110/220V Taps (Long)

[ron944]

Hi, I have a Metaxas Preamp that I am sending to Paris and need to tap the transformer for 220V. It has a seperate power supply and when I opened it there are two bundles of wires for the primary each consisting of a Black, Brown and Orange wire. As it is wired now for 110v it uses the Black and Brown on one bundle. When I measured the resistance it showed 7.6 Ohms. I was told that the 220v taps should measure almost double as they will be in series. I measured the Black and Brown from the other bundle and it also measured 7.6 Ohms. The Black and the Orange from the non used bundle measured 9.4 Ohms, the Brown and Orange on this bundle measured 2.1 Ohms. Would the 9.4 Ohm measurement be the 220v tap? I have a Variac but it only goes to 140v. Is there anything else I can do to verify I have the right taps. Thanks in advance.
Ron

[imfree707]

Originally Posted by ron944

Hi, I have a Metaxas Preamp that I am sending to Paris and need to tap the transformer for 220V. It has a seperate power supply and when I opened it there are two bundles of wires for the primary each consisting of a Black, Brown and Orange wire. As it is wired now for 110v it uses the Black and Brown on one bundle. When I measured the resistance it showed 7.6 Ohms. I was told that the 220v taps should measure almost double as they will be in series. I measured the Black and Brown from the other bundle and it also measured 7.6 Ohms. The Black and the Orange from the non used bundle measured 9.4 Ohms, the Brown and Orange on this bundle measured 2.1 Ohms. Would the 9.4 Ohm measurement be the 220v tap? I have a Variac but it only goes to 140v. Is there anything else I can do to verify I have the right taps. Thanks in advance.
Ron

This procedure should not result in human injury or property damage, if
executed responsibly. Measure and note the voltage of one of the
transformer secondaries, while in place, running on 110v setting, plugged
into a normal outlet. Carefully change the transformer wiring to 220v.
Using a safely constructed test adapter, connect a light bulb of similar
watt rating to nameplate wattage rating of your preamp, in series with the
incoming power. If the rewiring to 220v is correct, your previously
measured secondary should have about half the voltage it had before,
while testing on 120v. Testing at half voltage to get a voltage reading,
then promptly turning the preamp off, should not harm it. Improper wiring
will almost always cause the bulb to brightly light, limiting incoming current
to protect your transformer. A properly wired and insulated test fixture is a
must! I keep one in my toolbox and it has been a valuable safety and
troubleshooting aid that has kept me out of trouble many times!

[DF96]

It sounds like you have two primaries, which may currently be wired in parallel. Each has black-brown-orange, probably 0-110-130V (or 0-100-120V?). You will need them in series for 220V: black1-brown1-black2-brown2 i.e. connect brown1 to black2, then mains input to black1 and brown2. Then test as described by imfree707.

[ron944]

I was thinking that the two Black and Brown needed to be in series but did not know how to wire them to create it. Should the resistance then be almost double the 110v reading? Thanks for you help.

Ron

[DF96]

No, it should be four times the 110V reading - if the two primaries are currently in parallel. If it is just using one primary at present (which would be unusual) then the resistance will double.

[ron944]

When it was wired for 110v only one of the Primaries was being used, the other along with the Orange wire from the used primary were heat shrinked and tied off. I rewired the Black from one primary to the Brown of the other and then used the remaining Black and Brown from the two primaries as the mains. I tested it and had a reading of 15.6 Ohms, double the reading for 110v wiring. Thanks for the help I will use my Variac and the light bulb and see what the output is tomorrow.