Mods to G3ORY 80m receiver.
Although this receiver works brilliantly I found that it suffers when the battery begins to drain. The audio chirps and it could drift off tune. Predominantly the voltage regulator needs to be of a low dropout type, the original 78L06 needs over 7.6V input for regulation. For good measure I also changed the series switching FET for one with a lower channel resistance. More detailed notes are shown below.
In summary
1) Remove the regulator IC3 and replace with a LK112M60TR
2) Reduce R19 to 3.9K. (3.9K to 10K should be OK)
3) Change FET, TR3. Either ZVP2106A (not actually tested) or IRFU5305PBF (tested but more awkward to fit)
The present FET will drop approx 250mV, ZVP2106A will drop about 80mV, the IRFU5305PBF drops vitually nothing. All to achieve a better battery lifetime and limit instability on current peaks.
4) Add 10R in series with C25 to cure audio instability
5) Unknown viability. Cut track leading to pin 6 of IC2, Vcc of the audio amp. Link a wire from pin 6 to the input of the regulator (drain of the FET). Gives higher voltage on audio amp leading to less distortion and reduces any negative affects on regulation on current peaks.
Perhaps the FET is overkill (I used a IRFU5305PBF RS542-9951 Rds=60milliohm) the ZVP2106 may be better since its easier to fit. I think the ZVP2106 is a 5ohm part so at 16mA extra voltage drop will be 80mV, big deal!!
The regulator is a LK112M60TR, RS686-9004. To fit I turned it upside down and bent pins 1,2,4 and 5 down to meet the pads, leave pin3 alone, it'll naturally point up and out of the way when inverted. Pins 2,4 and 5 are soldered to the existing IC3 pads, pin1 will solder onto the +ve input line running underneath this pin. Just scrape off the solder resist and tin.
Although it's small the pads align pretty well and the job is quite easy for those with a small iron and a steady hand.
You can see my lazy change of R19, I had to hand a 3.9K so I just placed it in parallel with the existing R19, I didn't bother removing it. Upon further examination it is obvious why the voltage on the gate of TR3 doesn't go as low as it ought to. The voltage will always be a split across R19 and R18, since they are originally equal approx 3.5V is about the lowest it will go.
This is too close to the threshold of the fet, I'd make R19 somewhere between 3.9K and 22K.
You can also see I've cut the track to pin 6 of the audio amp and put this to the input of the regulator.
All the above was done without removing the board including removing TR3 and IC3. The lid still fits, I've tested it. You need to be careful that the battery will not short out on the tab of the modded fet, careful placement is all that is required.
The only drawbacks are that it does involve relatively fiddly surface mount devices and the regulators can only be bought in 10's off (10 for £4).
I also suffered a little audio instability. The only solution I found was to add a resistor in series with C25. I added a 10R, a little higher than I'd prefer as it would affect the volume into the earpeice, however it seemed to make very difference and since I didn't have any other convenient value to hand I left it at that.
The circuit should now give solid regulation down to 6.2V.
G4KWQ