Hi,
below is the circuit diagram of a old Pioneer receiver SX 1500. I am planning to reuse the transformer. In the picture (the thick black lines on the right top) pin 11 seems to be center tap and 13 and 9 are the other 2 ends of the winding. Between 11 and 13/9 I measure 33 V. But I don't need 33 V, all I need is about 18V which is what I measure between 12/10 and 11 the center tap. Can I use 12/10 with 11 for the PSU for a 50 W amplifier?

The fuse rating for the old amplifier says 2.5 A and is designed for about 100 W. My doubt is is there any current limitation on the different secondaries of the same transformer or can all of them supply similar currents.

Thanks

 

Generally, they are different. You'll need to get a datasheet to find out what they are.

 

I would say yes, if you use the two diode full wave rectifier method (D601,602) like on the circuit of IC601 its got 23V input, (top right on circuit,) this method doubles the current of the transformer windings.

 

So I won't be able to power a bridge rectifier to get a dual +/- 24 to power about 50 W amp using the 12/10 and 11 the center tap ?
Thanks

 

If you can figure out the VA rating of the transformer you should be able to take it out of any tap combination you want

 

Hi,
below is the circuit diagram of a old Pioneer receiver SX 1500. I am planning to reuse the transformer. In the picture (the thick black lines on the right top) pin 11 seems to be center tap and 13 and 9 are the other 2 ends of the winding. Between 11 and 13/9 I measure 33 V. But I don't need 33 V, all I need is about 18V which is what I measure between 12/10 and 11 the center tap. Can I use 12/10 with 11 for the PSU for a 50 W amplifier?

The fuse rating for the old amplifier says 2.5 A and is designed for about 100 W. My doubt is is there any current limitation on the different secondaries of the same transformer or can all of them supply similar currents.

Thanks

View attachment 96194

How big are the wires going to 10, 11,12? Are they the same size as 9 and 14?

 

The 24V tap is probably feeding the preamp and tuner section, so it wont need to be large, the 33V is for the power amp, that winding will have more current capability,

Look at the transformer windings see if they are the same size.

 

How big are the wires going to 10, 11,12? Are they the same size as 9 and 14?

Yes.
Also when I measure resistance between them they all come to almost a short.

 

You cannot use any single winding in order to utilize the max Va of the transformer without taking into consideration the current rating (gauge) of the winding you intend to use.
Max.

 

How big are the wires going to 10, 11,12? Are they the same size as 9 and 14?

I don't in terms of wire gauge but they are all the same size, even on the primary side from the mains the size is the same. Also all of the points 9 - 13 are show up as short ( < 1ohm) on the multimeter.

 

You cannot use any single winding in order to utilize the max Va of the transformer without taking into consideration the current rating (gauge) of the winding you intend to use.
Max.

Ok. thanks.

 

Measuring the resistance of transformer windings is difficult because the resistance values are typically so small.

A way to more accurately measure low resistances is to do a 4-wire measurement.
If have a DC voltage supply, put a known current (say 1A) through the winding and measure the voltage drop directly across the winding leads. From that you can calculate the winding resistance using Ohm's law.