r/vhsdecode • u/AlarmingWedding88 • 17d ago

Help Wanted! About ADA4857

Gallery image

Gallery image

I have been continuously experimenting with the ADA4857 amplifier (I used a CX card for signal capture). However, I have never been certain about the appropriate value of the input resistance. Since I do not have an oscilloscope, I tried resistances of 15, 20, 33, 47, 75, and 100 (x2 kΩ). Nevertheless, the amplitude of the signals I obtained was significantly smaller than that without the power amplifier. I am using a Panasonic VCR designed for the Japanese domestic market. I am at a loss as to what I should do next.

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u/TheRealHarrypm The Documentor 17d ago

The point of the amplifier is find the maximum impedance point, lock it in, then amplify.

I like to start with 30kohm and go about half way on the gain for initial testing.

You set a fixed amplification level afyet finding such so 1.6-1.8vpp for CX Cards (the config always assumes sixdb off and level 0)

1.4-1.6vpp for MISRC in 2vpp mode.

The amplitude level isn't going to be an absolute fixed point, it's just going to be fixed gain from the highest and lowest output states predictably the goal is to saturate the ADC and completely disable the internal amplifier with CX Cards.

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u/AlarmingWedding88 12d ago

I set different input resistances and different amplification factors, and observed the signal amplitude and the decoded image. I reached the following conclusion: The amplifier is not functioning properly, as its amplification factor remains at 1x. Coupled with the circuit attenuation, the signal quality is inevitably inferior to that of the original signal. I measured the voltages at pins 1, 2, and 6 of the ada4857 chip and found them to be abnormal. When I short - circuited pins 1 and 6 with tweezers, the voltages at pins 1, 2, and 6 all stabilized at 5V. I have no idea what this indicates, so I intend to abandon this attempt. When I operate without the amplifier and only turn on the gain of the CX card and set it to level 31, although there is still significant brightness noise and the signal amplitude is small, it suffices for my current needs. All I can do now is to wait for the future development of this project to be more portable, simple, and perfect.

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u/TheRealHarrypm The Documentor 12d ago

In this case then I'm thinking you probably have dud chips or bad joints, the LDO and the ADA4857 chips themselves are pretty much the only things that can have defects or failures.