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Old 04-21-2017, 02:41 PM
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Modifying RF channel of RS modulator

The local Radio Shack is closing in a few days. Their cheap NTSC modulators were marked down 70%, so I bought a couple to play with. I wanted to try changing the RF channel so I could put at least two on a cable to feed vintage sets.

The unit opens by finding screws under three feet, and removing the F-connector nuts, followed by a sort of Houdini escape routine to wiggle the board out of the enclosure.

The RF oscillator coils are under an easily removable shield cover. They are accessible through the hole in the upper left, but are covered in what seems to be beeswax for mechanical stability. They are tuned by knifing them (spreading a winding or two). I removed the the cover and scraped away enough beeswax that I could do some extreme knifing of the channel 4 coil, which you can see in the photos.

I connected the modulator to a TV with a synthesized tuner, and knifed the coil first to channel 5 and then continued to channel 6, which is about as far as it will go without replacing the coil.

The bottom shield is soldered in place with a big glob of solder, so it would be difficult to replace the coil. Besides, I am leery of doing any component replacement because the bottom is covered in thin traces and surface-mount components.

i don't know if the frequency is exact, only that the AFC can pull in when changing to channel 6 from above or below, so it's probably within 100 kHz or so.
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Old 04-21-2017, 07:26 PM
user181 user181 is offline
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Pretty cool.

I picked up a lot of good deals before our Radio Shack closed a few weeks ago, and one of the things I got was an HDMI-to-RF modulator. I hadn't seen one of those before, and at 70% off, I figured "why not."
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Old 04-22-2017, 03:59 PM
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Well, to illustrate the perverse nature of inanimate objects:

I hooked up the modulator and a D/A converter through a reversed splitter to my CTC-5. Worked fine, except the channel 6 frequency was far enough off to require a good rotation of the fine tuning knob. In the process, the fine tuning broke (doesn't do anything on any channel), so now I'll have to fix that. (It had been binding to the channel selector, and still does, so the break is somewhere beyond the binding point. I'm hoping that there is minimal damage and I just lost some sort of coupling clip or the like.)

Also, the modulator board is fastened to the TOP of the enclosure, which means I can't re-tune the channel 6 frequency without taking it all apart again. When I do that, I'm going to drill an access hole in the top of the enclosure.
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Old 04-25-2017, 06:15 PM
zeno zeno is offline
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You probably cant get this to ch7. after ch6 you hit FM radio
then aircraft ( AM ) then public service hi band. If it can
be forced to work it will probably be crappy.....

73 Zeno
LFOD !
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Old 04-25-2017, 07:52 PM
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Quote:
Originally Posted by zeno View Post
You probably cant get this to ch7. after ch6 you hit FM radio
then aircraft ( AM ) then public service hi band. If it can
be forced to work it will probably be crappy.....

73 Zeno
LFOD !
Probably true. Apparently, from the fine tuning result, I didn't get all the way to channel 6.

There are multiple possible problems going higher, such as, the oscillator might not run at the higher frequency; the modulating element may be partially bypassed by stray capacitance, reducing the modulation depth; and so on.
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Old 04-26-2017, 08:32 AM
kf4rca kf4rca is offline
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Part of the problem might be output filtering. It probably has a crude VSB filter that would need tuning.
The only experience I have with modulators is the UM1381 modulator which uses the Motorola MC1374 chip.
From the data sheet if you want to move the frequency, you have to retune the output filter also.
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Old 04-26-2017, 11:20 AM
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These devices typically have NO output filtering, and are full double sideband.
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Old 04-26-2017, 05:56 PM
kf4rca kf4rca is offline
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The UM1381 appears to have TWO fairly sophisticated filters in the output circuit. I suspect one for each channel. In reality, they are probably bandpass filters. Filtering out harmonics on the high side and lower sideband components on the low side. There is also a resistive pad that can be jumpered out if you want to broadcast around the neighborhood.
The UM1381 is from the 80's. I used to see them frequently at thrift stores. They were part of the TI computers that were popular at that time.
Could be the requirements for modulators have changed since then.
The MC1374 modulator chip is also used in the Blonder-Tongue CAVM series of (fixed channel) modulators.
Not sure about the one you're working with but a sure way to tell if it has no output filtering is to tune to the lower adjacent channel and look for modulation components.
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