channels.

        I took the zeners out and checked them with a couple 9v batteries and a resistor in series. They both output 16v right on the money and both have about .6v when reverse biased.

        Unlike other Devilles and the like- there is none of the charring you usually see. The traces all look good and I can see the 10v and -5 all the way to the ICs.

        What else could be pulling the rails down?

        Again, thank you guys for your time and help.

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        • #5
          Originally posted by Wes View Post
          What else could be pulling the rails down?
          Bad IC, bad filter cap.

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          • #6
            Put your finger on the top of each IC, is any getting hot?
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              I will check that, Enzo.

              I did, in the meantime, pull up the two wire jumpers that distribute the positive rail..... testing with that rail unloaded revealed a healthy 16 and some change volts, and it raised what was -5v down to about -11v on the negative rail....

              I had already changed the filter caps before the 5 watters and the ones after the zeners..... I forgot to mention that....

              Onto the possibly not so icey ICs!

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              • #8
                Yup- U2, a TL072 was bad. Replaced that guy and everything works fine......all except the horrible feeding back of the reverb on the clean channel. As soon as you select the dirty channel- the reverb behaves as it should.

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                • #9
                  aaaaaand now I have red plating too. Before this the power tubes were idling nicely down in the 30mA range....Now they're over 80 and the bias voltage is about -39.

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                  • #10
                    Set up the dirty channel to be real clean (master full), do you now get the reverb squeal?
                    Have you tried other tubes in the reverb spots?
                    You bias voltage is weak, maybe a bad power tube pulling it down? Do you get more like -50V with the power tubes removed?
                    Originally posted by Enzo
                    I have a sign in my shop that says, "Never think up reasons not to check something."


                    ​

                    Comment

                    • #11
                      I fired the amp up in standby to track down that bias issue. It turned out to be a shorted bias supply cap. It decided it didn't want to be a cap anymore- it saw all the cool stuff resistors get to do, so it decided to become one..... Sorted that out.

                      Per that crazy reverb issue-

                      reverb with clean channel= squeal city.....feedback onset, completely uncontrollable...

                      dirty channel at any setting- no prob- huge reverb

                      I found that if you jumpered the pre-amp out and power amp in with a patch cable- reverb works both channels.

                      Hmmmm........

                      Got to looking at the crazy arrangement of the pre and power amp in/outs with U1, a TL072.

                      Could it have to do with impedance differences with the different signal path selections when channels change and as well- the way the signal path changes once you start using the in/out jacks?

                      I then looked down at the reverb tank itself. I noticed that someone had opened that bag- the tell-tale second and third holes at the fold....

                      I opened that up and sure enough- wrong damned tank. I luckily had one on hand and everything worked out.

                      Son of a....... always check the tank.

                      These stumpers really bog you down and the more you get frustrated, the less clearly you can see.

                      Thank you all for your guidance, guys. It means alot.

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