I have searched the web thoroughly and find very little info on smps transformers. How to take them apart ... bare bones operations in Power supplies etc. I have a smps that still doesn't work after replacing ALL burnt components. So I'm thinking the transformer bit it as well. AFter blue ring testing seems the thing is toast. Took it apart to rebuild. Noted as much as I could and remembered the rest. This transformer is pretty simple ... 2 primaries and two secondaries ... primary one (pins 1 & 2 ) is from the mains measuring approx 165V and is part of the main switching transistor to produce +18V and -18V on it's secondary which of course has a center tap on secondary for the split supply. The 2nd primary is for the 5V supply for the digital supply. It's pins 8 & 9 on the schematic ... it's primary input voltage is the same as the 1st primary on pins 3 & 4. So when I took it apart the winding ratio's didn't make any sense for the respective ins and out voltages.

Primary 2 ... pins 3 & 4 was approx 44 winds @ 28 awg. It's secondary was 4 winds @ 23/24 awg. This one sorta makes sense. This primary was the innermost wind ... it's secondary was the outermost wind.

The 2nd primary was only 2 winds (possibly 1 wind) and it's secondary winds were not counted but was at least say 20-25 winds. Insulating tape was used between all winds but not sure how thick each layer was so subsequent winds after the 1st weren't known exactly.

So problem is I need secondary voltages of +18 and -18 on the 1st primary and secondary then need 5v from 2nd primary and secondary ... according to what I know about turn ratios they don't line up.

SMPS power supply that is in question

After rebuild 1 I had 5v supply working but found out that priamry one should be attached to pins 3 & 4. Got that fixed ... haven't finished the rebuild at that point. BUT the windings for second primary and secondary don't make sense for +-18V One or two winds for primary and say 20-30 winds dual for split secondary. +-18V

Any help would be appreciated

  • \$\begingroup\$ One odd thing that has happened using the blue ring tester tesing the winds as I do them. Wind the 1st primary and it rings ok the wind 2nd primary and it rings ok but then 1st wind is bad again ? WTF ? \$\endgroup\$ Commented Aug 11, 2018 at 12:20
  • \$\begingroup\$ Of course with the ring tester the primaries without the secondaries don't ring like they should but I do see the ringers pulses. \$\endgroup\$ Commented Aug 11, 2018 at 12:21
  • \$\begingroup\$ Also note ... when taking apart the unit I broke the lower and upper portions of the E part of the ferrite core ... I am hoping crazy glue can reattach these arms and still function. I have done this with ferrite cores for magnet motors without issues. \$\endgroup\$ Commented Aug 11, 2018 at 12:24
  • \$\begingroup\$ Since this is an audio piece of equipment should I just build a linear supply for it ? Problem is a linear won't fit inside the old box ... will have to have a separate box for it. \$\endgroup\$ Commented Aug 11, 2018 at 12:26
  • \$\begingroup\$ It's an SPX90 effects unit for studio or live effects \$\endgroup\$ Commented Aug 11, 2018 at 12:33

1 Answer 1


It seems to me that you are confusing some things. The two primary windings are on the LEFT in the schematic, pins 1-2 and 3-4. The two on the RIGHT, are secondary windings, pins 5-6-7 and 8-(9).

The 2nd primary is for the 5V supply for the digital supply. It's pins 3 & 4 on the schematic

It doens't work that way. In a transformer, all the coils influence each other. If I understand the schematic correctly, the primary coil that 'does all the work' (power goes IN) is the Pin 1-2 winding. The 3-4 primary winding is for feedback and power for the primary switching transistor/circuit. So although it's on the primary side, power doesn't go IN, but OUT. I'd call it a primary winding, because it's on the live side of things. But maybe to call it that is wrong, and it should be called a secondary winding? I'd be interested to learn how that works actually. I guess it depends on how you look at it.

primary one (pins 1 & 2 ) is from the mains

Primary 1 ... pins 3 & 4 was approx 44 winds

Let's stop the confusion, and call Pins 1-2 Primary 1 (P1), and pins 3-4 Primary 2 (P2). Pins 5-6-7 Secondary 1 (S1) and Pins 8-(9) Secondary 2 (S2). Maybe you should edit your post to match...

So count the windings of P1. Then count the windings of S1. This should be something like 9:1:1 (for 50% duty cycle at 165V). So for every 9 winding on P1, S1 Pin5-7 has 1 winding and Pin 6-7 has 1 winding (2 windings in total). Of course there are guestimates, the actual amount depends on the desing of the SMPS. Also take note that the direction of the turns matters!! The turn direction is marked by a dot in the schematic.

Then for S2, you still compare with P1. This should be something like 33:1 (again, for 50% duty cycle). Update: Apparently this SMPS has quite a low duty cycle at 165VDC, see corrected values at UPDATE below.

I don't know about your broken E-coil, but I'd say to glue it is a long shot, and you'd be better off with a new transformer... But I might be wrong.

Also note that not all broken parts will seem 'burned'. Usually the main switching transistor blows, taking more parts along with it. You can't always find the faulty parts by eyesight.

I hope you'll be able to fix it. Just make sure you insulate the primary windings from the secondary ones properly. Use the right tape for it, as your safety depends on it!

Hope this helps. Good luck!

UPDATE: So the correct number of winds should be:

  • P1 (pin1,2) has 44 winds
  • P2 (pin3,4) has 2 winds
  • S1 (pin5,6,7) has unknown winds
  • S2 (pin8,9) has 4 winds


  • Up1:Us2 = 165:5 = 33:1
  • P1:S2 = 44:4 = 11:1
  • 33/11=factor 3


  • Up1:Us1 = 165:18 = 9:1
  • divided by factor 3 gives you:
  • P1:S1 = 3:1

Since P1 has 44 winds, 44/3=14.7 (approx. 15). That would mean that S1 should have 15 winds for 18V (so 2x 15winds for 2x 18V -or- 15 winds between pin6-7 and 15 winds between pin7-5)

You can read more on how it works here

About turn direction:

Look at the dot in the schematic. If you connect the start of the wire to the dot, start turning in one direction, say CLOCKWISE. Then for the other coils, also connect the start of the wire to the dot and start turning in the SAME direction - in this case I chose CLOCKWISE. This way you'll do the direction right. You can also choose to turn CCW, but then you should do the other coils also CCW.

The dot on P2 is in the wrong position. The dot in the schematic should be on pin4, and NOT on pin3!

About dot notation:

Yeah, the dots... there's so much confusion about them, and I've looked at many schematics online only to find so many dots in the wrong place.

  • P1 is the coil that creates the magnetic field. The + is connected to the dot, so current flows from the dot terminal to the non-dot terminal. Remember that.
  • S1 is a coil that does the opposite. It turn the magnetic field into a current. Here, the current flows in the SAME direction, so from dot to non-dot. To make that happen, the + must be at pin 6.
  • Same for S2. Current flows from pin9 to pin8, so the + must be at pin 8. That matches with D8 and D10, otherwise nothing would happen.
  • For the + to be at pin3 of P2, current must flow from pin4 to pin3. So the dot must be at pin4.
  • \$\begingroup\$ Hi Martin, thanks for pointing out my errors in the post. I corrected it. Your tips were interesting especially about that wind at pins 3 & 4 ... that one is throwing me for a loop. Feedback sounds like a good conclusion but who knows ? About the ratio you stated, shouldn't it be Hi:Lo instead of Lo:Hi ? To step down ratio should be for example 4:1 no ? I found another core same dimensions as the old one so can build a second one after I rebuild old one and see if I can get it going. My insulation tape is glass cloth class B insulation. It seems to be doing the job since I ran it once already \$\endgroup\$ Commented Aug 12, 2018 at 15:28
  • \$\begingroup\$ No sparks flying or magic smoke getting out. I couldn't find anything else at the electronics supplier in town and they really didn't have any idea what kind of tape would be used for transformers anyway. Looked online and didn't find out much either. Things like paper, oil and one other used in large power transformers ... nothing about small stuff. Although I know for flybacks that use high voltages one needs something like silicone or similar to insulate from arcing. \$\endgroup\$ Commented Aug 12, 2018 at 15:29
  • \$\begingroup\$ After the 1st rebuild, I found a note I made about the most inner wind ... seems it was connected to pin 3 & 4 so I had 1 & 2 and 3 & 4 reversed which would definitely cause a problem. Weird tho is that the secondary for the 5v supply worked, had just over 5v when powered up but nothing on pins 5 to 7. Currently correcting that error and rebuilding again will get back here with results. \$\endgroup\$ Commented Aug 12, 2018 at 15:44
  • \$\begingroup\$ Some extra note about number of winds I know for sure. Pins 8 & 9 had 4 winds. pins 5,6,7 were not counted ... forgot. Pins 3 & 4 had approx 44 winds and Pins 1 & 2 had 1 to 2 ... I'm leaning towards two due to the actual length of the wire then when wound it was 2. So I'm trying to number crunch your suggested ratios and they don't seem to jive. \$\endgroup\$ Commented Aug 12, 2018 at 15:59
  • \$\begingroup\$ Some extra note about number of winds I know for sure. Pins 8 & 9 had 4 winds. pins 5,6,7 were not counted ... forgot. Pins 3 & 4 had approx 44 winds and Pins 1 & 2 had 1 to 2 ... I'm leaning towards two due to the actual length of the wire then when wound it was 2. So I'm trying to number crunch your suggested ratios and they don't seem to jive. If pins 1 & 2 is the main/only primary at 2 turns and pins 8 & 9 was 44 winds then the ratio ends up being 1:22 not 1:33. \$\endgroup\$ Commented Aug 12, 2018 at 16:11

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