VK3YE AGC Success in uBITX


With the help of a couple ops here I figured out my wrong integration of this circuit into the uBITX.  I was trying to use it is series with the volume control, it needs to go in parallel.  Simply explained - do not disconnect any wires or parts in the uBITX.  Attach the LDR across the volume control pot as described, and the bias to the 12-13.8 volt DC supply (I wired a SP2T here to experience the difference, honestly I find no need to turn off this circuit).  The AGC 'sense' into the series (nominal) 10 uF capacitor is fed from the center of the volume control, via a series resistor.  I started with this resistor being 3.3k but noticed the AGC behavior varied with the volume control setting - which implies this circuit is loading the main path.  I increased this resistor to 27k and now it works nicely - the volume control acts independent of the AGC sense circuit. 

The result is quite nice with large signals being squashed I estimate 15-20 dB.  I had needed to run the audio gain at about 5% or less to copy these large signals in headphones - now I can routine tune across the band with the volume control at around 25%.  As I also operate a K2 I am familiar with a soft AGC response requiring some operator audio gain adjustment for copying weak signals - but rarely needing more than 50% audio at least on 40 meters.  I am thrilled with how this circuit works.  Two NPNs, an LED and an LDR are certainly within the spirit of the uBITX design. 

A couple parts comments - the LDR has a dark resistance of over 100k ohms and a few k ohms in dim ambient light.  Perhaps the latter may be more important, as in the box it may offer comparable resistance to the volume control.  I am using a small but very bright LED with a clear body but emitting red light.  Reading about photoresistors they are said to have a little lag in their response and it seems to act nicely as I tune across the band or listen to a pile-up of signals of varying levels.  It is a keeper. 

There are several other AGC circuits to explore.  The one in the Jackal seems to be a derivative of the W7ZOI hybrid cascode, albeit simplified.  Worth exploring for homebrewers wishing a more complex circuit to try.  Likely I was misapplying the ND6T AGC.  Enjoy your pursuit of AGC in the uBITX. 

73 Curt

Tim Helming

Hi, Curt

I'm about to build this circuit, and have a question. It looks like there's a 10k pot on the board. How do you know where to set this pot?



Tim Helming

Oh, one other question. I'm planning to use the version of the circuit shown here. In your writeup you say "connect the LDR across the volume pot." The diagram here has the overall circuit connected across the volume pot, not the LDR itself. Same difference?


Rajendran Mayilsamy

hi dear all, the components value of the schematic is not clear...73 de Raj VU2SMM

Sunil Aruldas

Well, I used the circuit in series with the volume control and it works quite well.
Basically the signal coming to the high end of the volume control is routed via this unit. A DPDT switch enables bypassing it completely.

Sunil VU3ZAN 



I just saw your message - that pot is adjusted by you to provide your judgment on best AGC behavior - I had forgotten it was there.  You will pretty much know when you are happy with its setting.  I am remembering that one resistor value is incorrect in the VK3YE video, and I think I have a similar error in my write-up on the K3PZN website - so email me mycall@... and I can check these details.  I am very happy with this circuit as it addresses S9+ signals, and the flashing LED even inside my case can be seen. 

Note with this circuit there is nothing to move in the ubitx transceiver.  As shown the volume mid goes to the wiper or center of the pot.  Find the hot side of the pot - a DVM will show it is not connected to ground - and this is where the other terminal goes.  then that just leaves connecting the bias and ground. 

locating a bright enough LED is critical - my ordinary LEDs did not do the trick, but I found a stash of bright white ones in a discarded appliance in my junk pile.  I also have a 200 uA meter and series resistor connected in the circuit not so much as a real S-meter, but to show me when AGC action is taking place.  I rarely turn the AGC off.

73 curt wb8yyy


LDR's have their most sensitive wavelengths at green so you should use a green LED