|
Yet anotehr update
I'm sure that this thread is pretty well played out, but there was one last problem I was chasing after for the past few weeks with the use of some of the TV modulator chips. In the CM7000. THese chips and associated circuitry I salvaged from old VCRs all ran off of +5V, and the CM7000 has a 5V supply, so naturally I used it. Well, turns out that one of the chips had good power supply ripple rejection (PSRR), but two other chips did not. And it seems that the CM7000 draws irregular bursts of current while it tries to cope with bad or non-existent reception on a DTV signal. Which makes the 5V and the 12V supplies bounce up and down around 50mV. The power supply tightly regulates the 3.3V supply, and the 5V and 12V not really regulated (feedback taken off of only the 3.3V). Well, that much ripple doesn't bother digital circuits, and analog audio and video circuits (mostly op-amps), but it made some of the TV modulator chip outputs flicker on the TV set screen. At first thought I had a poorly performing power supply board, and changed a few caps on it. Then I swapped power supply boards on both my CM7000's, and the problem stayed with the TV modulator, and not the power supply board.
Well, to make a short story longer, I decided to forget using the 5V supply, but to grab a 7805 regulator chip and use the 12V and make it into a clean 5V. Problem solved!
For added fun, you could extend this TV modulator mod trick to create a channel 1 for your late 40's TV sets that had a channel 1 on the dial, you need a crystal that puts the video carrier at 45.25MHz. You'll want to exercise some care that this channel 1 signal doesn't get into the DTV's IF strip, so keep it all out of the tuner can, and not route it to the RF output jack inside the tuner can.
Or you could produce a TV set IF signal at 45.75MHz, to feed directly into a TV set's IF strip (disconnect the tuner front end coax cable that fed the TV set IF, and have the modified TV modulator feed the IF strip instead. This would let you do minimal mods to a TV set, but let you hide the converter box inside the TV set, and make the TV remote controllable.
__________________
|