Bias supply on 1203-80

[wdelaney72]

I'm working on a build with a Metro 1203-80. There are supposed to be 2 blue wires for the 95vac bias supply. Instead I've been blessed with one blue and one green wire. I assumed the green was the ground and the blue was the hot.

I wired my heaters, grounded the green bias wire and taped off the alleged blue hot line so I can make sure my heaters were connected. The green bias wire connected to ground keeps smoking and gets warm when I turn the power on.

Does this mean I have a backwards assumption and that the blue should go to ground and the green is hot? I'm thinking 95v drectly to ground would cause a little smoke.

Just looking for confirmation and sanity check.

[SDM]

Either bias supply wire can be grounded, it doesn't matter (why they are often the same color). If the other bias wire is not connected, no current should be flowing through that wire to ground.

So guessing it is not a bias wire. To test, simply measure resistance from the blue wire to the green wire (lift green wire from ground first, and of course unplug the amp!). If they are the same winding they'll show continuity. If not, find the set of wires the green wire does read continuity to, to figure out what winding it belongs to. Just to be sure in case the blue and green do read continuity to each other, measure them to the other PT wires (with the green one green still disconnected from ground). Neither should have any continuity to any other PT wire if they are both bias wires, only to each other.

[wdelaney72]

Thanks, Steve. I'll take a look at that.

Based on what you said, I can already tell they intermixed the location of the heater and bias wires.... UGH. Where the heater wires are normally located, they have 1 heater, 1 center tap, and 1 one of the bias wires. Where the bias wires are supposed to be, they have 1 bias and the other heater wire.

UGH.

[wdelaney72]

Yep. As I described, they mixed up the location of heater and bias wires. Colors are correct. Heater voltage is good and the two heater wires showed continuity like you said and no continuity to either bias wire.

Just to be clear. The bias bias wires do NOT show continuity to each other, but I think that is correct?

This sucks. I gambled on the wires being colored wrong and located correctly. It was the other way around. It fried up my indicator lamp in the process.

[SDM]

Just to be clear. The bias bias wires do NOT show continuity to each other, but I think that is correct?

The bias wires should read some low ohms to each other, around 11 ohms ish maybe on that PT. So they should have continuity to each other, but not quite a direct short. They should have no continuity at all to anything else, no other PT wires (with neither wire attached yet).

[wdelaney72]

As always, Steve... you are right on the money. Thanks for your help.