I've finally found some time to get back to this project. When I switch the device on, there is a noticeable improvement in the quality of reception using one of the Radialva sets as the test radio. However, none of the tuning/Q controls seem to do anything.
As I'm not familiar with varicap tuning or the details of how dual gate FET's etc. work, I'd appreciate any guidance regarding how to establish if the varicap diode is working or how to check the circuit in general. The output pot certainly works and the quality of reception improves as the output is increased. The noise level also reduces as output is increased - which is especially welcome.
I went down a bit of a hole with regard to the FET. I used the component tester that many of us bought on eBay which seemed to indicate that the component layout drawing had got the device drawn the wrong way round. I later discovered that this little tester seems unable to tell the source from the drain. Swapping the S and D leads round makes no difference to the result of the test. I've never used the gadget for semiconductors before so I'd be interested if any of you have seen similar behaviour.
So, some progress is being made but still some way to go.
Cheers, John
I still haven't found mine
Alan
The varicap tuning should tune ok in use with your radio even if the FET doesn't work, that's assuming the FET doesn't have a G to Source or G to Drain short.
Check that the voltage on the varicaps cathode is varying between it's voltage parameters when the tuning pot is rotated, the tuned circuit should definitely peak up at the frequency your radio is tuned to.
The non working Q multiplier might be a wiring error or the FET, for it to work the feedback needs to be of the opposite phase to that of the tuned circuit, sometimes swopping the feedback winding connections can solve the problem if originally they happened to be connected in phase with the tuned coil.
Good luck.
Lawrence.
Thanks Lawrence, I did some more playing around this afternoon using a sig gen and scope, etc. Things are looking much better now. I can get it to tune across the MW band and the Q seems to work as well. I turned out that the variable resistors on the pcb needed tweaking - one to match the FET's characteristics for the Q multiplier, and the other to set the low end of the frequency range where it would still tune.
I didn't appreciate how sensitive the tuning is (even though the text warns that it is the case). I was moving the coarse tuning pot much too quickly to catch the tuning point. I'll need to make a decal for the front panel and calibrate the tuning control at least so I'll roughly know where to go to tune a given frequency. As expected the Q multiplier has an impact on the final audio tone.
I'm pleased with progress so will continue 'tweaking' until I'm happy with the results when using the main sets that I use regularly. I picked up a UK sports station earlier on - never did that before!
Cheers, John
Yes the tuning will be sharp when the Q multiplier does it's stuff, it's a bit like a regen radio, if you find it bites a bit to early during the Q pots rotation you could try increasing the distance between the single turn feedback winding and the tuned winding, the arrangement in that circuit is very similar to a pentode regen with screen grid regen control.
Sound as though you are nearly there with it John.
Lawrence.
(20-08-2014, 09:05 PM)pwdrive Wrote: [ -> ]Sound as though you are nearly there with it John.
Lawrence.
Hi Lawrence, yes I played around with it last night and it definitely makes a big difference. I'll need to work on the coarse tuning control. Mine is a small pot that is hard to adjust and it really needs to be a high quality one with no drag and a much larger knob to get the degree of control needed.
On a related track one of my mates in US suggested I try this as well - can be set to cover either MW or SW.
http://www.kaitousa.com/KA33.htm
Cheers, John
A helipot (multiturn) might help, good quality pots are normally the best when used for feedback control loops.
Your project aside, an active broadband magnetic loop is also a possible option, no tuning required, I have heard that the Wellbrook loop works well, some years ago I spoke to the man who designed them, a chap called Andy Ikin, nice fella and knowledgeable, the loop is balanced with an unbalanced output, the active amplifier has a certain amount of impedance tracking that follows the impedance of the loop, it covers the normal broadcast band through to 30 mhz.
Lawrence.
Or you could mount a 3:1 slow - mo drive on it.
http://www.wellbrook.uk.com/
I have heard about magnetic loops and always intended to experiment with them ... just never found the time to get around to it !
Gary
I made a square one years ago (tuned) a definite improvement signal to noise wise.
Lawrence.
The electronics must be in another discussion thread? I could give you a 10-turn pot, and for that matter, a ceramic switch too. Lots of them in the junk box.