Pin Confusion #u3s


KI7MWA
 

Page 15 of the U3S construction manual says: "To run the transmitter using the 5V supply, be sure to connect a wire between the +5V and PA pins. This is shown by the short red horizontal line in the diagram (below)."  But, in the diagram there are actually TWO short red horizontal lines, one between the second and third pins of the 4-pin header next to the capacitor labeled C1, and another between the second and third pins of the 11-pin header at the edge of the board, the pins labeled +5 and Heat.

With no jumpers installed, my DVM shows continuity between the +5 and Heat pins.  Is this correct? Or do I have a short somewhere?

With the Si5351 module removed I have NO continuity between the +5 and Heat pins.  Does this mean I have a short somewhere on the Si5351 module board?


N3MNT
 

For the PA 5V, heat pins there are two of each connected together.  The lower jumper connects 5V to the heater if using the TCXO.  The upper jumper connects 5V to PWR for the final power.  If you power the finals via 12V you do not use the upper jumper. 


KI7MWA
 

N3MNT: "The lower jumper connects 5V to the heater if using the TCXO."

So, in the case where one is NOT using the TXCO, the synth board makes a connection between the +5V pin and the Heat pin...just for the hell of it?


N3MNT
 


From Schem.


KI7MWA
 

Another question:  The diagram on page 15 shows +5V supply voltage applied to the PA pin, with the (upper) jumper connecting the PA pin to the +5V pin.  But, with the (upper) jumper installed, the +5V supply could just as well be connected to the +5V pin on the board, because with the (upper) jumper installed the +5V pin is in common with the PA pin. 

You could argue that the +5V supply HAS to be connected to the PA pin if one wants to install the (lower) jumper between the +5V pin and the Heat pin when you want to supply voltage to the oven of the TXCO (should you be using it). 

But if the synth board itself makes a connection between the +5V pin and the Heat pin (my DVM shows 0 ohms resistance between the +5V pin and the Heat pin when the synth board is in place), why would you need the (lower) jumper?


KI7MWA
 

N3MNT:

Okay, but on page 3 of the synth assembly manual, the PCB tracks diagram shows that pins 8, 9, and 10 are connected together...and my DVM agrees.  Pin 8 is not independent.


Mont Pierce KM6WT
 

On Sat, Feb 6, 2021 at 05:19 AM, KI7MWA wrote:
Page 15 of the U3S construction manual says: "To run the transmitter using the 5V supply, be sure to connect a wire between the +5V and PA pins. This is shown by the short red horizontal line in the diagram (below)."  But, in the diagram there are actually TWO short red horizontal lines, one between the second and third pins of the 4-pin header next to the capacitor labeled C1, and another between the second and third pins of the 11-pin header at the edge of the board, the pins labeled +5 and Heat.

With no jumpers installed, my DVM shows continuity between the +5 and Heat pins.  Is this correct? Or do I have a short somewhere?

At the bottom of the U3S webpage (click here) there are two diagrams from the PCB layers showing the traces in the board.

In the left image, you can see what pins are already connected, so helps to understand how the jumpers are described in the assembly manual.

The PA track is separate and thicker, to allow applying a higher voltage to the PA only.

The +5V Heater track is thicker to allow more current.


73
km6wt


KI7MWA
 

KM6WT:

Okay, thanks.  The purpose of the +5V-Heat pin jumper is to provide for the greater current flow needed by the TXCO oven.

And, by the same token, if one is not using the TXCO, and one is using a 5V power supply, one could connect one's power supply to the +5V and Gnd pins, so long as one has the jumper installed for the connection between the +5V and PA pins.  Right?


Mont Pierce KM6WT
 

On Sat, Feb 6, 2021 at 09:19 AM, KI7MWA wrote:
if one is not using the TXCO, and one is using a 5V power supply, one could connect one's power supply to the +5V and Gnd pins, so long as one has the jumper installed for the connection between the +5V and PA pins.  Right?
Yes, correct.


73
Mont - km6wt


Mont Pierce KM6WT
 

On Sat, Feb 6, 2021 at 06:41 AM, KI7MWA wrote:
the synth board makes a connection between the +5V pin and the Heat pin...
Since the Synth board already connects the Heat track to +5V, you could also do the following:

Jumper all 3 as circled in Red below.  Thread a wire down one hole, up the next, and down again in the last hole.
Then you can apply +5V to any of the 3 pins circled in Blue.  Or even use one of those pins to power something else.



73
km6wt


Mont Pierce KM6WT
 

On Sat, Feb 6, 2021 at 10:11 AM, Mont Pierce KM6WT wrote:
Then you can apply +5V to any of the 3 pins circled in Blue.
Oopss... I meant "Yellow"

(should have proof read)


N3MNT
 


This does the same thing with easier jumpers to install.


On Sat, Feb 6, 2021 at 01:11 PM, Mont Pierce KM6WT wrote:
On Sat, Feb 6, 2021 at 06:41 AM, KI7MWA wrote:
the synth board makes a connection between the +5V pin and the Heat pin...
Since the Synth board already connects the Heat track to +5V, you could also do the following:

Jumper all 3 as circled in Red below.  Thread a wire down one hole, up the next, and down again in the last hole.
Then you can apply +5V to any of the 3 pins circled in Blue.  Or even use one of those pins to power something else.



73
km6wt