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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Aug Tue 15, 2017 4:52 pm 
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BobWeaver wrote:
I tried that modification, but wasn't satisfied with how it worked. It wasn't possible to get the modulation even remotely linear unless you kept it below 30%.


Here's a topic I started about the mod I listed.

viewtopic.php?f=12&t=303162


I have yet to do it, but later this year when I have the money I'll buy one.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Tue 26, 2017 2:00 am 
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Hello Everyone, My name is Dan. I started getting in to old radios earlier this year when I picked up and restored a Transoceanic G500. I'm now looking to build a transmitter to broadcast to the set. I found this post after seeing the AMT3000 is no longer available. I 'm hoping I can get a couple questions answered about this transmitter.

Is this probably the best option (seems to be so far) for a DIY solid state transmitter?

If so is the schematic linked on page 41 the most up to date version?

With the exception of c2 and c6 are all the cap values in micro-farad?

Thanks
Dan

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Tue 26, 2017 5:31 am 
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Dan, try one of these, reviews have been good and price is right...https://www.ebay.com/itm/DIY-655kHz-131 ... Swk6pZfyS3

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Wed 27, 2017 9:03 am 
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Thanks for the Suggestion ke4nu. I had come across that link here along with several others in my search for a transmitter. I don't think that is quite what I'm looking for in regards to learning and DIY. I have located most of the parts for this transmitter and would very much like to build it. Though to do so I'm sure I'm going to need some assistance as a lot of this is new to me. currently I'm hung up on not knowing how to select a bridge rectifier. Any help would be appreciated.

Dan

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Wed 27, 2017 4:59 pm 
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Retrotube wrote:
currently I'm hung up on not knowing how to select a bridge rectifier. Any help would be appreciated.

Dan

Most of us order from places like Mouser or Digi-Key but if you have a Radio Shack near by, they have suitable bridge rectifiers... One I linked is overkill but at $1.25 should live through even moderate shorts...

https://www.radioshack.com/collections/ ... -rectifier

Another one

https://www.radioshack.com/collections/ ... -rectifier

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Wed 27, 2017 7:29 pm 
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Thanks Tom, most of the parts will be coming from Digikey. I'm wondering if this one linked below will work. I should have mentioned my T1 is 12v 150ma. Just so I can learn, the bridge rectifier is selected based on the max/peak values of the 12v side?

https://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=247614876&uq=636499730430959116

I got 2 more parts to locate, L1 & L2. Again I'm having trouble due to my lack of knowledge, I'll be using a 1000kc osc.

I'm wondering if anyone can comment based on my T1 if R4 should be 220 ohm?

Thanks for the replies, I'm really looking forward to building this.

Dan

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Thu 28, 2017 5:44 pm 
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Yes voltage rating of rectifier is based on peak of supply(output of xformer)... I doubt you can find anything below 50v, I mostly use parts from junk TV PCB boards...

For R4 I assume you're not planning on using a 9v regulator, in P/S??

Should be able to find your coils in this listing...

https://www.digikey.com/products/en/ind ... ageSize=25

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Thu 28, 2017 9:09 pm 
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Thanks Tom,

If I understand correctly, you are correct. I am intending to start with 12v and see voltage drops similar to the schematic going through R4 and the LED if I'm reading it correct, with only whats listed.

While searching of my final few parts I have ran into a few more questions.

Does the LED need to a 12v one I'd assume?

Does it matter if R3 is linear or logarithmic?

Does the wattage of any of the resistors matter, I'd guess 1/4 or 1/2 watt would be ok? Would there be any benifit to lower/higher watts on the audio in resistors?

Thanks
Dan

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Thu 28, 2017 9:41 pm 
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Can you provide a link to your version of schematic?? I seem to be missing that one... No doubt it's on my old computer...

This unit uses the LED for tuning and voltage drop of approx 1.7v, all that's necessary is any general purpose LED without a current limiter resistor...

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Thu 28, 2017 9:55 pm 
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Tom, Here is the schematic I have.

http://antiqueradios.com/gallery/main.p ... ewsIndex=1

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Thu 28, 2017 10:33 pm 
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Ahhh I've seen that somewhere before... :lol:

For R4 it'll probably need to be approx 500-700 ohms, when rectified a 12v transformer generally gives 16-17v DC... Go high in resistance, if the voltage to xmit is 6 or 7v it'll still work, can use less resistance if needed... If voltage is high can always use additional LED(s) in series, each one will drop approx 1.7v...

The resistor wattage isn't going to make any difference... Too low and they'd overheat but 1W isn't any better than 1/4...

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Fri 29, 2017 5:55 am 
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Thanks so much for the assistance Tom. I was able to get everything ordered earlier in one go with the exception of a final enclosure. Picked up some extra stuff to play around with and answer some of my own questions including extra resistors and LED's. Parts will probably arrive Tuesday, pretty exited. Just picked up my second radio today too I happened to come by for $5, I might be hooked now. Though before I get to the breadboard I'd have a couple more questions.

To figure out the rectified voltage, should it be measured under load or can it be measured before I hooked everything else up.

This will probably show how green I am. Do all the grounds in the schematic go to chassis or do they end somewhere else? I'll be using a 3 prong plug and steel enclosure when it's finished.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Fri 29, 2017 4:01 pm 
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Unloaded voltage will be higher than loaded but these transmitters current draw will be so small it isn't going to make a huge difference... What's the unknown is what voltage for a given xfomer will be, roughly figure 1.4x the AC voltage... I cheat and use a resistance sub box, so select value that gives voltage I'm looking for... In the real world I find these usually modulate best at 8 to 8.5v...

All circuitry has a common ground... You can use the AC ground or ground rod, water pipe etc... Usually the AC ground is OK, but can induce hum... Keeping antenna away from AC wiring helps minimise hum... Modern wall warts with switch mode circuitry create all sorts of issues... My Amazon Fire TV Stick is so noisy I keep it shut down unless I'm using it...

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Fri 29, 2017 6:53 pm 
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Tom, if a guy wanted to try a voltage regulator to take the guess work out of R4 , where would it go in relation to D1, C8 and C9. Would you go with a 12v output or 9-10v and eliminate R4? Probably worth mentioning I will not be using the optional Mp3 player power shown in the schematic.

Dan

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Fri 29, 2017 10:24 pm 
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The schematic on first page shows version with a LM7810 regulator in B+...

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Sun 31, 2017 8:32 am 
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Got my transmitter on the breadboard. Haven't got any music yet. I may have damaged my trim cap trying to jam it in the breadboard. I stuck it in a protoboard and hooked it up with wires but didn't get a response from the LED. I'm going to order a different style to replace it. It was pretty crowded between 800 and 1000 tonight so that probably didn't help. Usually I cant find anything on.

I'd like to ask about the voltage around the LED. When I measure the voltage from the negative lead unhooked with the multi-meter between that and ground I get 8.55v but the LED does not light up (10v on positive side). When I have everything hooked up the LED is fairly light and seems to be responding slightly to normal interference. When I touch the multi-meter to the side of the positive lead with everything hooked up I get 10v, when I touch it to the side of the negative lead I get 3.25v. My question, is that a normal voltage for just touching it to the side of the negative lead when everything is hooked up?

Dan

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Sun 31, 2017 4:54 pm 
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It'll work without the tuning cap, range will be reduced...

Voltage should be close to indicated on schematic, 10v on LED positive should be approx 8.3v on it's negative...

Check for approx 4v on pin 5 of LM386 & same on pin 14 of osc(or 8 with eight pin IC)... The osc xtal is powered by the audio IC...

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Sun 31, 2017 8:21 pm 
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Thanks Tom, without the trimmer cap should I still have a ground between L2 and the antenna? I notice on the schematic without the voltage regulator it lists L2 at 150uh. On the schematic with the voltage regulator L2 is listed at 150-690uh. Whats the effect of changing the coil value. I started over on the breadboard double checking everything, here's what I have so far.

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Dan

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jan Mon 01, 2018 12:12 am 
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No connection to ground without cap...

Value of coil is determined by freq being used...

Dunno if it'll help but here is my breadboard unit... Was running from 14v so extra LEDs were for voltage drop, the three were in series...

Image

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Jan Mon 01, 2018 6:58 am 
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Tom, Thank you for the assistance! I got a working transmitter, was able to tune into it 1050 right away this time. Confirmed the trimmer cap was bad. I'm going to play around with it on the breadboard for a while, do some experimenting. I'll report back with some pictures when I get it in a final enclosure.

Attachment:
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Dan

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