WD5DZG's Ramblings and Rantings

The last half of July and August so far has been hotter than the henge's on the gates of hell here in Central Tennessee, high nineties to low hundreds most days an not much relief at night. My shop is a 12' by 20' prefab metal building and the insulation is inadequate to deal with such temperatures. I do have a small window air unit and with reasonable outside temperatures it manages but not for the last month to six weeks. Needless to say I have chosen to stay out of the shop on hot days.

That said , I have had some problems with the digital frequency readout, initially the problem turned out to be a damaged TCXO. I have no idea whether it was a bad unit to start or if I accidentally damaged it, but it failed during the calibration process, no joy! However , Neil at AADE helped with the problem and this week I was able to complete the assembly and set the If Frequency into memory. As I stated in an earlier post the receiver now works very well on 80 meters. although I am less than thrilled with the IF and Xtal filter. When the  digital frequency readout has no input from a converter crystal Oscillator the display reads from just below 3.5 MHz to just over 4MHz. When I adjust the BFO crystal adjusting capacitor. The frequency is displayed followed by either LSB, CW , or USB depending upon where the BFO is set in relationship to the IF frequency. I am still not sure how the converters will  display on the readout , got to figure that out before I run out ad buy a bunch of rocks.Well it is two days later and we finally got a break in the high temps,
Made considerable progress on the rig. Got the counter mounted and a face plate fabricated and mounted as well. Still have some tinkering to do getting the counter to read the correct frequency when the converters are added, Here is a pic of the rig as it is now. The band switch when added will be located below the S meter and above the on /off pull switch with no knob. I hope to score a suitable Knob at the Huntsville Hamfest this coming weekend.

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Did some fine tuning on the receiver today. I was pleased that the receiver knocks your ears off when tuned across my signal generator using its lowest setting, (250 milli-volts to 50 ohms) using about 2 feet of  wire lying on the bench as an antenna. All symptoms were good , there is a noticeable increase in the noise level when an antenna is connected.there is plenty of IF and audio amplification everything should be hunky dory right? Well not so, I waited until this evening when 80 meters came alive again and tuned across the band, I heard a few signals but not as many as should be. I decided in the interest of science (and air conditioning) to bring the rig into the house and work on it from my operating bench. Big difference, the band was filled with signals both sideband and CW when connected to my operating antenna (wire in the trees). I obviously have a problem with the shop antenna. I wonder how many of the module rebuilds were unnecessary, oh well, my learning curve may be low but it does curl up eventually.

I tried another version of the crystal filter, this time a three pole filter designed for sideband use. I am not pleased with the bandpass characteristics but it may be about as good as I am going to get with these crystals.  Lately I have been leaning toward trying the sideband filters I mentioned earlier 5.6 MHz That would only require a VFO change of some 300 kHz and that is easy enough to do. Perhaps I will give that a try but first I want to rework the four pole filter and see if I can work out some of the rough spots in it I am of two minds about incorporating an rf amp in the receiver. Normally judging from tonight's listening session I don't need one but there are times when one could be handy.. Perhaps one that can be switched in or out would be nice. 

Tomorrow is another day but unfortunately there are appointments. One of the things about getting older other than the obvious is the need to watch after ones health more so than when we were young, of course if we had looked after it better when we were young then perhaps the need would not be as great  who really knows... the thing to do is LIVE forever or Die trying I guess. 

an added note, I just went back and ran a spelling check on all the entries of this blog, For those who may have struggled through them let me offer my sincere apology, I do know better...I am sorry for putting you through all that misspelling.

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It has been almost a month since my last entry in this category. No excuses except to say that Life has a way of getting in the way and there has not been any news worthy break-through to write about. A lot has happened, Progress has been made and a lot of blind allies have been explored.  At this point I think I have rebuilt every module in the rig at least three times except for the VFO. some builds worked badly , some not at all and a few worked well. Some of the mediocre ones after trouble shooting have turned into keepers and ones I thought to be keepers early on have turned into "also ran". If there is anything that stands out in attempting an effort like this is how much there remains to be learned, even about circuits I though I understood well. Some of the circuits that I have rebuilt were brought about by me having a better understanding as to how they should work and usually the rebuilds have been worthwhile. Here is where we are now

I now have a functioning 80 meter receiver. It has much room for improvement but it is a long way from where we were at my last blog entry. The copious bird calls I reported have been tamed, there are no more shirks and whistles and ear splitting heterodynes heard in the phones. Those were eliminated by careful bypassing, shielding and  placement of modules.  The bandpass filter that I settled on is a version of the bandpass filter used in W7ZOI and K5IRK's Progressive Communications Receiver (QST Nov 1981)  The secret to building this filter, if there is one, is to read the instructions carefully and measure every component used in the circuit. I used combinations of standard values of capacitors to arrive at the values specified. During the process I also discovered I had been misreading the number of turns called for, on the coils in the tunable section of the filter. When the correct values are used it is amazing how well this filter performs. As an experiment I built a version of this filter using a circuit board (commercial) and a bag of parts sold at one time as a kit for this filter. The kit used standard closest value 5% capacitors and not the values called for in the original article for example 4700 pf caps were used for the two 5000 pf caps called for in the article. While it is possible for 5% caps of this value to be very close to the required value these were not and the end result was  a very poor filter with a lot of insertion loss and weak peaking. I built my filter Manhattan style and used capacitor combinations (as measured on my ALDE LCII Meter) equal to the values called for. Likewise the coils were wound and measured for correct value. The result is a very good front end pass band filter with under three db of insertion loss ( if my measurements are correct)and with the peaking capability off setting the losses. What was especially satisfying is that I now know why previous versions of this filter that I had built, had sometimes performed so poorly. 

I had problems deciding upon an  IF amp circuit, I built several all of which worked after a fashion but most had issues.  Two were built around MPF131 dual gate MOSFETS and two around cascode arrangements. The MPF131 units were OK , however one had far too much gain and both had stability issues. The red LEDs in the source biasing were fun to look at. The stability issues could likely been overcome with use of Ferrite beads but I had none on hand.  Same story with the cascode circuits using J310 FETs and 2n3904s in cascode. TOO much gain in both cases and not enough control. I Finally settled on the cascode circuit used by Frank Harris (From crystal sets to sideband ) Available as a free download on line ) using 2n3904s only. It is easy to build, works well, is reasonably quiet,is easily controlled and seems to be immune to oscillations. Until I find something a lot better, it is my choice. 

I am still not satisfied with the crystal filter. I measured and tested 50 crystals (5.905 MHz a ham fest lot) for several parameters using several pieces of home brew gear (that I had to build in order to use) and found several sets that appear to be suitable for filter construction. I chose the Minloss arrangement (Cohn) using matched rocks but it seems that they either do not have adequate q for filters or my home built test gear is not suitably accurate to measure them correctly as the filter seems to have excessive loss and is a bit more narrow than I expected. I am using a four pole filter designed for about 2.4 kHz bandwidth. I may redo it with more careful attention to layout and see what happens.  More on that later. If all else fails I have acquired a couple of Drake upper and lower sideband filters in the five mhz range that I can use although I wanted to use a home brew filter in this rig.

That is where we are now, I will try to get some more photos when I have a chance but for now I am concentrating on building the best 80 meter receiver that I can build after all the band converters will only be as good as the rig they convert to.      

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Got around to actually hooking the modules together and doing a road test.It works in a sense, sounds something like an aviary at the zoo at feeding time, more squeaks, squawks and whistles than a Mockingbird on drugs. One of the problems is that the IF system has far to much gain.  That is fixable several ways but after all is said and tried I may just go for a system with at least one fewer stages. At this point I am not happy with the front end filter, it introduces to much loss  and I am not at all certain about the Frequency scheme I have chosen,  The VFO works fine as does the PD and audio amp and the mixer is satisfactory but running with the barn door wide open ( no filter installed) there are too many birdies for my liking. I am hoping that an IF filter will do away with some of the more annoying chirps and whistles which should be beat notes of harmonics well outside the desired pass band pf the receiver. Time to build a filter before rebuilding the various modules, more later!

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Checked out the modules so far and they all seem to work except for the IF system. By connecting the output of the first mixer to the input of the Product Detector  I can detect signals from my signal generator  Of course there is far to much loss in the diode rings to enable on the air signals to be detected although both  PD an First Mixer work perfectly well as DC receivers when connected individually to an antenna and  to the audio amp. The VFO and BFO are operational as well. Still I need a lot more gain to overcome the mixer losses and bring the weak on air signals up to a usable level. To that end I started on another IF strip. This one was published back in the 80's by a french ham F6CER and published in QST Magazine (August 1981) under De Maw's by line) It uses three dual gate MosFETs as the gain element and an AGC system using MC1350P and 741 ICs plus a couple of FETs. Here is a pic of my effort using Manhattan style  Construction. As I build, I like to use a red ink pen to highlight the parts on the schematic as I go. I find this helps to eliminate omissions of parts and allows me to start and stop as the need arises. The transformers use here are units 10.7 MHz units salvaged from defunct transistor FM radios. Here is a closeup of the partially completed if strip this is the section with Q3 and T3 and T4. No attempt has been made to miniaturize this section since doing so would make testing and repairs difficult and I have no need to do so in this receiver. In the original article 80673 or 3N211 MosFETs were used, at over $6 each when you can find them I elected to use MPS131s at $1.75 from Dan's Small Parts and kits. Tags: Untagged