PT reverse engineering problem

[mhuss]

I have a Partridge PT supposedly from a SC200, and the secondary windings and voltages it mostly check out as expected - but the three big wires (black, blue, green) coming out of the secondary, which one would assume are the heater winding, measure ~62VAC across two pairs and ~3.1VAC across the other. I double- and triple-checked the readings (it's an auto-ranging meter, so the only setting is "VAC", and the decimal point is clear and obvious).

There is a separate bias winding that provides about 42VAC.

Any of the three wires measure open to any other winding wire.

The black wire is solid core, the other two are stranded.

I confirmed I'm supplying mains power to the correct primary winding (the other windings autoformer to the expected other voltages).

Any ideas why this would be? I know unloaded windings always read higher than nominal but 10x seems excessive!

[martin manning]

Why are some of the voltages censored in that schematic?

[mhuss]

You'd have to ask Sound City.

The upper HT winding is ~250VAC and the lower is ~325VAC (both no load).

Here's the 'left hand' side of the schematic fwiw

[martin manning]

Have you ID'd the shield wire?

[Stevem]

The solid core black wire must be the shield attached to the core.
Other then that you got me!

[mhuss]

There is very low resistance (< 1 ohm) between the three big wires.

[sluckey]

I would try a different meter.

[TUBEDUDE]

Odd they used brown, blue.and green for.a.secondary winding. Being generally used for primary wiring. Is the xfmr damaged? Wires less than an Ohm sound shorted, no?

[R.G.]

Try sticking a very mild load on the windings as you measure. Maybe something like a 10K across the meter leads. Your meter could be measuring stray capactively coupled voltage.

[mhuss]

I think R.G. is onto something. I put a 250 ohm 25w resistor across all three combinations, and read 3.2vac (again) on one, but the other two now read almost nothing (0.007xx vac). These three wires are the only "8 amp" (6 x EL34) sized wires coming out of the transformer, so they must be the heater winding.

It seems as though two are connected together, but 3.2vac indicates we're only seeing half the winding.

I also double-checked the primary connection, putting in 120vac on one, the other three read 100v, 220v and 240v as expected.

[R.G.]

Condolences.

Loading the windings a little wasn't needed in the bad old days of analog meters, but today's meters have input impedances so high that they'll pick up stray voltages from anything.

[mhuss]

Yeah, little high gain antennae.