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SVT classic transformer color code

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  • #16
    Thanks Enzo. I guess what you're saying is the standby switch closes the relay allowing high voltage to flow to the tube plates. I have heard no clicks at all from the relay. I removed the relay and jumpered across the solder pads. Brought the power up to 20 AC volts on a variac and now I have voltage present at the tube plates. Now before I bring it up to full 115 volts, am I about to fry something I'm not aware of?

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    • #17
      No, but you didn't follow my story. I recommend you take the jumper off and solve the relay circuit. Look at the circuit, the mains hot line comes in at top left, through power switch at J12/13, from ther down and to the right to the standby switch, J25/26, and immediately through the relay contacts, pins 8/9. When the relay closes pins 8/9 connect to pins 6/11, completing the circuit to the PT primary J27/28. The mains cold - neutral - comes in upper left, straight across then down to J31/32, the other end of the PT primary.

      The standby switch does NOT pull in the relay, the relay is pulled in by that MSPA13 transistor, which in turn is energized by the DC heater voltage from the preamp. SO again, the heaters have to be working before anything else happens. Once heater power is present, THEN the relay turns on. The standby switch is in series with the relay contacts, so you need BOTH to be on for high voltage to occur. If the standby switch is off, no power reaches the power transformer regardless of relay, and likewise with the relay off, it doesn;t matter what the switch does, the circuit is not complete. Only when the relay is on AND the switch is on can power reach the PT primary. But the standby switch does not control the relay.


      SO look at the schemo, see at the bottom J36, a two pin connector where the heater voltage FROM the preamp comes in. Look left to the layout, the J36 is lower left corner. is there about 6vDC between the two pins? If so, we move on. The relay is near J36, the boxy thing, and right off the end of the relay is diode D1. D1 is parallel the relay coil, so while it is not convenient to get at the relay pins, we can measure any coil voltage across D1. If there is something like 5vDC across the diode, then the relay SHOULD be energized, but is likely open. If we get zero volts, move to the transistor. That is just to the right nearby. DC volts from emitter to collector? If you get about 5v, the transistor is either open or not turned on. If you get zero volts, then either ther is no power in the circuit or the thing is ON.

      That leaves the control to the MPSA13. That signal comes in from the power amp. Look on the power amp page, bottom center in the fault circuit. See where it says to AC board? Normally the +15v charges up C12, which sends a positive voltage through R44 on to the AC board. That turns on the MPSA13 there. If there is a fault in the power amp, IC2B goes to -14v, which hauls that line to the AC board low, turning off the MPSA13, and thus the relay. SO the fault kills high voltage. You can check the voltage at J35, the wire terminal on the AC board bottom left, not far from the diode and transistor. You have some positive volts or close to zero or negative?
      Education is what you're left with after you have forgotten what you have learned.

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      • #18
        I was half asleep when I read your post this morning. I need to take my time and read your last post carefully. It's a very thorough post and it opens up a lot of mysteries for me. My little jumper wire did work. Anyway the power trans is working. That's why I did the jumper wire. Back to the problem. I thought the to 92 transistor only affected the fault mode. The output tubes test good. If he did plug this amp into a 220 volt outlet,like he said he did, I'm guessing any damage done would be confined to the iec and surrounding components. Let me re read your last post Enzo,take some notes and check the things you suggest and I'll report back later. Thanks.

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        • #19
          And here is the link to those schematics you mentioned:

          Index of /schematics/Ampeg/SVT/Classic/
          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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          • #20
            Since I had the relay out anyway I tested it on the bench and it does engage. The mps a13 transistors I have in stock so I'll just pop in a new one. The diode in parallel with the relay coil tested good. This amp has not had the screen resistor and diode removal mod done yet. I will take care of that,install tubes again and check the voltage going to the mps a13 and see what I get there. I'll check the cathode resistors while I'm at it. I think I found cracked solder joints at the output tube sockets. Those will get resoldered. That's the plan for now. Thanks.

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            • #21
              Really, before throwing parts at it, just check for voltage at J36. If there is no 6vDC there, then the transistor and relay cannot do anything. And if ther is voltage there, check the control signal coming in on J35. The point is, we need to isolate the problem to a specific place. What if the problem is a bunk resistor on the power amp causing a loss of the relay control signal? We don't need to replace every part on the relay board to decide the issue is elsewhere.

              That is the whole point of systematic troubleshooting - to isolate the problem. Once we know where the problem lies, it is usually simple to correct it. The two voltage readings I suggested above take a mere couple of seconds to do and they tell us tons about the circuit condition. Really, this is more efficient than swapping a lot of parts.
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                Yep, and unless I missed it, we don't know if the filaments are lit? If they're not, that'd be a clue.
                "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                • #23
                  Honestly Enzo, I didn't know an SVT operated like this. I thought it worked like any other tube amp,power switch on to get voltage at the primary side,heater voltage,low voltage and maybe bias voltage. Then close the standby for hi voltage to the plates. I didn't know all this other stuff comes into play. You did very good job of explaining how it does work. Anyway I just finished with doing the Ampeg update to the screen resistors and removing the diodes. I did take care of some cracked solder joints at the output tube sockets. I checked the grid stoppers and the fly back diodes. They were all good. The reason I replaced the mps a13 was because I have a drawer full of them and I just don't trust my troubleshooting skills with solid state devices. If the transistor cost 15 bucks, that would be different. I don't work on many solid state amps. Honestly my schematic reading is challenged on more complicated circuits. Let me get back to it. Thanks everyone.

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                  • #24
                    The journey of a thousand miles begins with a single step. And proceeds with step after step. We learn incrementally. Make each repair an opportunity to stretch your knowledge and experience. I try to hide little electronics lessons in all the discussion. Just keep plowing the field, you will eventually come to realize you know what you are doing.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #25
                      Thanks Enzo. Even my wife knows problems sometimes are a pain to figure out. I will say this,you guys are helping me learn. Every time I post a problem on this forum,I walk away with a wealth of knowledge. Again, thanks to everyone on this forum for chiming in. My band is gigging this weekend so the SVT is going to wait til Monday. I have one last question before I leave,why does Ampeg use those Allen head screws for the o/p tube retainers? I always replace them with plated Phillips head screws. Number 6 I believe. Have a great weekend everyone.

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                      • #26
                        Allen screws? You mean the clutch head screws? The hole looks like a little bow tie?

                        Clutch heads have a real solid grip on a driver, unlike phillips, which allow the driver tip to climb out of the hole.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #27
                          Originally posted by Enzo View Post
                          Allen screws? You mean the clutch head screws? The hole looks like a little bow tie?

                          Clutch heads have a real solid grip on a driver, unlike phillips, which allow the driver tip to climb out of the hole.
                          Besides Ampeg, the only other place I've heard of clutch heads being used is on Studebaker and maybe Hudson car interiors. I guess Ampeg must have known what a good fastener they are, not likely to be removed or tampered with, and/or they must have gotten a knock-down discount on a train car load of them.
                          This isn't the future I signed up for.

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                          • #28
                            The story given in the Ampeg book was that they switched to clutch drive when the factory was equipped with pneumatic tools. The square drive didn't strip as easily as Pips head, which backs up Enzo's assertion. Of course, if you use a pips head or Allen wrench on them or the wrong size square bit, they strip like anything else. But any assortment of drill-type screwdriver bits comes with a few square drive bits in different sizes, so the tools are fairly easy to get. I usually replace the screws, too.

                            Justin
                            "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                            "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                            "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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                            • #29
                              I think the RV industry used clutch heads too.

                              And didn't GM use them in places?
                              Education is what you're left with after you have forgotten what you have learned.

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                              • #30
                                This is a SVT-CL, so a re-issue of the classic. I'm pretty sure he means real allen head screws here. Not sure why they use them but I do find them to be better quality and less prone to stripping than phillips screws.
                                I wonder if it has to do with robot/mechanized assembly? An allen will stay on a bit much better than a phillips will.
                                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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