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Sunn 200S Power Supply Help

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  • Sunn 200S Power Supply Help

    Picked up a Sunn 200S head that does not turn on. 2 amp fuse was blown, these should have a 3 amp fuse. Looking inside I found a solid state rectification mod. Primary and secondary at the PT seem okay. The secondary 5V tap is not in use and taped off.

    I also found filter caps in series pairs. The first series pair are 220 uF 350V. Schematic calls for 30 uF 525V. In series the effective capacitance would be 110 uF. Does the voltage handling double to 700V? Is this cap value okay here?

    I also found that the connection between the two series caps was broken. The second cap lead wire broke off. I tried to solder a new wire onto it but I cannot get the solder to stay there.

    Before discovering this disconnect at the first series pair, I brought the unit up slowly on the variac and bulb limiter. The bulb got brighter as I turned it up and stayed lit. At full voltage the entire amp was drawing about 1.5 amps. This was in standby mode. No tubes installed. I measured only 14 VDC at the standby switch. I was expecting 500 VDC. As I write this I am wondering if I have the full 120 VAC at the primary. Although I did see very little resistance (about 2 ohms) across the PT primary.

    I am seeing about 60 ohms between the high voltage secondary windings, and about 30 ohms each wire to the center tap.

    I next plan to clip in some appropriate cap where the first series pair is disconnected and test VDC again at the standby switch.

    Before I do I was hoping for some feedback. Thank you, MC


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  • #2
    I don't see where you measured the transformer secondary voltage.

    Comment


    • #3
      Disconnect the secondary windings and test again. If no current drain, check the bridge rectifier diodes and then move along the chain to the first tank capacitors then the choke etc.
      Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

      Comment


      • #4
        At 120 VAC on the variac I am only seeing about 4 to 5 VAC the primary with the bulb limiter connected. I bypassed the bulb and now seeing the current peg when dialing up just 4 volts AC on the variac. Something is drawing very high current.

        Do I disconnect all the secondaries or just the high voltage pair before testing again for current?

        And, standby mode is on so I am not seeing the filter caps or choke in the above testing.

        Thank you

        Comment


        • #5
          Disconnect the high voltage windings first. Test again. If still high current, replace the mains transformer.
          Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

          Comment


          • #6
            The voltage doubles and capacitance halves for two capacitors in series. They need voltage balancing resistors in parallel with each capacitor, otherwise the voltage can be unevenly distributed and cause failure due to excess voltage appearing across one capacitor in the pair. if it fails short, then the other will be subject to over voltage and quickly fail as well. Your amp does not appear to have these resistors fitted (unless they're out of sight), so once you've established what's going on with the PT and fixed the connection it would be worthwhile checking the voltages across each capacitor.

            The first filter cap value was specified for a tube rectifier to keep within the tube design specs. With SS rectification you can go higher, but the increase places addition strain on the PT so I like to keep values roughly similar to the original spec - maybe a change from 30uf to 47uf but for me, no more than this.



            Comment


            • #7
              Originally posted by Jon Snell View Post
              Disconnect the high voltage windings first. Test again. If still high current, replace the mains transformer.
              Done. No more current draw. Looks like a problem with the string of diodes.

              Diode test;
              • One string reads about 1.6 V in one direction. OL in the other.
              • The other string has about .5 V in one direction and .6 V in the other direction.

              I think they are 1n4007, replace with same?

              Yes those caps have balancing resistor beneath them, out of sight. I will see what I have to bring that first filter capacitance lower.

              Thank you

              Comment


              • #8
                1N4007 is ideal, I would replace all of them.
                Good to hear you found the fault.
                Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

                Comment


                • #9
                  The other string has about .5 V in one direction and .6 V in the other direction.
                  Dead short.
                  Juan Manuel Fahey

                  Comment


                  • #10
                    Originally posted by Mick Bailey View Post
                    The first filter cap value was specified for a tube rectifier to keep within the tube design specs. With SS rectification you can go higher, but the increase places addition strain on the PT so I like to keep values roughly similar to the original spec - maybe a change from 30uf to 47uf but for me, no more than this.
                    All diodes have been replaced and high voltage is now working. I just found a 30 uF 600V cap that I will use to replace that first filter cap pair and balance resistors. Keeping the same spec okay for the solid state rectification?

                    Thank you

                    Comment


                    • #11
                      Progress but not done. Diodes and first stage filter cap replaced. Amp is operating now.

                      Amp is drawing around 1.25 amps. Is this normal or high?

                      Amps has a 120 hz hum that gets louder with the volume control. Filter caps or something else? All the filter caps look to have been replaced at some point. I did remove the polarity switch connections and death cap. Unit has a 3 prong plug.

                      Thank you

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                      • #12
                        Maker sure the input jack is a switching jack and shorting input to ground when there is nothing plugged in. If not, that would cause hum. Total current draw seems a bit high if that measurement is at idle. Check bias current.
                        "Yeah, well, you know, that's just, like, your opinion, man."

                        Comment


                        • #13
                          Input jack #1 is a switching jack and it is grounded. The noise is also present with a guitar plugged in.

                          This amp uses a pair of 6550 output tubes.

                          So I think the high voltage is too high. I am seeing 585 V at the first filter cap where the schematic shows 500 V. Is this because of the solid state rectification? Do I need a dropping resistor here and if so what value and type would be the recommendation?

                          Thank you

                          Comment


                          • #14
                            If I dial down the variac to see 500V at the stand by switch, and adjust the grid bias to -55V, the hum reduces greatly. Barely noticed below 4 volume setting. But as you dial up the volume past 5 the hum gets louder. Better than it was but still unacceptable.

                            I am left wondering if the previous 110 uF first filter cap was compensating for this hum. Would increase in capacitance help here?

                            Also not sure how to calculate a resistor to drop the voltage from 585 to 500. I can experiment but are we talking small values or large value resistors here? My thought is to have a dropping resistor at the stand by switch.

                            Am I on the right path to solve this or am I not considering something?

                            Thank you

                            Comment


                            • #15
                              More thoughts and observations.

                              I thought filter cap hum was independent of the volume control. This hum gets louder as you dial up the volume.

                              I used a signal listener at the first through fourth filter caps. I hear noise on the first and second filter only. Third and fourth are quiet.

                              I used a signal listener at the preamp plates. No noise.

                              With variac dialed to 500V at the standby switch I observed the following voltages at the four filter caps.

                              First cap 495 V (schematic says 500)
                              Second cap 494 V (schematic says 480)
                              Third cap 310 V (schematic says 425)
                              Second cap 223 V (schematic says 320)

                              Values are way off from the schematic as I go down the chain. Is the filter choke doing anything (only a 1V drop), how can I tell a good from bad choke?

                              6550 Output voltage at one tube. Remember variac is not at full 117 VAC in order to keep 500V at the standby switch.
                              • Pins 2,7 read 5.6 VAC
                              • Pin 3 read 492 VDC
                              • Pin 4 read 482 VDC
                              • Pin 5 read -55 VDC
                              • Pin 6 read 492.7 VDC
                              Anything else to test? Still not sure where to go next. Thoughts please!

                              Thank you, MC

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