Help requested with a regenerative receiver

Discussion in 'Homebrew and Kit Projects' started by KC2TAU, Nov 28, 2018.

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  1. KC2TAU

    KC2TAU Ham Member QRZ Page

    Hello everyone,

    I have been working on building this iGen regenerative receiver kit and I've been having a bit of trouble getting it to operate as it should. It's a regenerative receiver using a 1T4 as an RF amp, a 1T4 as a regenerative stage and an op amp as an audio amplifier. It uses plug in cards with leaded inductors in both the RF and regenerative stage. The power source is 12vdc. With the supplied inductors the receiver should tune the AM broadcast band.

    After building the kit and using the supplied 1T4 tubes I applied power to find that the set worked but did not have a great deal of sensitivity. A 10 foot or so section of wire for the antenna yielded two local stations at a rather low volume. I live in an area with a decent amount of loud local stations and so I was surprised by the lack of performance. In addition, I was unable to bring the set into oscillation no matter how far the regeneration control was advanced. Upon connecting the receiver to my 40m end fed antenna I was able to bring in a great deal more stations but was still unsuccessful in being able to make the set oscillate.

    The only alignment required with this set is the position of the two inductors that are mounted on the plug in cards. You are instructed to mount the inductors so that they sit proud of the PCB so that you may adjust the gap between the two inductors upon final assembly and testing of the receiver. Specifically, adjusting the gap between the two inductors for the regenerative stage should allow you to adjust where in the regenerative potentiometer pot's travel oscillation should occur. However, no amount of adjusting brought about any oscillation.

    I contacted the gentleman who supplied the kit and he was kind and helpful in his asssitance and suggested that I try connecting the antenna to the input of the regenerative stage, thus bypassing the RF amplifier section, and see if performance improved. Upon doing so with the 10 foot section of wire I noticed a marked increase in performance. There was still no oscillation but sensitivity and overall volume was up to the point where I was receiving stations 100 miles distant without much issue.

    Thinking that there may be an issue with the plug in inductor card for the RF amplifier section I switched it around with the one from the regenerative stage and did not see any increases in set performance or the presence of oscillation in the regenerative stage.

    At this point I thought a tube might be at fault and so I went into my collection and found every 1T4 tube that I could find. After locating five of them (not including the two included with the kit) I set about swapping them into the two sockets, powering on the set and noticing if there was any change in operation. Out of the 7 total I found two in particular that stood above the rest bringing about quite a bit more sensitivity and volume. In addition, when I went to adjust the gap between the inductors on the plug in board for the regenerative section I was greeted with the squeal of oscillation!

    So, the set was now able to oscillate but one issue that remained was the fact that it would oscillate through only about 1/4 of the total travel of the tuning potentiometer (this is a varactor tuned set) and then the set would drop out of oscillation. Adjusting the gap between the two inductors so that they were closer together would cause the set to howl at the top end of the dial. There would be no smooth progression into oscillation, just a loud howl.

    At this point I am quite perplexed as to why the set would be so picky about tubes. Only two of the 7 I have on hand allowed for any oscillation. I tested all of the tubes on my Sencore TC-142 and all of them tested fine. The only difference was that the two hottest tubes measured just a bit higher than the rest. Still, the best test is in circuit, right?

    So, I'm not sure why the set is so picky about tubes and I'm also unsure how to get the set to oscillate over the entire tuning range of the AM broadcast band. I have included images showing the schematic, the parts list, a photo of one of the plug in inductor boards and the set itself. Any ideas would be greatly appreciated!

    Parts List:

    https://i.imgur.com/NixpyOz.jpg

    Schematic:

    https://i.imgur.com/YEweEit.jpg

    https://i.imgur.com/wXe1G3r.jpg

    https://i.imgur.com/lvIy2Ri.jpg

    Plug in card:

    https://i.imgur.com/GUEjZ0e.jpg

    https://i.imgur.com/FVJE3jJ.jpg

    Receiver top:

    https://i.imgur.com/BnciwK0.jpg

    Receiver bottom:

    https://i.imgur.com/0PCXtb4.jpg
     
  2. K6BSU

    K6BSU Ham Member QRZ Page

    It seems strange to me that a variable mutual inductance scheme is used to adjust regeneration (positive feedback). All regen radios I have built require an adjustment of the regeneration control (a potentiometer or a variable capacitor) for every change in station frequency. The two controls are used together.

    IOW, the regeneration control needs to be continuously variable.
     
  3. K6BSU

    K6BSU Ham Member QRZ Page

    OK. Now I see the regeneration is a pot in the screen grid circuit. Adjusts tube gain to the point of oscillation.

    Those "inductors" are probably very low "Q". My suggestion would be to replace them with solenoid inductors wound on as large a form as possible.
     
    KB4QAA likes this.
  4. KC2TAU

    KC2TAU Ham Member QRZ Page

    Do molded inductors generally have a relatively large manufacturing tolerance? I ask because this was purchased as a kit and so you would assume that, when assembled, it would work without issue. I was wondering if perhaps the inductance value used in the kit is not correct? I was thinking that perhaps an inductor might be faulty but when I swapped the plug in boards the behaviour stayed the same leading me to think this might not be the case.
     
  5. W9ALD

    W9ALD Ham Member QRZ Page

    I may be all wet here, BUT, your two plug in cards are oriented differently as they are plugged into the main pc board. If you look at the schematic for the plug in board socket pins 4 & 5 are tied together. It appears the plug in boards are the same in layout and in part value and placement. However I can not help but wonder if the orientation of the plug in boards makes a difference. There are four possible combinations. Have you tried them all? Secondly, I am assuming the boards have plated through holes. I can see some components where the solder has not flowed through both sides making for possible open connections on the pc board to component connection. Is that a solder bridge on the brown ring end of R-7?

    I really do not have much to add. Put it aside for a day or two and inspect the boards under magnification, there is likely something there you are not seeing, even though you may have looked at it a dozen times. Good Luck with the project.
     
  6. K6BSU

    K6BSU Ham Member QRZ Page

    Some variation in molded inductor values would mostly affect tuning range and/or ease of obtaining oscillation.

    Since the coil card is separate, you could easily fabricate your own, and use a coil form at least 1" diameter. Both coil windings could be on the same "tube form". I think you will find performance greatly improved over those molded inductors.
     
  7. K6BSU

    K6BSU Ham Member QRZ Page

    High "Q" in regen circuits is essential. Most all schematics show coils wound on forma at least 1" diameter, and larger is better.

    This shows a typical antenna/tuning/feedback coil for a regen. It is wound ion a styrene form, but cera regen.jpg mic is better.
     
  8. KC2TAU

    KC2TAU Ham Member QRZ Page

    K6BSU - I appreciate the suggestion regarding the coil form but having the coils on 1" forms would require completely redesigning the connection method for the coils to the board. One of the things that attracted me to this kit was the small size. I understand about the differences in Q between coil sizes but in the past I built a regenerative receiver from Vectronics, the VEC-102, that used molded inductors and it worked quite nicely.

    W9ALD - Thank you for your suggestions. I took a look at the plug in boards and the connections are mirrored meaning that it does not seem to matter which way you plug in the cards. I did try just to see and the behaviour remained the same.

    I looked at it a bit more last night and through some more adjustment of the coils on both boards I was able to extend the range in which regeneration occurs through more than half of the regeneration potentiometer's travel. What struck me, though, is that there is no gap now between the coils on both boards and yet two things I notice is that the set will still not oscillate on the lower half of the band and that the sensitivity drops off markedly past about the same point as well. This almost makes me think that the set is either wanting for more/better coupling or more inductance. What I'm not sure about, though, is which inductor to increase. I'm having difficulty figuring out which inductor does the tuning and which is the equivalent of the tickler coil.
     
  9. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    You might check whether switching the direction of one of the inductors might help. I imagine they should be positioned with the winding direction the same for each, and you do have them installed the same according to the marking pattern but maybe the marking direction is random and not fixed.

    73,


    Mark.
     

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