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 Post subject: Subminiature Receiver Issues
PostPosted: Sep Tue 03, 2019 1:17 am 
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Joined: Feb Sat 16, 2013 5:08 pm
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Location: Ft. Collins, CO.
Over the past week I've been slowly playing around with some soviet subminiatures I had lying around. I'd like to get a receiver working, able to play through a 16 ohm speaker. With just a detector, I can hear the stations faintly through headphones. I added an audio amp stage, and it was pleasently loud in the headphones. I added a second amp, and then that's where I lost the ability to actually tune the radio. I can place signals in the grid circuit of the detector and have clear sound through the speaker, but I can't pick up and tune stations. The most powerful local station comes through extremely faintly, regardless of whether I even have a variable capacitor in the circuit. Before this, bringing a hand close to the coil would change the frequency, now almost nothing. I've disconnected/reconnected, and tried all sorts of things before bringing it here. I am also encountering some signal distortion, as shown on the oscope. The first image is the input, 1000 Hz at 1V on the grid of the detector. The second is the 10V out of the last stage before the transformer. It is still at 1000 Hz, but note how the wave has become more jagged.

Any suggestions would be much appreciated.


Image
Image
Image
Image

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Last edited by RCAProduction on Sep Tue 03, 2019 10:09 pm, edited 1 time in total.

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 Post subject: Re: Subminiature Receiver Issues
PostPosted: Sep Tue 03, 2019 4:19 am 
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Joined: Sep Mon 16, 2013 2:42 am
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Location: Tucson, Arizona U.S.A.
First of all, my experience with those white breadboards is that parts have a way of falling out while you aren't watching. Make sure everything is still connected like you want it to be.

Second, you don't have any bias on the grids of the audio tubes. That will cause distortion. the fact that the AC voltage on the grid of one tube is lower than the voltage on the plate of the preceding tube indicates that the coupling capacitor is too small or you are making a measurement error, like using an instrument with too low input impedance.

30V RMS at point D is likely to mean that V2 is overloaded with too much signal. What does the waveform look like at that point?

Driving a speaker with only 45V of B+ and battery type tubes is shaky. Portable radios generally use 67 1/2 V or 90V on the output tube. 45V greatly limits the power that can be produced, not that 90V would give room shaking volume with these tubes. For that matter, is V3 even an output tube? If not, it may not be capable of driving a speaker without a lot of distortion. Is the impedance ratio of your output transformer suitable for the tube?

Your lack of reception could mean that one of the tubes is oscillating. This could be the result of using the white breadboard and long wires. They aren't very suitable for high gain amplifiers.

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 Post subject: Re: Subminiature Receiver Issues
PostPosted: Sep Tue 03, 2019 5:39 am 
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Joined: Feb Sat 16, 2013 5:08 pm
Posts: 957
Location: Ft. Collins, CO.
Thanks for the reply!

I can try bumping the plate voltage up to 67 when I figure out the rest. 45 was just a number I threw at it, no real reason other than 22 would be too low.

V3 is the same as the other tubes, not an output tube, just a 1J29B. I think these are for VHF or UHF ranges, so this is not their intended purpose at all.

I can take a look at point D tomorrow on the scope. I think it was about halfway between the two waves shown.

As for bias, I'm trying to avoid adding a "C" power source if possible. I can also try some larger capacitors as the coupling capacitor. I think I may have tried that at one point, but it completely cut off the output. I have no idea if the impedance of the output transformer matches the tube, but I'd make a good guess and say it's not. I pulled it out of an old aircraft radio, I don't know the original purpose. I assume it was the audio output transformer for a 12AX7 or similar. Most of this has been pieced together from scrap radios laying around.

For that matter, I am not entirely sure of the values on the coupling capacitors. They are some old micamolds with the dot designations, and I couldn't find my sheet on how to read them. They came from grid circuits in other radios, which is why I picked them. I'd agree that they may be too small for this application.

It's also possible one or more tubes is oscillating, as you said. I don't know what I would do to verify that, but a lot of these jumpers are much longer than I'd like. I might solder everything together directly, and see what happens.

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 Post subject: Re: Subminiature Receiver Issues
PostPosted: Sep Tue 03, 2019 5:57 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: SoCal, 91387
You might consider designing a regen; I've gotten great reception from mine. speaker only, 67.5 volt B supply. I only wound one coil, for the BCB.


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 Post subject: Re: Subminiature Receiver Issues
PostPosted: Sep Tue 03, 2019 8:04 am 
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Joined: Mar Thu 21, 2019 3:53 pm
Posts: 128
Hi,
Perhaps you should rewire V1 as a pentode not triode. If the stage is oscillating as Jim suggested, then this configuration might stop oscillation. Triodes tend to oscillate easily and pentodes give more gain.Try different resistor values for the G2.


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 Post subject: Re: Subminiature Receiver Issues
PostPosted: Sep Tue 03, 2019 10:01 pm 
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Joined: Feb Sat 16, 2013 5:08 pm
Posts: 957
Location: Ft. Collins, CO.
I upped the voltage to 67, and replaced the coupling capacitors with new ones with larger values. I found my capacitor sheet, and I was using a 220 pF cap. Now they are 0.003 uF. This brought the voltages between B-C and D-E to be about equal, just as was mentioned earlier. The final output is now at 40 volts (Point F).

However, this appears to be where the majority of the distortion comes from. It's bad enough that the input wave essentially becomes a square wave going into the speaker. Is this because of a lack of bias?

I have also not figured out why the tuning circuit is not working. I haven't shortened any leads, just made sure everything was properly attached.

Point A (Input):
Image
Point B:
Image
Point C:
Image
Point D:
Image
Point E:
Image
Point F:
Image

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 Post subject: Re: Subminiature Receiver Issues
PostPosted: Sep Wed 04, 2019 3:45 am 
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Joined: Sep Mon 16, 2013 2:42 am
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Location: Tucson, Arizona U.S.A.
The ratty waveforms could be caused by overloading from a too large input signal. You didn't say what scale factor your scope was set to so it is hard to guess. I assume it changes from one picture to the next; if it doesn't, your amplifiers aren't doing much.

Another thought occurred to me is that you don't have an RF bypass capacitor on the output of the detector. Perhaps there is enough RF coming out of it to overload the audio amplifier and prevent anything else from getting through. You might try 470pF from point B to ground.

Do you have a link to a data sheet for the tubes (in English)? If so, please post it.

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 Post subject: Re: Subminiature Receiver Issues
PostPosted: Sep Wed 04, 2019 6:14 am 
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Joined: Feb Sat 16, 2013 5:08 pm
Posts: 957
Location: Ft. Collins, CO.
Yes, the scale changes for each image. This is how I was measuring the voltages shown on the schematic.

I can give an RF bypass capacitor a shot.

Unfortunately I don't have an English version of the data sheet. All I have information for is the voltages and current draw. THis also has some good information, but still short of a full sheet. https://www.radiomuseum.org/tubes/tube_1j29b.html

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http://www.FortCollinsB17.org


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 Post subject: Re: Subminiature Receiver Issues
PostPosted: Sep Wed 04, 2019 7:09 am 
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Location: Tucson, Arizona U.S.A.
One thing I notice even from that, is that the filament voltage should be 1.2V. The 2V shown on your schematic is excessive. It won't prevent the circuit from working but it will shorten the life of the tubes. They look like they were designed to run from a zinc-carbon battery where the voltage drops with use.

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 Post subject: Re: Subminiature Receiver Issues
PostPosted: Sep Wed 04, 2019 3:59 pm 
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Joined: Feb Sat 16, 2013 5:08 pm
Posts: 957
Location: Ft. Collins, CO.
The 2 V is actually 0.4 V below the filament rating, because I am not using the center tap. Each half is rated at 1.2 V, and they are wired to be in series.

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http://www.FortCollinsB17.org


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 Post subject: Re: Subminiature Receiver Issues
PostPosted: Sep Wed 11, 2019 6:18 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10742
Location: Omak,wa,usa
Hello Eli,
what great build


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