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Sovtek Mig 100 U speaker output issues

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  • Sovtek Mig 100 U speaker output issues

    Hi, I purchased a nonworking Sovtek Mig 100 U for $30.00 as a electronics project. I fixed the amp by finding a loose wire and re-soldering it. I ran the amp through a 15" 200 watt Bass cab when working on it and it was really singing through the 15" with a tube screamer pedal after it was fixed. I then replaced all the rusted nuts and washers on the input and output jacks (aluminum washers, all I could find) and now the 8 ohm and 16 ohm output jacks have lost most of their output volume when at full on, but not the 4 ohm output, which is fine through the 8 ohm 15" speaker? I checked for any visible loose connections and found none?. Both the tip and ring of the 4, 8,and 16 ohm output jacks all show zero ohms to chassis ground. This really has me confused. I kinda cranked down on the torque with the nuts at first and have since loosened them back up, don't know if that has something to do with it. I'm trying to exhaust all leads before I take the OT out of circuit to do the load test on. I have been unable to find a schematic for the Mig 100 U modal but have one for the Mig 100 H modal. Any suggestions would be appreciated...p.s. I also cranked the nut on the line out as well and all of the jacks seemed connected in series. I hope I did something stupid I haven't found yet that I can discover and fix. I hope the OT didn't just mess up some how. Thanks

  • #2
    Here's the schematic for the Sovtek Mig 100 H
    Edit: I guess I can't load a pdf here? Let me see what I can come up with?
    Last edited by Jesse Pearson; 08-22-2015, 06:14 PM.

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    • #3
      The secondary (output side) of the output transformer is relatively few windings of heavy wire, and so it has a VERY low resistance for your meter to try and measure. All your meter can do is verify it is not open. It is unlikely the transformer is bad, though possible.

      Since the 4 ohm works, run a signal through the amp, and measure it with your AC voltmeter across the speaker. Now ground the meter, and measure AC voltage at the hot terminal of the speaker just to verify you get the same reading. Now measure at the 16 ohm jack tip contact. Is the voltage there or missing? If it is there, then try a clip lead from the ground terminal of teh 4 ohm jack to the 16 ohm ground terminal.

      Do the transformer wires solder directly to the jacks? Or are ther any connectors or solder junctions in between?
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Originally posted by Jesse Pearson View Post
        Here's the schematic for the Sovtek Mig 100 H
        [ATTACH]35367[/ATTACH]
        This link is bad.

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        • #5
          Thanks J P Bass...I tried to load a pdf. Working on it.

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          • #6
            Is the schematic attached the one you were looking at?
            The problem seems to be related to the work you did on the jacks, probably related to grounding. The tests & jumpers Enzo mentioned should get it sorted out.
            Attached Files
            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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            • #7
              Originally posted by Jesse Pearson View Post
              ...Edit: I guess I can't load a pdf here? Let me see what I can come up with?
              The forum software does support .pdf attachments. It will work for you once you figure out the method. You will find FAQs on the subject at Reading and Posting Messages

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              • #8
                We know that the output jacks are grounded as shown in the schematic. However, the schematic doesn't tell you how and where the ground connection is made. The speaker jacks on the Mig 100's that I have seen are plastic jacks. Are yours plastic jacks? If so they don't make the ground connection through the chassis mounting nut. I suggest that you trace the ground wires from the jacks and check the connections. You can also check continuity to the chassis and the speaker as has already been mentioned.

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                • #9
                  G1 thanks for posting the pdf. There is no schematic for the Sovtek 100 U on the web, so I'm in the process of mapping out my own. The Sovtek Mig 100 H looks real close. I think the difference between the two from what I've read is the U modal is a cathode follower and the modal 100 H is plate driven and has more gain. Still trying to understand what that exactly means however, haha. I'll do the tests that Enzo suggested and post the results when I get done. Thanks

                  Tom, everything jack wise is metal and seems connected to chassis ground with the ohm meter. I don't understand how both the tip and ring on the speaker outs are showing zero ohms to ground? Could someone explain this to me please, is this right or wrong?
                  Last edited by Jesse Pearson; 08-22-2015, 06:55 PM.

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                  • #10
                    The cathode follower vs plate driven thing is in regard to how the tone stack is driven.
                    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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                    • #11
                      Originally posted by Jesse Pearson View Post
                      I don't understand how both the tip and ring on the speaker outs are showing zero ohms to ground? Could someone explain this to me please, is this right or wrong?
                      It's ok, the low resitance of the OT winding is what you are measuring. (see post #3 from Enzo).
                      What you are calling "ring" is I think the "sleeve". Like Tom mentioned it is important to know if it has a grounding wire, or is just relying on contact with the chassis (if it is plastic type it will need a ground wire).
                      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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                      • #12
                        All the output jacks are metal. A resister is connected to a ground point on the chassis that then goes to one of the line in jack points and then is connected to the sleeve of the line in jack by way of another resister from that point. The black ground wire starts here at the line in jack sleeve and goes to the 16 ohm jack sleeve, then to the 8 ohm jack sleeve and then to the 4 ohm jack sleeve. From there it goes to the power caps/rectifier board to a ground point. I removed all the jacks and reinstalled them again and now the 4 ohm jack has very low volume like the other two jacks? The light bulb current limiter I have the amp going through shines the brightest with the 8 ohm output jack when things are turned up. I poked everything in the amp including the tubes with a chop stick over and over again but there was no change in the sound out put. I was unable to read any a/c or amps from the tips to ground on the jacks? I need a good sleep over this stuff and will pick up after work tomorrow. Thanks
                        Last edited by Jesse Pearson; 08-23-2015, 06:04 AM.

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                        • #13
                          First try at this. If all goes well, there should be an attachment with a schematic that may be of use to you.
                          Attached Files

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                          • #14
                            I'm wondering if cranking down on those nuts damaged the jacks. Disconnect the wire to the tip on each, then check resistance from tip to sleeve of each jack.
                            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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                            • #15
                              The metal Line Out and Speaker Jacks are all insulated from the chassis with plastic shoulder washers. The OT secondary common wire connects to the Line Out sleeve then a wire goes to the 16 ohm jack sleeve, then 8 ohm, then 4 ohm, then to pin 1, pin 1', or pin 1''. Pins 1, 1', and 1'' are all at the same ground point, but it is not connected to the chassis at this point.

                              The resistor from the Line Out tip does not go to the chassis. That little connector is insulated from the chassis and a wire hooks the resistor to the 16 ohm tip.

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