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Old 09-19-2017, 02:47 PM
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old_tv_nut old_tv_nut is offline
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DTV converter and UHF modulator units from MCM

Feeding my CTC-5 has been an interesting exercise. Its circuit design is scrimpy in every section including a cost-cut color section and single stage audio IF.

The weak audio IF in particular means that the sound from common DTV converters tends to be weak and critical for IF alignment and fine tuning.

I found a converter box that also had a USB input for playing media onto an NTSC output at, which has joined Newark recently as Newark Element 14:

This converter has a strong audio output (don't know if it's the modulation level, the carrier level, or both). Its digital reception sensitivity seems similar to a Zenith converter.

The box has DC power output on the antenna input jack, which is supposed to be switchable on or off in the menus, and is supposed to default to OFF. The first unit I received had the power shorted to ON, so I returned it for a replacement. The replacement worked properly the first time I used it, but the second time I powered it up, it shorted too. So, I have added a power-blocking connector rather than trying to get another replacement. Picture quality is good on the CTC-5 as well as sound.

The other unit I have just received and fired up is a UHF/cable analog modulator.

It has video and monaural audio input. This has allowed me to test the UHF tuner on the CTC-5 and switch between off-air and DVD by going between VHF and UHF. I have the two units going into a reversed 2-way splitter, and the TV is connected with a V/U/FM balun.

I see some moire that looks like random RF interference in the UHF pictures, and it is the same on all UHF channels. I have no idea if it's the TV, the modulator, or something else getting into the signal. It could be a clock in the DVD player leaking into the modulator, for example. Disconnecting the DTV converter does not fix it, so that's not it. Have to do some more chasing.
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Old 09-20-2017, 12:50 PM
kf4rca kf4rca is offline
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The sound carrier level is supposed to be 12 db below visual carrier level. But I would bet the problem is in the CTC5's intercarrier sound section. I'd play with the grid resistor of the sound IF to try to get the level up.
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