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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Thu 05, 2017 4:08 pm 
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One more comment, after thinking about this last night. I believe the original 6888 transmitter used an audio step up transformer, where more recent transmitter designs (like those using other dual control pentodes) used a triode gain stage to drive the transmitter tube. I would suggest using the version with a triode stage as the Magnavox MPX unit probably can't drive a high impedance load.


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Thu 05, 2017 4:44 pm 
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fifties wrote:
It's still not gonna be true stereo.

Tell me this; if a vocal group recording has the lead singer on the right channel, and the backup group on the left, how would anything short of an FM pilot circuit be able to separate them? Two radios in or out of phase, frequency, etc, might sound like a stereo effect, but that would be the extent of it.

In the above example, both channels would still be combined in each AM receiver.



He is going to use a multiplex decoder, which will give him true left and right audio channels for each transmitter.

If there is a phase problem, he can reverse the "polarity" of one channel.

Pete

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Fri 06, 2017 5:17 pm 
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Location: pensacola fl
Here you go true stereo decoder as real as stereo gets
http://www.ebay.com/itm/meduci-AM-STERE ... 2637382495
this board takes in I.F. and 12 volts and give you true stereo from am radio. It says won't work on tube sets but it will I have one in service now. Build the stereo tx and have ture stereo and not difficult to use. It gives line level left and right stereo out so connect to stereo amp and tuner and you can enjoy true stereo on am.


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 07, 2017 6:27 pm 
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Phase one is almost complete. I had two Pilot 601 tuners, so I cleaned them up and checked them out.
The older of the two apparently had never been opened up. The newer one had the electrolytic can replaced and one of the paper caps.
The newer one did not make any sound but the old one buzzed and seemed to be tuning something, so I repaired the older of the two.

First I replaced the rectifier, then the electrolytics, and then the paper caps. Testing revealed a high performance tuner!

Here is the newer of the tuners, the one that I have not repaired, yet.
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newer.jpg
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Here is the tuner that I decided to repair.
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older.jpg
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Finally, the repaired tuner.
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At first, I was considering remounting all components of this project to a common chassis, but I am so impressed with the little tuner I decided to build this project "componentized" and leaving the tuner intact.

Next step is the MPX unit.

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sun 08, 2017 12:34 am 
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If the tuner doesn't have AFC it is easy to add. Just needs a few compoments.

I can email you the schematic to a Magnavox tuner I added AFC to on Tuesday when I'm back at work.


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sun 08, 2017 1:11 am 
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Tube Radio wrote:
If the tuner doesn't have AFC it is easy to add. Just needs a few compoments.

I can email you the schematic to a Magnavox tuner I added AFC to on Tuesday when I'm back at work.

Here is the schematic.
Attachment:
601_schematic.jpg
601_schematic.jpg [ 144.82 KiB | Viewed 720 times ]

I was thinking of picking up the "pre-de-emphasis" signal at the 15k resistor above the 1500 uuf cap at the audio out.
Do you see it differently?

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sun 08, 2017 2:07 am 
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There's an easier way. Short across the 15K resistor and the 1500uuF cap.

That way the output is available on the terminal strip without much modding.

Also if a shielded cable is used inside the tuner to carry the audio to the terminal strip replace it with a good quality low capacitance shielded cable. An easy way to get some is to go by some cheap component video cables at wal mart.


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 14, 2017 12:09 am 
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kc5gym wrote:
Phase one is almost complete. I had two Pilot 601 tuners, so I cleaned them up and checked them out.
The older of the two apparently had never been opened up. The newer one had the electrolytic can replaced and one of the paper caps.
The newer one did not make any sound but the old one buzzed and seemed to be tuning something, so I repaired the older of the two.

One of the difficulties with FM tuners is that if a previous owner has messed around with the coil adjustments, it could very well have absolutely no output, even though the circuit itself is otherwise okay. I bought one of these a few years ago. The previous owner had made a mess of the adjustments, and It took a while for me to get it straightened out. After getting it realigned it's been a good performer.

I'm interested in seeing how you make out adding an mpx adapter to this tuner. As Tube Radio mentioned, the IF bandwidth could be a problem. If you get it working well, I may look at adding one to mine.


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 14, 2017 2:06 am 
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As usual, I am kinda getting the cart before the horse, but I believe that a good project should have a good cabinet.
There once was a piece of furniture that my grand father bought somewhere around 1950. It was known as a "hutch."
Although it had been refinished thirty or so years ago, it got left in the carport for a long time and weather took its toll.
I have some radios that need veneer and the sides of the hutch proved to be a good donor. The rest is good solid wood.
Since I had junked it down for the wood, I started sawing it up today to make my stereo transmitter cabinet.
I like to see the tubes glowing,hence the open sides.
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Attachments:
mpx.jpg
mpx.jpg [ 35.87 KiB | Viewed 659 times ]

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 14, 2017 2:11 am 
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BobWeaver wrote:
kc5gym wrote:
Phase one is almost complete. I had two Pilot 601 tuners, so I cleaned them up and checked them out.
The older of the two apparently had never been opened up. The newer one had the electrolytic can replaced and one of the paper caps.
The newer one did not make any sound but the old one buzzed and seemed to be tuning something, so I repaired the older of the two.

One of the difficulties with FM tuners is that if a previous owner has messed around with the coil adjustments, it could very well have absolutely no output, even though the circuit itself is otherwise okay. I bought one of these a few years ago. The previous owner had made a mess of the adjustments, and It took a while for me to get it straightened out. After getting it realigned it's been a good performer.

I'm interested in seeing how you make out adding an mpx adapter to this tuner. As Tube Radio mentioned, the IF bandwidth could be a problem. If you get it working well, I may look at adding one to mine.

The tuner turned out to be an excellent performer. Digging around in it with the scope indicated that the 19khz pilot signal was being received.

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 14, 2017 2:16 am 
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Tube Radio wrote:
If the tuner doesn't have AFC it is easy to add. Just needs a few compoments.

I can email you the schematic to a Magnavox tuner I added AFC to on Tuesday when I'm back at work.

Sure, I'd be glad to review an afc circuit that I may be able to implement. It may help in adding a stereo indicator.

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 14, 2017 7:56 pm 
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I'll email it Monday.


Just because the 19KHz pilot tone is being received doesn't mean the bandwidth is wide enough.

You need to modulate your RF RF generator with a 53KHz signal and see how well that is passed through the IF stages and detector. Will require removing the de-emphasis circuit.

If it does not adequately pass a 53KHz signal be sure the alignment is dead on perfect and try again.

If it still won't adequately pass a 53KHz signal then the tuner is not suitable for stereo.

With my Magnavox tuner I tweaked and re-tweaked until I got the IF, discriminator and RF sections as good as they were ever going to get.


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 14, 2017 11:26 pm 
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The bandwidth requirements for FM are more complex than for AM. The required bandwidth depends on the maximum modulating frequency to be transmitted (similar to AM), as well as the modulator's peak frequency deviation. For normal broadcast FM, the peak frequency deviation is 75 kHz. Carson's Rule is a formula for determining the required channel bandwidth:

BW = 2*(∆F+Fm)

where:
∆F is the peak frequency deviation, which is 75 kHz for standard broadcast FM
Fm is the maximum modulating frequency to be transmitted (15kHz for FM mono, and 53kHz for FM stereo)

That gives a bandwidth requirement of 180 kHz for FM mono, and 256 kHz for FM stereo.
In order to transmit just the 19 kHz pilot in addition to the mono signal, the mono bandwidth only needs to increase to 188 kHz. So, it's not surprising that a standard mono FM receiver would detect the 19 kHz pilot, but getting all of the stereo subcarrier signal is another matter.

Another characteristic of FM making things even more complicated: If the IF bandwidth isn't wide enough, you don't just lose high frequency response, the demodulated signal will be distorted as well.


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Mon 16, 2017 1:29 am 
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okay Tube, and Bob. I get it. I rummaged around and couldn't find anything that could modulate my sig gen at 58 khz.
So I spent money (just a little bit) so I can.
I did, however, find a power oscillator. It is an interesting test gear. Either 400 or 1000 Hz @ up to 1800 v!!
Nope, not gonna modify it.

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Mon 16, 2017 7:08 am 
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Hi Mark, I wasn't trying to discourage you, but thought I should warn you in advance that hooking an MPX to an older FM tuner may not work out as planned. The only way to know for sure is to hook it up and see what it does. I just thought I should mention it so that you could test things out before you get too far along in cabinet construction etc.


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Tue 17, 2017 11:55 pm 
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Just to add a little to what I mentioned way back in my first post, it looks like WA2ISE has already done this and posted his results here:

http://audiokarma.org/forums/index.php? ... 6a.413041/

Sounds like it can be done, but might take a little tweaking to make it all work. At least we know, it can be done.


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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Wed 18, 2017 1:55 am 
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classicelectronicsguy wrote:
Just to add a little to what I mentioned way back in my first post, it looks like WA2ISE has already done this and posted his results here:

http://audiokarma.org/forums/index.php? ... 6a.413041/

Sounds like it can be done, but might take a little tweaking to make it all work. At least we know, it can be done.

Thanks for the link
I will never give up.
It is a ratio detector.

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 21, 2017 6:50 pm 
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Okay, if the Pilot works out okay, this is what it will look like.
If the Pilot doesn't work out, then something else will sit on top.
Now that I have my scope back in service I will know more soon.
Attachment:
combo_front.jpg
combo_front.jpg [ 96.44 KiB | Viewed 503 times ]
Attachment:
combo_side.jpg
combo_side.jpg [ 126.44 KiB | Viewed 503 times ]

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 21, 2017 6:56 pm 
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I'm not too crazy about the crochet pattern it's sitting on... :wink:

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 Post subject: Re: Stereo Transmitters
PostPosted: Oct Sat 21, 2017 7:01 pm 
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fifties wrote:
I'm not too crazy about the crochet pattern it's sitting on... :wink:

LOL. To each his own... to me the crochet looks better than anything else I could find for a quick "backdrop" :P

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