YAESU YS-500 SCH

[W8UJX]

Hi Guys. 

I have a YAESU YS-500 SWR/POWER meter that needs repair.  One of the resistors on the PC board inside the little metal box attached to the two COAX connectors is burned so bad I cannot read the value.  I cannot find a schematic of this little board anywhere.

There are six resistors, two diodes and two small variable capacitors on this PC board. 

I wonder if anyone has a schematic of this board or knows where I can find this information.

Thanks much,
Jerry, W8UJX

jk.ashley@verizon.net

[Pete, WA2CWA]

The little board is probably a directional coupler. You can try searching some of Yaesu's other "YS" models to see if a similar schematic is shown or you can get a rough idea of a directional coupler design by checking the ARRL Handbook and/or other manufacturer's designs. Typically, similar or identical components are on either side of the toroid.

[W8UJX]

Hi Pete.

Yes, it is a directional coupler.  Does not use a toroid.  It uses a short heavy bus bar placed between two short heavy traces on the PC board.  Each trace connects to three resistors, a small variable cap and a diode.  Each end of the bus bar connects to a COAX connector.

I'll take a look in some of the ARRL handbooks.

Tks much,
Jerry

[Pete, WA2CWA]

If you look to the other side of the sense trace, you probably will find the same resistor value. At least it's a starting point.

[W8UJX]

Hi Pete,

Wednesday morning, just got up about 8 AM.  Fed the cats and turned on the PC.  Tks for the tip on the YS-500 problem.

That resistor I burned up was only 1/4 or 1/8 watt I think.  So I replaced both of those resistors with 33 0hm 1 watt resistors.  That tells me that the good resistor on the other side was 33 ohms or close to that value.

I remember what happened that blew up the YS-500.  In January 2007 the log shows that I built a 2 meter 500 watt amp for my Yaesu-736R.  My log shows I worked several stations in Europe on 2 meter EME.  I'm sure that I tried to measure the power of that amp with the YS-500 and blew it up.  It's rated at 200 watts max.  I probably tried to repair it with higher power resistors and then put it on the shelf.

I'm 79, retired now so I have time to repair some of the stuff setting in the shack or outside in the garage.

The YS-500 appears to work ok but I would still like to see a schematic of that directional coupler to make sure that I replaced those two resistors with the correct value. 

So, as they say, that's the rest of the story.

Have a good day.

Jerry, W8UJX

[AB2EZ]

Jerry

The manual (including the schematic for the box, but not the sensor) is here:

http://www.qsl.net/kb7tbt/manuals/yaesu/ys500%20manual.pdf

Does the sensor have a separate part number on it?

Separately, this reference should be helpful:

http://vk5ajl.com/projects/swrmeter.php

Note the sentence below the diagrams:

"The terminating resistors are critical but only in so far as they should be the same and somewhere near the correct terminating value for the width of the pickup strips.

The correct value can be estimated using the formula above the diagrams:

"Z₀ = 377 x (t/w) x E_r^-0.5

where t/w is the ratio of thickness of the thickness material to width of the strip (so inch, mm or cubits doesn't matter) and E_r is the relative dielectric constant of the material... which for a fiberglass PCB circuit board is about 4.5."

Edit by Stu:

I'm not sure that I agree with the reference's author regarding the values of the terminating resistors. For a symmetrical coupler, both resistors should be the same... but the two terminating resistor values should be chosen as follows.

1. Connect the coupler's RF output port to a 50 ohm dummy load, and apply power at the RF input port. Measure the voltage produced at the "reflected power" output measurement port.

2. Adjust the value of the terminating resistor across the "reflected power" strip line to cause that voltage to be essentially zero.

3. Use the same value for the other terminating resistor.
 
Stu

[W8UJX]

Tks Stu,

I have the manual for the meter.  The small metal box inside the meter enclosure has no number on it.  Just two coax connectors and two feedthru caps where the forward and reflected signals are sourced.

If I can't find a schematic I guess I will just replace the 33 ohm 1 watt 5% resistors with 33 ohm 1/4 watt 1% resistors.

I have several other SWR/Watt Meters that use toroids.  I will take a look at them and see how they are designed.

I tried to show a picture of the small box.  I have not done that on AMfone before so we will see if it shows up.  Hi...

I will read over the VK5AJL info.  That may help me better understand how the circuit works.

Jerry