Hello,

I have had some success modifying my three QCX Classics to have AGC to prevent eardrum overload. I've tried to keep it simple and so it uses only 6 additional components, 3 capacitors, 2 diodes, and a trim pot, all of which are soldered to the underside of the board. If you are interested take a look at the attached.

73, Jim AJ8S

Thanks for posting this, Jim. A very useful mod. indeed.

 

Cheers,

Ronan MM0IVR

Yes - it looks great.

Bob,

I love your whacky callsign, mine's just okay.

Trystan G0KAY

CAUTION: Do not use a conductive screwdriver to adjust the 10k potentiometer if the radio is mounted in a metal case. This can cause the wiper to short to ground possibly damaging the 5 volt regulator or the potentiometer.

Uh oh.  Sounds like there is a story there.  Judging by the 5:30 am post time, more coffee might have been needed?  I hope there wasn't too much carnage.  I liked your AGC idea!

Joe

Joe, I am naturally an early riser. Glad to say the story is that I realized that someone else might blow up their beautiful radio and now my conscience is clear, hi. Glad you like the idea. I am building my first QCX mini now and planning to incorporate the idea there and will provide another write up. 73.

Oh I'm glad to hear that Jim.  I look forward to seeing how it goes with the mini.  I'm working on another idea right now that belongs in the other thread which deals with AGC and speakers because it is a stand alone audio level control.  I may write something up too but there are a few niggling details that I'm not satisfied with yet.
Best regards
Joe

Has anyone done this on the QCX+?  If so would you mind posting a how-to?

73, Bill NZ0T

Bill,

I don't own a QCX+ so I can't help you but you should be able to find the same points in the circuit since it is the same circuit:  the output from IC10A pin 1, the junction of diode D5 and gate of Q7, the 5 volt regulated supply, and ground. Maybe another ham can look at the QCX+ and help you with this. Good luck.

73, Jim AJ8S

Hi Jim,

Thanks - I'm a tech idiot so hopefully someone else can figure the QCX+ out.  I'm great at following instructions and building kits - reading schematics and understanding circuits not so much.

73, Bill NZ0T

Bill,

I took a look at the manual for the QCX+ and it is not so different from the QCX. There is plenty of room. The 10k potentiometer could be mounted to the underside of the board where the 6 pin header is just at the end of the CPU socket. I have attached a depiction of the circuit board with the four attach points marked. The gate of Q7 is close to where the potentiometer would be so can be wired to the junction of D1 and C2 but the connection to the output of IC10A would need a short extension wire to the plus end of C1 from the other end of the board.

Maybe this helps,
Jim AJ8S

For a test I tired the mod it works but the interaction between RX and tx was
not pretty.  One change was the fix., another 2n7000 in parallel
(drain and source only) and the gate connected to the AGC.

Yes it adds one part but is actually simpler and less troublesome.

Now TX mute is, as was, circuit.  Agc also works and they do not interact.

No more ear blasting.

Allison
-------------------------------
Please reply on list so we can share.
No private email, it goes to a bit bucket due address harvesting

Allison, I like this mod and plan to add it to my QCX40.  Would you tell me the process you used to set the agc pot? I see the interaction between the volume level and the threshold setting for the 2n7000 and I’m scratching my head. Thank you. 

Jim Linder, AJ6ER  

Jim,

Great job! Great idea!

Do you think that your AGC would work on a QCX mini or do you think I’m asking too much because of the tight squeeze?

Thanks in advance, 73

Robert, Ag6lk 

On Tue, Mar 9, 2021 at 03:02 AM, Robert AG6LK wrote:

Do you think that your AGC would work on a QCX mini

There's a separate thread where Jim has posted the instructions for adding the same mod to a CCX mini. I've added it to my QCX mini along with Allison:s extra 2n7000 and it works very well indeed.

Thanks to both Jim and Allison for a useful addition to the radio.

Cheers,
Ronan

Allison,

I really like your idea! I will modify my QCX mini today. Then the other three QCX classics when I get time.

73, Jim AJ8S

Today I tried the addition to the QCX AGC proposed by Allison, KB1GMX. It works great except that a 120k resistor is needed to return the level back towards the unattenuated state. I have attached a revised schematic. Note that a BS170 will work in place of the 2N7000 if that is what you have on hand. It is now up to 8 parts but I think well worth it to have the receiver come back up right away.

I did the updated mod on the QCX mini and there is a photo attached but it is the same circuit for any of the QCX models.

Thanks Allison for your suggestion. I hope I have correctly understood it and would be interested in your comments re the 120k resistor.

73, Jim AJ8S