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Old 04-26-2017, 04:56 PM
kf4rca kf4rca is offline
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Join Date: Feb 2015
Location: Atlanta
Posts: 737
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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