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I just got a Soundcraftsmen CX4200 preamp/eq at a pawn shop for cheap. It's in rough shape, and needs 220V.
I opened it up, and it appears that the small, simple power transformer has half the primary shorted together. There are two white wires and one purple wire of the same gauge (about 18 ga) and two green wires and a yellow wire of about 20 ga that connect to the circuit board. The smaller wires are pretty clearly the secondary.
One of the white wires is connected to hot. The other white wire is connected to the purple wire and to the return. Only using half the primary makes sense for 220V. But shorting the other half of the primary? Can anyone hazard a guess?
thanks,
Lee
Follow Ups:
A power transformer with a 3 wire primary, wired as you say, is most probably the manufacturer shorting together voltage taps on the end of the winding (the one with the least amount otf turns). This would be my best guess of what you describe.
I would not believe that the transformer has a center tapped primary wired as you suggest as this would place the windings out of phase and damage the transformer.
A power transformer designed for universal 120/240 VAC operation would have 4 wires on the primary, or two separate windings color coded.
Look at the back panel and look for information that shows the fuse value for 120 and 240V operation, a sign that the Preamp could be configured for 120/240V operation.
If this was a European model, then you will probably need a 120/240 VAC step-up transformer to operate it properly.
There's a note on the unit saying that the unit is convertible to 120V and to contact the factory for instructions. The manufacturer, of course, is out of business. So I'm pretty sure the thing is convertible, and probably without replacing the transformer. I'm going to try using half the primary and see what happens. If it smokes, I'll junk it. If it works, I've learned something. I got it cheap, it's in lousy condition, so what the heck. I'll report back on how it comes out.
I think it's risky to proceed like this, the note could imply one of two things.
1) The factory would re-wire the primary of the already installed transformer. Lucky you!
2) The factory would swap the 220V transformer for a 120V version.
I would note down the wire configurations and de-solder the primary.
Then with an Ohmeter, measure the wires and see if you have a split primary (roughly equal DCR between one the wires and the other two).
Or a tapped primary to adjust for +/- mains voltage deviations from 220V.
This would result in un-balanced DC resistance readings.
If you had access to a signal generator and an oscilloscope, you could de-solder the transformer and apply a signal to reverse engineer the winding ratios to determine the AC Voltage parameters.
Another option is to replace the existing transformer for a close substitute that has a 120V Primary.
You can measure the secondary if you get hold of a proper step-up tranny.
The current capacity of the secondary must be a few amps DC. You can measure the rectified DC with an ammeter and reverse engineer.
Also the secondary AC voltage is simple to determine with an AC voltmeter.
The safest would be to buy a step-up transformer unit and be done with it, or borrow one for testing.
The saddest thing that can happen is that the preamp is broken and you find out only after applying power properly.
It's a preamp, so the current draw is steady. Shouldn't affect the sound or performance unless placed under a sonic microscope :)
Good luck!
Thanks for the thoughts.
I like your idea about using a signal generator to determine winding ratios. I think I'll do that.
I won't be substituting a power transformer. This guy has been ridden hard and put away wet, and it just isn't worth much. If I can get it cleaned up and working properly I'm going to sell it for $50. It's just too ugly to ask more, no matter how well it operates.
I'm looking at this as a learning piece, where if I totally hose it I'm not out much.
Thanks again for the great ideas.
I wish I was at home as I would have lent you my 120 to 220 VAC step up transformer I bought while in Stockholm Sweden some years back.
Since Sept 13th have been in temp housing and all of my "stuff" will be in storage until Spring. Moved north.... from the Caribbean.
I use the step-up to service 220V equipment for troubleshooting purposes or to convert vintage USA tube equipment to 220VAC for overseas buyers.
The last piece I changed was a Dyna PAS4 Preamp that went to Italy.
This PAS4 was fortunately designed for dual 120/240 VAC operation by re-plugging a jumper board and changing the fuse on the back panel.
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