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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Fri 05, 2021 2:55 pm 
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Tom,

Can you explain the difference between the new and old oscillators? Are they not all analog?

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Fri 05, 2021 3:09 pm 
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anchorman wrote:
Tom,

Can you explain the difference between the new and old oscillators? Are they not all analog?


Dunno, just that earlier osc modulate maybe 10% at best. Of course may not apply to all brands, but some unmarked and the Dale units I mentioned give similar results.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Fri 05, 2021 5:27 pm 
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anchorman wrote:
Tom,

Can you explain the difference between the new and old oscillators? Are they not all analog?

Newer oscillators have a wider operating voltage range. For example with an operating voltage range from 2.7V to an absolute max of 6V, you can set the no modulation voltage to the mean 4.35V. This will give a max modulation of 38% without the risk of killing the oscillator chip.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sat 06, 2021 7:39 pm 
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The only osc I've killed was one output at chip shorted to ground.

Here's my 386 with ECS 1meg osc, will modulate easily to 80% +.

Also mod at half Khz.

board of xmitt, connectors plugs in.


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386mod501.jpg
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386xmitt.jpg
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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sun 07, 2021 4:59 pm 
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Here's a Dale 921Kc osc in my 386 tx.

Pictured is max modulation. Higher input results in extreme overmodulation.


Attachments:
386dale921.jpg
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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sat 13, 2021 2:50 am 
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I got a little power supply built to plug into the wall, and so I’d have an easy ground for this. I am still working on laying out part locations in the enclosure, which is a Heathkit phone patch. Probably will move around a bunch of stuff in the final version to keep the oscillator, the audio, and the power separate. I’m going to keep the VU meter, and have it sampling the audio through a .1 uf capacitor from the output of the lm386, akin with a more permanent trimmer cap to keep from overloading the meter. The idea is to figure out how loud my audio needs to be while it still has a clear signal, and then use the 5k pot to adjust level in for different sources. If the meter is loading things needlessly, I’ll drop a small hi impedance input op amp buffer at its input.

The large LED does the best out of all of the ones I’ve tried so far. I’ve also got some other led indicators in strange form factors that I picked up at the local electronics store near my friend’s place in Munich last time I visited there that also perform well and give good contrast between tuned and not tuned.

At the moment, I’m a little worried about the possibility of over-driving the oscillators if the supply voltage goes too high, so will probably build in an lm317 adjustable regulator in there on the empty space of the power supply board to get maximum voltage into the lm386 and oscillator, without overdriving. The little transformer I’m using has a large amount of sag. Off to give it a listen and see how it does, then I need to play with the scope and see if I can figure out how to check modulation levels with that.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sat 13, 2021 2:55 am 
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When I was using just a 9v battery, you could see the input voltage getting modulated as the led would flicker with the music. Hopefully now with a proper supply, I’ll get a little usable distance too.

I made it so that the antenna loading inductor could be swapped out easily (it plugs into a connector on the board. The plan was to try out a few different frequencies and see what works best for me. I’d ultimately like to build at least one transmitter that has switchable crystals/oscillators/loading inductors, and as mentioned before, to make a few of them and place around the house so I can have different “stations” to listen to.

Thanks to those who have contributed to the general knowledge on these! I recently got a bunch of 6888 tubes in, so will hopefully have time to start building some transmitters with them before too long. First, I’m going to wrap up a few of these, though!

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sat 13, 2021 4:49 am 
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ok, I managed to talk my scope into working by laying the probe tips onto the antenna. new to reading modulation, but I'm assuming that when the troughs on my sine wave touch in the middle, that's 100%? when I gave more signal than that, it first made the troughs brighter, and then started to visibly square up the peaks, which I assume = overmodulated.

I managed to get the VU meter adjusted so it is at 0 on 1khz tone when the troughs are just touching on the scope. playing music through that, I had to crank it up a bit in order to get it to peak at 0, and I could start to see bright spots in the modulation pattern at the center of the waveform. cranking it up way too high started to make obvious flat spots top and bottom on the modulation picture.

I noticed also when I drive it deep into distortion territory, that my LED starts to flicker with the music, as I was seeing when running it with the battery. Currently when tuned, I've got about 3.5V on the oscillator, and about 5.15v without the antenna peaked. I realized while messing about, that the power supply still sags a fair bit when I'm tuned properly, and I can see the voltage change when I measure at the output of C9 before my LED, and also at the junction of c8 and R4 , as I tune. If I regulate the supply, will I still see the LED peak as I tune it? It seems like I'm loosing a fair bit of power to the voltage sag of this tiny little transformer that I'm using.

I'm off to plug this in at home and see how it sounds now that I've got power and a ground, and a bit more voltage than the battery was offering. please let me know if the regulated supply with the LM317 is worth the trouble! I'm thinking if I add that to just ditch R4, and C9 and take power off of C8 directly.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sat 13, 2021 5:29 am 
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Sounds like you're having fun...

For scope coupling wrap 4" of wire around the antenna lead, connect probe to that. For best modulation, set LM386 voltage input for 8-8.5v max. At higher voltages, most won't have the audio power to cleanly drive the osc.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sat 13, 2021 6:10 am 
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I’ve got something like 1khz howl happening with no audio. Also managed to make my LED go poof somehow, but everything else keeps working. Audio quality was surprisingly good, aside from the howl. I was on 1.2288 MHz, and there’s a station that comes in clearly there. I’ll have I try some other crystals tomorrow and see if a different frequency makes it go away. I know it’s probably more likely to be the jumble of wires I’ve got in the case at the moment, and something feeding back.

I’m only broadcasting from the kitchen to a bedroom on the far side of the house, maybe 30-40 feet? I’m cheating a bit, though, as the radio has an antenna that runs the length of the attic. There’s more than enough power to overload the front end of the s-40b, but it’s got sensitivity control that can be cranked down if needed.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sat 13, 2021 7:00 am 
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For one of my test items I got some little "level" (VU?) meters to replace the LED. Use them with a very small value shunt resistor & the supply drop is much less than the LED causes. Maybe see what the sensitivity of that Heath VU meter is?


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sat 13, 2021 7:47 am 
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Are kits for this transmitter still being sold?


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sat 13, 2021 4:50 pm 
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ai6vx wrote:
Are kits for this transmitter still being sold?

Unfortunately the ARF member that stocked parts has passed away.

hwhall wrote:
For one of my test items I got some little "level" (VU?) meters to replace the LED. Use them with a very small value shunt resistor & the supply drop is much less than the LED causes. Maybe see what the sensitivity of that Heath VU meter is?

Current draw should be around 10-11ma.

I use the satellite meter with shunt resistor as current indicator. Used the resistance sub box to find value needed for approx 3/4 deflection. Case LEDs are power indicator only.

BTW the Dale osc I pictured earlier pegged meter.


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386meter.jpg
386meter.jpg [ 448.94 KiB | Viewed 1205 times ]

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sun 14, 2021 9:49 pm 
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Quote:
Are kits for this transmitter still being sold?


I have some of the parts that are used in these but not complete parts like a kit.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sun 14, 2021 11:18 pm 
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hwhall wrote:
Quote:
Are kits for this transmitter still being sold?


I have some of the parts that are used in these but not complete parts like a kit.

I have some parts too, but not the important bits. I have several LM386 chips, general diodes, resistors and led's, but no oscillator, inductors and definitely no variable cap. The oscillator I could get from Digikey (is there one that's the 'best' one to get? 1.000Mhz on my dial is clear). The variable cap? Not sure where to get that.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Mar Sun 14, 2021 11:22 pm 
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I have several diff oscillator frequencies and some variable caps (they're the polyvaricon type that I used in my own transmitter). Probably some suitable chokes, too. And a handful of the little meters that I tried out.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Sun 04, 2021 10:36 am 
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anchorman wrote:
Retrotube wrote:
To follow up on my last post, here is an updated schematic. I think it sounds good, especially on my ZTO G500 that seems to pickup a lot of HF noise. Not sure what's up with my inductance (osc or ant wire) but it's the same for the original design in my current setup.

A 10pf can be used in place for C1 for a slight frequency change on the high end. Anything in between 10 - 1000pf (that i have) adds some funkiness on the scope.

C3 can be swapped for .1uf for a slight frequency change on the low end.

C4 can be a 10 or 47uf, I liked 22uf or 47uf.


Any reason to use the non-inverting vs inverting input on this? retrotube's version does this, and adds a 10uF cap across pins 1 and 8, which are the "gain setting" inputs. according to the LM386 data sheet, leaving these unconnected gives gain of 20, with 10uF across them it has a gain of 200. R4/C1 on this circuit from pin 5 to pin 2, these are shaping the tone of the audio coming in, or something else?


I believe the the inverted input was used originally due to picking up stray RF the other way, Tom can correct me on that if I'm wrong. The 10uF cap does raise the gain while R4 lowers it and does filter out HF, I believe the gain on that should be around 26. That one sounds AWFUL compared to one I came up with after that. My later one, before I got busy again, is still partly on the breadboard. Going to get that going again and post it. You can really beef up the sound on the original design with just one more cap.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Sun 04, 2021 10:51 am 
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anchorman wrote:
I’ve got something like 1khz howl happening with no audio... I was on 1.2288 MHz


I have this also so I only use the 1MHz osc.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Mon 05, 2021 1:49 am 
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Thanks for the update! Looking forward to see what the final design is.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jul Tue 06, 2021 7:37 am 
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anchorman wrote:
Thanks for the update! Looking forward to see what the final design is.


Here it is.

Attachment:
LM386.png
LM386.png [ 16.63 KiB | Viewed 196 times ]


C1 .01uF
C2 47uF
C3 10uF
C4 .1uF
C5 .1uF
C6 470pF
C7 1uF
C8 .01uF
C9 Variable
R1 2.2k
R2 2.2k
R3 10
R4 1k
L1 680uH
L2 390uH

Notes:
The value of L1 is important.
C7 is the cap that really fills in the sound, a 1.2uF will pretty much make it sound like FM. C5 & C6 fine tune C7.
In my Design most caps are PP film and I trend to buy C0G ceramics, this is important on C6 for sure that it is a time independent cap or "S's"
will roll off a bit sharp. The only ceramics on mine are 5 & 7, I don't *think* they are C0G rated based on the size, I'd have to check the invoice's to know for sure.
Nothing special about R1 & R2, just what I use. My variable cap is a 1-9 pF high Q air cap.

Edit, Additional notes: for testing the usable range on C6 is 10 - 1000 pF. The usable range on C7 is .64 - 1.5uF with 1.5 boosting it on the high end, .64 on the low end. C5 to me, either use it or don't. For every step I raise C7 my ears tend to want to lower C6 a step. I make no claims that above is the best config, in fact its a compromise for voice and music. If someone is doing only music they may want to use a 1.2uF C7 and omit C5.

Edit: Called C6, C8 in the notes, updated to reflect C6.

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Last edited by Retrotube on Jul Tue 06, 2021 6:04 pm, edited 2 times in total.

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