While i have Papua before, this is the first to confirm it both in Eqsl and Lotw.
So today I populated the 2nd IF mixer, the 2nd mixer and some of the switching circuitry and gave all those elements a little bit of a test.
To follow on from the test of the first IF amp, I put a 10mv signal into the first IF amp and tested the whole IF strip through the Xtal Filter and out the 2nd IF amp buffer and it all seems to be working quite well.
But there is one issue and I am not sure what the cause is. The 470R resistor from voltage divider on Q6 does not go straight to ground and trying to find its ultimate path to ground has been elusive.
To make matters worse there are q Q10 in the schematic and Q10 and 21 on the board, there are 2 diode on the board not on the schematic and I cannot for the life of me work out what the actual cause is here. Either one of the Q10’s is not being biased on or there is some part missing or either VR7 or VR2 are not providing the emitter ground path on the emitters of those Q10 transistors.
Anyway, I think what i will do is pull both the Q10’s out and related circuitry, part of which is for a signal meter and just hard wire the 470R voltage divider to ground. That will at least leave me to be able to complete the receiver and actually test it.
At one point today I had fed a 9mhz signal into the IF strip and was getting a 600Hz tone out the audio amp, so that in itself is promising as I was worried about the dodgy design 2nd mixer. Which looks like a ring mixer but does not have trifilar windings on the inputs and outputs, but some crazy bifilar winding on one side and a variable resistor and cap divider on the other. I am not sure about this mixer design and I have not done any google foo to see if i can find it, but it does work
Oh and for the diodes in the mixer i did not use the typical 1n4148 diodes that everyone uses, but rather I used a Schottky Diode 1N5711, it probably means i have way to much drive from the BFO as the switch on of these are much lower, but its working and for now, I call that a win, particularly so when i really do not understand the mixer topology being used.
Oh and as far as audio goes, of the 2 AF stages, I am only using the LM386 first stage because the TDR2003 is motor boating or oscillating or just acting like a tard and I am not sure why here either
I do hope the next post will be a video of it receiving signals. Failure is always an option here, but so far this build has been very challenging, but its been rewarding and even if it ends up being a pile of junk, i will have learned a lot.
I did a bit more on this today, went through and added in the first IF amp and tested it to ensure it was working. After sorting out some probe issues with my scope, it was all systems go.
After working out where the input to the first IF amp was and also buzzing it out back to the first mixer, I fed a 100mv signal into the amp and scoped the output on the other end of the xtal filter to ensure signal was going in and coming out. As you can see from the oscilloscope output above it works.
The blue trace is the 100mv signal from function generator and the yellow trace is the signal on the other side of the xtal filter. Now to build the 2nd IF amp and test it also.
I thought before i added in the IF Amps i would build the xtal filter first and see how it looked. The values of 33pf seemed very small and would end up with quite a wide filter. Typical values for a 2K wide filter are usually around the 100pf mark, so i started there.
Obviously i was wrong, but i am thinking 50pf might be closer to the mark after looking at the bode plot. Its not going to break any performance records and the skirts are kind of garbage for a 5 xtal filter. But, I will see how it goes on receive once i have the rest of the radio built.
As far as xtal filters go, this is one of the better looking ones I have built HEHE, so I guess the others have been really crap. It is kind of symmetrical looking and only 3db-ish of ripple. Might be ok. We will see.
Ok, so after playing with cap values i got the filter about the right width, but there is a lot of variance in the passband. 8db to be exact. Way to much. But as there is no way to impedance match this, there is not a lot i can do. It is what it is as they say and as I am only building this as a receiver, I can live with it.
Not really a big job, but I stuck the PI and DMR modem into a case so it is kind of protected. But hey, its something ticked off the list already for this year.
I have about an hours work to do today and then clean up my bench a little and then it will be onto finishing off the Bebek as much as i can. There is still a transformer that I do not know what the turns ratio should be that I need to work out, or what will likely happen is that I will mount a small carrier board where the 2nd mixer is and use an ADE-1. I never could work out why they would have a homebrew mixer in one spot and an commercial one in another. Go figure.