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Thread: SVT classic transformer color code

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    SVT classic transformer color code

    I have the schematics I need for this amp but I can't find any reference about the power transformer wiring color codes. Does anyone know the colors of the primary windings of the power transformer? I've looked all over and can't anything. Thanks.

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    Supporting Member The Dude's Avatar
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    Is it this?

    svt-pt.png
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    Thanks. But it's blurred to the point I can't read any lettering. This amp has 6 wires going into ,what I believe,is the primary side. The chassis has two holes to allow the power transformer wires to pass through to the circuit. What I consider the primary winding is the hole closest to the back of the chassis. It has 6 wires. The secondary has 4 wires, 2 red and 2 yellow pairs. I think that is the secondary. It's the other hole in the chassis. Then there is the filament transformer with the wires coming off of the AC power entry module to it. I'm getting voltage out of the isolation transformer. I need to check the primary windings of the power transformer for an open winding. But with 6 wires I don't know which is which. The owner said he accidentally plugged into a 220 volt outlet. Don't ask me how he did it. Also I checked the voltage off the AC entry power board and I'm not sure if I'm getting voltage to the power tranny. That's why knowing the wire colors would help. Also it has the relay on the AC entry p c board for the fault circuit. Will that relay have any effect on the incoming AC voltage? Again thanks Dude for the diagram but it's pretty blurred.

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    Supporting Member The Dude's Avatar
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    If you click on the picture it will expand and should be plenty legible at that point. No matter. If yours has 6 primary wires, it's not the right one anyway. Can you post what colors your primary has?
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    Page 3 of the pdf shows the colours of the PT wires and the numbers of the tabs they connect to on the AC termination board.
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    Thanks G1. I'll verify the colors tomorrow but what you posted looks right. I guess my question, which two colors tie into the primary winding? I want to check it to see if the winding is open. Does the primary have a center tap? Or am I totally overlooking something here that should be pretty simple to figure out ? Thanks Dude and G1.

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    Supporting Member The Dude's Avatar
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    The primary won't have a center tap. It's most likely several taps for different voltages. I'm not sure if these colors match your particular amp/transformer, but see the last post in this thread. It might be what you have.
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    Supporting Member The Dude's Avatar
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    Some checks:

    1) No power applied, measure the resistance across the blades of your AC plug with the power switch on. Do you get anything? you should measure the transformer primary if nothing is open. If you measure infinity, either the primary or something before the primary is open.

    2) With the amp unplugged and power switch on, meter from the blades of your AC plug to the transformer wires and find which wires are connected to the power cord. If one or both is missing, something is open in front of the transformer. If you find both blades of the AC plug connected to the primary, the primary is probably open. Double check directly across those transformer wires for continuity to verify.

    You can figure out what's going on even without knowing the transformer wire colors.
    Last edited by The Dude; 06-08-2017 at 05:22 AM.
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    From what I can tell there is no AC voltage going to any of the wires for the power trans primary. I checked the thermistor and it's ok. Fuses and solder joints ok. Does the fault relay have anything to do with the primary winding of the power trans. Thanks.

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    No. (edit: incorrect, appears relay does have to do with PT primary)
    There does not seem to be a schematic showing the primary side, just that layout with the connectors.
    You will have to trace out the wiring from the blades of the AC cord to the tabs that the various PT primary wires go to.
    Last edited by g1; 06-13-2017 at 04:20 AM.
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    Supporting Member The Dude's Avatar
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    Check also the solder on the IEC connector.
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    I know a few days ago I did find a schematic on the power entry module. Drawn out showing where everything went,what did what. Now I can't find it again. It seemed to me that relay tied into the primary side. Before I open my big mouth anymore let me see if I can locate the schematic. I checked voltages again today and on one set of relay contacts I had full AC voltage. I removed that relay and I'm going to jumper across and bring voltage up via a variac and then see if I have any kind of secondary voltage. If I do that tells me the primary is good.Hopefully. I already resoldered the iec connector and every other solder joint on the board. I'll know more tomorrow. Thanks.

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    Was it the SVT2 Pro schematic? Seems to have the right board number.
    I don't understand why no one can draw up complete schematics anymore. Why not show the transformer?
    Not sure what exactly is going on with the relay, but it has to get power from somewhere to engage, so if not from PT it must be from filament transformer. So sorry for saying earlier that it wasn't switching the PT primary, looks like it does.
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    I found the schematic at tangible technology. It is titled SVT classic. I've heard the SVT 2 is close to the classic but since this schematic says SVT classic on it, I'll have to go by this one. I don't know either why it was so difficult to find. Thanks everyone. Let's see what I can do with this.

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    Senior Member Enzo's Avatar
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    You don't have any AC mains voltage on the primary of the main power transformer? Before going nuts, consider that an open primary would still have voltage present, it just wouldn't draw any current for a load.

    The relay breaks the circuit to the power transformer, so does the standby switch. The main power switch powers the heaters, through the small fuse and through the thermistor. Note the relay is controlled by sensing the DC heater supply. Only if it sees that will it turn on the relay to enable the power transformer. No heaters, no high voltage.

    The standby switch is nothing more than a power switch for the main transformer. If you are in standby it is normal for no voltage at the primary wires of the power transformer.

    So you need the standby switch on AND the relay on for any voltage to reach that transformer primary. The AC heater supply goes to the preamp board, and is rectified and sent back for the relay circuit to sense.

    SO...

    Is the standby switch closing? And is the relay clicking on?


    The SVT2Pro is the same board and circuit, they just crammed the speaker jack board onto the same page with it.
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    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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    Senior Member Enzo's Avatar
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    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?
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    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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    And here is the link to those schematics you mentioned:

    Index of /schematics/Ampeg/SVT/Classic/
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    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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    Senior Member Enzo's Avatar
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    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.
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    Yep, and unless I missed it, we don't know if the filaments are lit? If they're not, that'd be a clue.
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    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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    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.
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    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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    Senior Member Enzo's Avatar
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    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.
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    Quote 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.

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    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.

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    Senior Member Enzo's Avatar
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    I think the RV industry used clutch heads too.

    And didn't GM use them in places?
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    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.
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    Senior Member Enzo's Avatar
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    Also allen screws can be tightened with ball end wrenches at an angle, or simply use really long ones to clear possibly constricting transformer bulk.

    I haven't seen an original SVT in a long time, I forgot they used allen screws - or at least didn;t remember that.
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    I don't think the originals used allen screws there.
    The one catstrat is working on is the re-issue.
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    Senior Member Enzo's Avatar
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    Cool.

    I stay with my premise that a skinny allen wrench, especially a ball end type, can access much tighter paces than conventional phillips or posidrive.
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    I just checked the voltage at j 36 and have 6.3 volts. J35 is O volts. Enzo, you mentioned ic2 b. I'll check that. The -14/15 volts you talk about are at what pins on ic2b? I measure heater voltage at the driver tubes. I see your point. Every SVT I've worked on before always seems to be an output problem of some kind. Thanks

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    That icb2voltage should be at pin 7 I believe. The schematic shows 15 volts at a resistor then another resisto that ties into pin 7 of ic2b. Positive 15 volts means it should work. Negative 15 volts mean not working,as you and the schematic both say. Then you have two diodes coming off of pin 7 circuit to ground. Maybe I can check voltage at the resistors coming off of pin 7. If it is a shorted diode to ground then that will bleed off any voltage to ground. I tend to think the AC board is probably good. I'm about to find out. Thanks

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