Double Checking a Power Transformer

[Dubis7]

I'm working on rebuilding an RCA 1YB11 tape cartridge recorder. On my first power up after replacing the paper caps, I melted my power transformer. It was running on the old electrolytics, which, yes, lesson learned. 80 dollars later, Heyboer sent me a new transformer, which I'm about to wire in.

My issue is the resistance values on the transformer coils aren't matching the schematic. The schematic shows three coils - one at 28 ohms, one at 75 ohms, and one without a resistance marked which I assume means it has little to none. My transformer's three coils are measuring 90 ohms, 20 ohms, and less than one ohm. Close, but not exact. It's not wired in, and I'm measuring it cold.

Am I risking damage when I wire this in, or is the discrepancy within tolerable levels?

[Electronic M]

Resistance of power transformer coils does not always match especially replacements from a different maker/production run. I'd power the transformer with nothing connected to the secondaries and measure secondary voltage....If the leads are color coded and you have the voltage and current specs for the replacement, then I'd compare that to the original.

[Dubis7]

One side of the new transformer has four leads, meaning two windings, while the other side only has two leads meaning it connects to one winding.. I want to assume the side with only two leads is the primary, but the primary on the schematic is the 28 ohm winding, and the isolated leads on new transformer show .3 ohms.

Should I assume that the 20 ohm winding is the primary, as it's closest to the primary on the schematic?

When I say primary, I'm talking about the side that's directly wired to the power switch and wall plug. I could be thinking about this wrong.

[Electronic M]

If you don't know which set of wires are the primary I'd be leery of connecting it.

If this is for a tube based deck, one of the secondaries should be 6.3V or 12.6V (check tube types/tube data and heater wiring to know which). Connect the heater secondary (lowest resistance) to a heater winding of the same voltage on another transformer. The winding of your replacement transformer that produces closest to 120VAC is the primary...You can measure the other secondaries while you are at it.

Take care while measuring the secondaries. The line and B+ secondaries can produce lethal voltage and current. Keep one hand in your pocket and don't touch any metal or concrete while measuring it. I've done a LOT of work with mystery transformers like I describe, and not got hurt, but I did not do anything stupid.

[Dubis7]

Yes, this is a tube deck.

Originally Posted by Electronic M

Connect the heater secondary (lowest resistance)

If the lowest resistance is always for the heaters, then I know which winding my primary is. Of the two choices, I'm reasonably confident that the one with a 20 ohm resistance is my primary, especially since it's much close to the schematic resistance of 28 ohms than the 90 ohm winding.

If I'm understanding this correctly - I have two secondary windings. One has very little resistance and transmits a higher voltage to the heaters to warm the tubes. The other has a high resistance and drops the voltage significantly before it goes through the rectifier diode and grids.

Or am I completely off the mark on this?

[Electronic M]

Originally Posted by Dubis7

If I'm understanding this correctly - I have two secondary windings. One has very little resistance and transmits a higher voltage to the heaters to warm the tubes. The other has a high resistance and drops the voltage significantly before it goes through the rectifier diode and grids.

Or am I completely off the mark on this?

Yes and no.
On a transformer low resistance=low voltage/High current (high current needs thicker winding wire to not burn open...Thick wire with low turns count like a winding has low resistance), and high resistance=high voltage/low current. The B+ winding, if it produces the same voltage or higher than line, should have a higher resistance than the line winding.

If the wires still have readable color code (often they fade to beige) red is usually B+.

Do you know for a fact this transformer is correct for your deck, and if so HOW do you know it is correct for it?

[Dubis7]

OK. Then I can definitely trace which leads go where. My 20 ohm should be primary, 90 ohm B+, and .3 ohm heater.

[dieseljeep]

Originally Posted by Dubis7

OK. Then I can definitely trace which leads go where. My 20 ohm should be primary, 90 ohm B+, and .3 ohm heater.

I make a temporary connection to the primary of the transformer and power with the Variac, at a lower voltage to do an initial check of the output voltages. That way, you don't have to worry about damaging the new transformer.

[zeno]

Back to basics. Transformers have turns ratios.
If you got a 20 ohm primary & 90 secondary it will
be 1/4.5 so you get apx 450V
give 450 V, nice for tube B+
The .3 winding gives gives 6V

This is rough due to losses, load, line, RMS vs P-P & other
minor variables but usable still.

73 Zeno

[Dubis7]

Yeah, I've heard that tube based circuits are more forgiving than solid state, so as long as it's close enough, I should be able to wire it in without blowing anything up.