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SVT-CL: Protection at Turn-On

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  • #1

    SVT-CL: Protection at Turn-On

    It doesn't even wait for me to take it out of standby.

    The tubes all test OK (Orange Tester).

    I checked all of the components on the output tube board.

    I touched up any funny looking joints on the output tube board.

    I checked all of the big (physically) resistors and all of the diodes (that I could see) on the main board.

    I touched up any funny looking joints on the main board.

    I 'exercised' all of the multipin connectors.


    It does it with all tubes in, the 6550s out, and all amp section tubes out.


    What do I look at next?
    Tags: None
  • #2
    Have you tried disconnecting the output transformer?

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    • #3
      If you are not familiar with a circuit, one thing you can do is look in the schematic to see what turns on the protect, and work back to see which thing seems to be triggering it.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Aside from blown screen resistors or cathode parts I have found a few where the comparitor circuit that monitors bias current were bad. I forget which chips off hand.

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        • #5
          On the main Chassis PCB, IC2B is the Fault Detector. One of the mod's needed on SVT's is adding a 0.1uF 50V or greater cap between Pin 7 & 6, to slow down false triggering. That comparator is always looking at the summed voltages of all the cathode circuits, comparing it against the heater voltage, half-wave rectified. If IC2 is bad, that will definitely fire the Fault circuit inhibiting the relay from closing....J12 on the Relay board.. C13 in the fault circuit could also be at fault, as well as the absence of the 0.1uF cap, to fire IC2B, assuming it is ok. The output of IC2B goes thru R31 (4.7k), then feeds the LED status circuit in the preamp (IC1A/B, as well as a diode network to the relay control....goes out on J12 of the main PCB, and comes in onto the AC Mains/Relay board on J35.
          Logic is an organized way of going wrong with confidence

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          • #6
            I pulled the main board and found that C13 had a broken leg and a wimpy leg. I don't know how this could have happened, unless it was junk to begin with and it finally broke from vibration. I replaced it. Now, with no 6550s in, it now powers up properly. Fingers crossed.

            Thanks nevetslab.

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            • #7
              The plot thickens.

              I put it back in the cabinet, and it tripped at turn-on. And again. And again. Then it was OK. I pulled it back apart.

              The chassis has an L brace front to back (that is notched for the filter caps.) This brace runs right where C13 is, so the brace may have been pressing on the original C13, causing it to fail mechanically. The new C13 was made from two series caps (I was out of 10uf), and where their leads met was some of the black foam that's on the L brace - so there could have been some conductive monkey business there between the caps and the brace. Lesson: Pay attention to stuff! So I put C13 on the bottom, and installed the 0.1uf Pin6-Pin7 cap on IC2. It powered up properly several times. Now to put it back into the case.

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              • #8
                I find it easy to re install by putting the head case on some chassis blocks I built, any short two by fours will do then slide the chassis in and you have room to install the screws from underneath.
                Then you can pop in the preamp and connect all the harnesses.

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                • #9
                  Sound like a plan. I put my amps on a table in the middle of the shop, one of those wheeled things. That way I can spin it around without lifting anything. When it comes time to install the chassis, I usually hang one end of the cab over the edge of the table, so I can run a couple screws up into that end's holes. Once those are finger tight, I slide the other end off the other side of the table, and do the same. Once that is done, the rest of the holes should be lined up, and I can flipthe thing up on its end to tighten all the screws.
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment

                  • #10
                    It behaved itself through several power cycles, so....

                    I flip them on their face to put the chassis screws back in. I do a little something that makes it easier the next time I have to work on the amp.*

                    *
                    Put the screws in, but just run the heads up - don't tighten.

                    Place the unit on its feet.

                    Push the chassis back in the cabinet.

                    Make a pair of blocks that fit between the grill panel batten and the chassis with about 1/16" of slop remaining. Put some glue on the batten side of each block and slip them in place.

                    Roll the amp on to its face and tighten the chassis screws.

                    The next time I put it together (with the amp on its face) the screws will like right up.

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