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Input voltage for QDX Digital Receiver

 

When ordering an assembled QDX Digital Receiver, the default configuration appears to be for using a 12V power supply. (There is a 9V option when ordering. What is the acceptable input voltage range with the default 12V configuration? Would 9V be too low?
Thank you!
 

It will work just fine. Your output power will be less, but hey lots of contacts to be made.

Happy New Year.

Dave

On Dec 27, 2024, at 10:16, Robert Rabin via groups.io <r.rabin@...> wrote:


When ordering an assembled QDX Digital Receiver, the default configuration appears to be for using a 12V power supply. (There is a 9V option when ordering. What is the acceptable input voltage range with the default 12V configuration? Would 9V be too low?
Thank you!
 

Hi Robert, all the information you require is in the QDX manual.
If you look at section 4.3, Hans has provided a graph of output power against voltage for both the 9 and 12V versions. From the graph 9V would give roughly 2W output from a transformer wound for 12V. The critical factor is not exceeding 6W output on TX.  Receive isn’t an issue as the linear 5V regulator is quite robust and it’s absolute maximum rated input is 35V, however it would probably have shutdown due to thermal overheating before you reached that figure, plus if you activated transmit, your output transistors would have turned into three legged fuses.
The recommended minimum input voltage for the 78M05 to maintain regulation is V out plus 1.8, so 6.8V. So 9V is not too low. 73 Nick G0OQK. 
 

On 27/12/2024 15:59, Nick Garrod via groups.io wrote:
section 4.3, Hans has provided a graph of output power against voltage
Looking at the charts I'm not sure Hans has said how to get it to work at 5 Volts when the regulator must have an output a fair bit less than that. I guess maybe the QDX will still work?

I run mine at 7.5 Volts and have series diodes to feed the PA dropping that to around 5 Volts, I Watt output.

I do not think I ever ran them at 12 volts, 8 Watts output seems scary!

73 Alan G4ZFQ
 

On Fri, Dec 27, 2024 at 08:21 AM, Alan G4ZFQ wrote:
I run mine at 7.5 Volts and have series diodes to feed the PA dropping that to around 5 Volts, I Watt output.
I just dusted off my QDX the other day.  I built it from kit a couple of years ago and wired it for 9VDC.  I wanted
to see how low I could get the output by lowering the input voltage (I plan to use it with my HardRock-50+, which
can be driven to full output by about 2 watts, depending on band).
 
I found that at 7VDC the QDX ran fine.  At 6VDC, the QDX really wouldn't boot up; the red LED came on solid ...
no blinking ... and nothing seemed to happen.  When I set the input voltage back to 7VDC, the QDX came up in
firmware update mode.  Turned it off and turned it on again and back to normal.
 
That 7VDC number is consistent with responses to my question a few weeks ago about lowering the output
on my slowly-getting-built QMX+.
 
Alan ... I'd be interested to hear more about "series diodes to feed the PA" ... I might still need to do this.
 
Cheers -
 
Bruce K1FFX
 

Thanks Nick! From the blue curve in the second figure (section 4.3), I assume that the expected output power is 4 W using the wiring for the 12 V version versus 5 W with the 9 V windings. Am I interpreting this correctly?
73
Bob KA5MIZ
 

On 27/12/2024 17:10, Bruce K1FFX via groups.io wrote:
I'd be interested to hear more about "series diodes to feed the PA"
Bruce,

Thanks for your test results. I thought about 7 volts would be the minimum.

I found enough room to fit 3-4 1N4007 (or equivalent) diodes in series with the "12 volt" feed to L14.

I do not remember exactly what I did. In any case I think the rev 1 boards I have are slightly different around that area.
You need to maintain the C37 connection to L14. I think I just needed to cut a track.

The bar of a 1N4007 goes to that junction, the other 2 or3 in series fitted in space available to take power from the jack.

I did something similar to a rev 5 QDX but fed the PA from an external regulator.

Like all mods I spend a long time wondering how to do it. Make notes, a mistake can waste a lot of time.

73 Alan G4ZFQ
 

Yes Bob, you are interpreting the graph correctly.
 
As you have found out Bruce, if you venture below the drop out voltage of that 5V regulator, there be dragons and strange things happen.
Cheer's Nick G0OQK. 
 

Hello.  I just assembled 2 QDX-Ms.  If you download the assembly manual,  there is a chart that plots input voltage vs power output to answer your question across the spectrum of power inputs.  That being said, if your device was built for 12v, you should power it that way.  I am using rechargeable lead acid batteries from Amazon that were about 20 USD.

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