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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: May Sun 26, 2019 11:59 pm 
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Location: Costa Mesa, California
One problem with home brew stuff is you can’t be sure it left the factory in perfect working condition. Sure, commercial radios do have the rare wiring mistake and those do tend to turn up on the used radio market because they have a problem and have always had a problem. With home brew, wiring errors have to be guarded against. This week new crystals came in the mail (I buy them for $5 each) and I hoped that would solve my 15 meter problem. When it didn’t, I knew it had to be a wiring mistake. I set out to check for the 50th time. The big problem is that the crystal oscillator and first mixer module works like a champ on every other band, just not 15. Like previous inspections, I started with the few circuits unique to 15 meters and there it was—the nut for the miniature bandswitch wafer was touching the 15 meter contact. It was sort of hard to see exactly, but it looked like I had my culprit. I added some insulating washers to get the nut clear and hooked everything back up. Still no 15 meters. This time I sat down with the DMM on buzzer and buzzed through all the possible pathways. Then I switched to the Ohms screen and started through the resistance paths, doing resistor math in my head. During this process, I checked the oscillator tube plate to ground and came out 60k high. I had a 75k where a 15k was supposed to be. Purple looks a lot like brown in those little colored stripes. When I had wired the socket, I was using parts from the original and a few random resistors. I wasn’t pulling parts from the sorted cabinet and the little bags with labels. Oops. I replaced the 75k with a 15k. As soon as I applied power, the scope screen lit up with a sine wave. I had my 15 meters. The scope probe is attached to a small coil of ten turns draped over the oscillator tube. Plate to screen 15k. Screen to ground 15k. Now the tube works for 15 meters when it wouldn’t before. My conclusion— and this is a guess— the oscillator needs a somewhat resonant screen (at least at higher frequencies) as well as plate and the 75k was interfering with that. Either that or just the lower screen voltage from the 60k more drop was enough to prevent oscillation at the higher frequency.

On to adding some bands around 6 MHz. These will use existing crystals so that simplifies the whole thing—or not.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: May Mon 27, 2019 12:41 am 
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Do you still have patches of hair where you hadn't torn it out yet? I know I would've been in that position....


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: May Mon 27, 2019 1:04 am 
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I have a full head of hair. The other two hairs fell out years ago. Actually, I love puzzles. My favorite is a difficult Kakuro puzzle. When I finish it there is a rush of satisfaction—then I start the next one. It is the same with radios. I have very little use for a working radio. Sure, I have my license, but that is so I can try out all the radios to see how they work. Now, a radio that doesn’t work. That is a puzzle that begs to be solved.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: May Mon 27, 2019 3:50 am 
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Norm Johnson wrote:
....It is the same with radios. I have very little use for a working radio. Sure, I have my license, but that is so I can try out all the radios to see how they work. Now, a radio that doesn’t work. That is a puzzle that begs to be solved.

Norm


Exactly!

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Jun Sun 09, 2019 6:39 pm 
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This weekend I tried to add a segment of frequencies starting at 9.4 MHz and going up to 9.9 MHz. The coil for the crystal was easy and the crystal oscillator oscillated nicely—except if I injected a signal directly into the mixer, it took way too much amplitude. At other bands all was normal. Not wanting to spend more time wrestling with that problem, I moved on to the RF amp stage and wound two transformers and wired them in. The result was a lot of parasitic oscillation and I then spent the rest of the weekend trying to eliminate it. I early on hooked my scope to the output of the RF amp module. This is allowed because I have isolated the final tuned RF circuit with a 6C4 and the signal moves out from the cathode of the 6C4 to the cathode of the first mixer. The signal at the plate of the 6C4 is dumped to ground. My scope showed that for all the upper bands, parasitic oscillation occurred as the tuning capacitor approached minimum. The 9 MHz band was especially problematic and parasitic oscillation occurred broadly across the range.

My fist approach at a solution involved adding a vernier reduction dial for the tuning capacitor to reduce any wobble and allow finer tuning. This didn’t solve the problem but helped. My next step was to make a shield for the tuning capacitor that separated the gangs as much as possible. The capacitor is already in its own enclosure. This also didn’t solve the problem. Finally, I googled RF parasitic oscillation and found a discussion that provided a clue. I tried adding a 1K resistor before the grid of the 6C4. Amazingly, this killed the parasitic oscillation on all the bands. It did nothing to hurt the gain. Before, I just had the secondary of the 6DC6 RF plate transformer feeding directly to the grid of the 6C4. The resistor is called a “grid stop resistor” and can be between 1K and 10K. The 6C4 is in the circuit to allow a cathode path that is low impedance and won’t be affected by a shielded cable for the signal to move to the first mixer/crystal oscillator module.

I am still having issues with the 9 MHz band, so that will have to wait until next week.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Jun Sun 16, 2019 7:57 pm 
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Correcting a wiring error solved the 9 MHz band problem. I now have a band for 9.4 MHz to 9.9 MHz and another for 9.875 MHz to 10.375 MHz. This gives me WWV at 10 MHz. I can’t decide if I want anymore bands which means I am focusing on adjusting the performance of the 10 bands I have now. First thing I did was try and shift all the band inductances up a bit so the variable preselctor tuning cap was operating more in its center range. Next I spent a lot of time with my antenna connected, listening to actual stations and judging the performance—and comparing the performance to my HQ-140-XA.

I am pleased by the results except for one glaring problem that appeared on the 6 MHz band. With my preselctor tuning capacitor adjusted to more capacitance, I received a mystery WWV signal. The crystal is 3.750 MHz and my resultant variable IF is 2400 to 2900 kHz. Normally this would be for a band at 6.15 to 6.65 MHz. I also use it for the upper BC band at .850 to 1.35 MHz. What WWV was I hearing? Double 3.75 plus 2.5? I tried a signal generator tone at 10 MHz and got nothing. Then I tried a tone at 5 MHz. Yep. I tried a tone a little higher in frequency and had to tune backward to get there so I knew what was happening. I was hearing double the crystal minus the variable IF when it was at 2.5 MHz.

My question is where is the frequency doubling? My scope is connected to a coil of wire around the outside of the crystal oscillator tube and sees a very clean 3.75 MHz. I know that is only the plate I am seeing. Does the doubling occur in the mixer tube? There is a 4p cap between the oscillator plate and the mixer grid. A 100k resistor connects the mixer grid to ground and the RF signal comes to the mixer at the cathode. When I adjust the preselector higher in frequency to resonance at the 6 MHz range, I can’t quite kill the WWV. It still bleeds through. I think I need to make the 6 MHz band a little more selective by changing the padder I placed in series with the tuning cap. Or I may need to wind another set of coils for the band.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Jun Mon 17, 2019 12:59 am 
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Location: Saskatoon
Any nonlinearity in any of the tubes will generate a 2nd harmonic. It could be in the crystal oscillator itself. Even if the sinewave looks clean on a scope, there could be enough to cause problems.

How difficult would it be to attenuate the crystal oscillator signal before it goes to the mixer? You may have too much LO signal.

BTW, Congrats on the impressive work you've done on this receiver.


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Jun Mon 17, 2019 2:10 am 
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Joined: Sep Wed 10, 2014 2:01 am
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Location: Costa Mesa, California
Thanks, Bob. That gives me the thought that I may try a different 6BJ6 oscillator tube. I will also try and "sniff" around the plate coil of the oscillator with my scope hooked to my coil of wire I slip over the tube. As far as attenuating the oscillator signal, I have thought of trying a "grid stop" resistor between the signal and the grid of the triode connected 6AH6 mixer. I have a couple birdies I would like to eliminate and suspect the source is the mixer for at least one of them. Maybe a 1K or 2.2K resistor there would help both issues. Another option is to reduce the coupling capacitor down to 2p from the plate of the oscillator to the grid of the mixer. If the frequency doubles in the mixer tube itself, then I will need to address the issue at the RF stage by increasing selectivity of the tuned circuits.

Bob, your help early on with your website tools for coils and bandspread were certainly instrumental in my getting past that stage of construction.

Norm

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