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Sovtek MIG 100H power transformer wiring

statzern

Member
Messages
37
Hey guys, first time poster to the forum - really good info going around here.

I'm rehabbing a Sovtek MIG 100H head and I'm stuck on this last detail. This may sound incredibly dumb, but I'm having a hard time figuring out how to wire the AC to the power transformer. Pretty standard, the AC comes in at the IEC connector, one limb goes to the fuse and on to one pole of the power switch, the other straight to the other pole. Then, the other side of the power switch has three wires that are supposed to connect - two of which go to the voltage selector switch, and one white wire that goes straight into the transformer. I'm not sure how to connect these to the power switch. Unfortunately, there are no colors marked on the schematic, and no info on the net regarding this I can find.

My guess is the wire going into the transformer is one end of the primary winding and should be connected to one of the wires going to the voltage selector switch, and the other wire to the voltage selector should be on its own. Main reason I'm confused is why there are three wires going to the switch and not two as the schematic would indicate. I attached a really bad drawing of my proposed wiring. Hopefully someone can help me out here!

Thanks!
Power%20wiring.png
 

pdf64

Member
Messages
8,404
It seems unlikely that the voltage selector would have 120V AND neutral going to it.
When you've wired it up, I suggest you power up via a light bulb limiter.
 

UsableThought

Member
Messages
1,649
It seems unlikely that the voltage selector would have 120V AND neutral going to it.
When you've wired it up, I suggest you power up via a light bulb limiter.

I might also suggest, don't wire up the transformer yet. Do a partial wire-up that you can read with your meter without yet going to the tranny. Or even simpler than that, just use the continuity setting on the meter - you ought to be able to make more sense of things than that diagram is showing.

I'm only suggesting intermediary stages & not going full-bore head-on because it seems to me there's no point in subjecting the tranny (or anything else that can be damaged, including you) to mistaken wiring, especially since these things can usually be puzzled out first.

You could also try posting some pictures of the gear involved - these would be more helpful than the drawing, I think.
 

Jeff Gehring

Gold Supporting Member
Messages
6,907
Yes, the neutral from the IEC and the PT primary low side should be terminated on the two lugs of one pole of the DPST power switch. You've drawn it that way, but you also added a wire going from the switch to the voltage selector, and as has been noted, that would serve no function.

uc
 

statzern

Member
Messages
37
Thanks for the replies everyone.

You've drawn it that way, but you also added a wire going from the switch to the voltage selector, and as has been noted, that would serve no function.

I should add, I replaced ONLY the switches in this section of the amp, and my mistake is that I did not take a photo of the wiring before I took everything apart. I know, major rookie move. However, there were definitely three wires connected to one side of the power switch when I took it apart, not two. I just can't tell what this mysterious third wire is supposed to be.

I think what is going on here is that the wire from the transformer is the COM wire as in this diagram: http://www.joepopp.net/jcmtwins/jcmtwins003big.jpg
The voltage selector switch appears to be a DP4T switch, which seems unnecessary, and is not what is shown in the schematic. If I were just following the schematic, there should be one wire going from the switch to the primary and another going to the voltage selector, but that is not how the amp was wired before I hastily took it apart.

I will post photos ASAP. I am out of town until Monday. Will update then.
 

statzern

Member
Messages
37
Thanks for the replies everyone.
I will post photos ASAP. I am out of town until Monday. Will update then.

Here are the photos I promised.
Switch%20full.jpg


Switch%20Left.jpg


Switch%20Right.jpg


switch%20wiring.png


This is how I believe the switch is wired up. I am not 100% sure about the dashed blue wire, and I believe it may be connected to pin 10 next to it, but I cannot see back there to tell.

All I basically need to know is if the dashed white wire (mentioned above, coming from the PT) should be attached to the power switch lug going to the top pin 5-6 or the bottom pin 3-4. It was one of these when the amp was working.

Thank you so much in advance.
 
Last edited:

Jeff Gehring

Gold Supporting Member
Messages
6,907
With that much of a switch (looks like maybe a 4 position 6 pole?) Try to identify where the common lugs are for each pole; they frequently are on a slightly different 'plane' in the switch decks. I wonder if the three coils on the primary side are separately brought out windings, and rely on the switch to combine them? How many wires are coming from the PT through that hole, seven?
 

Kyle B

Silver Supporting Member
Messages
5,233
Have you considered omitting the voltage selector and hard-wiring the primary you need to the power switch?
To simplify life -- ^^^^ is the right answer! I do it to my own rigs usually. Unless one is an international touring musician (I'm not) or if one intends someday to sell their amp and ship it across an ocean (I don't), then the amp will only ever see "one" supply voltage in it's entire lifetime. The voltage selector can not add anything to the usefulness of the amp, and may in fact detract from it (via switch failing, or somebody monkeying with it when you're not looking, etc...)
 

statzern

Member
Messages
37
SOLVED!

I poked around more with a multimeter and discovered the correct wiring is with the wire going to the bottom pins 3-4 on the switch should be tied to the white wire from the power transformer (In other words, OPPOSITE to the drawing I posted above!!!). If it is the other way around, the primary gets shorted in 100 and 120 v settings. I finished wiring it up, biased the power tubes and it is DONE! Sounds beautiful. Very happy with it.

-- Overall, I replaced in this rehab:
-- All filter caps with F&T caps
-- Bias supply filter caps with Nichicon Fine Gold
-- All coupling caps with Mallory caps
-- Almost all ceramic caps with silver mica
-- Input jacks and all input wiring
-- Power and standby switches
-- Removed some modifications that had been made by previous owner, back to stock circuit

FullSizeRender%20%282%29.jpg


IMG_0277.JPG


IMG_0278.JPG


IMG_0279.JPG


IMG_0280.JPG


Once again, a huge thank you to everyone here! Hopefully somewhere down the line this helps someone else.
 




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