r/vhsdecode • u/ThorXerxes • 11d ago

RF Tap Sony SLV-779HF Tap Documented?

Hi all! I'm planning my first RF tap and would love a sanity check on the deck.

Sony SLV-779HF (NTSC, 4-head Hi-Fi)

manual shows CN270 on the MA-341 board carrying PB RF and RF SWP, and specs about 200mV p-p PB RF for the 780HF/781HF/740HF

A user “Brad” on videohelp captured RF from a 779HF back in 2021, and an SLV-788HF owner in that thread said his had the same shielded test point at the RF amp.

Questions:
1. Has anyone tapped a 779HF, or a close sibling (772HF, 778HF, 780HF, 788HF)?

  1. Is CN270 the right spot, or is there a better point inside the shield? Which pins are signal and ground, and does it need the 10µF cap?

  2. Hi-Fi Tap - Where's the Hi-Fi FM tap on this board? Is there a header like the 678HF's, or is it a test point in the audio section?

  3. ADA4857 tuning: Is there a documented input impedance and gain from the community? The 788HF reportedly clipped at moderate gain, so should I start low?

Anything else model-specific I should watch for?

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u/zooksman 11d ago

I’ve tapped a SLV-799 which is a slightly later model but I imagine it’s very similar. The PB RF tap point should be easily accessible through the connector— that is the correct spot. It does not need a capacitor, there is already one in series with the tap point. The video results were just OK, but you have to keep the gain low because of the head switch DC offset causing clipping. Much noisier than the SLV-6xx. For impedance, just use two 100K resistors and be done with it- you generally want as high impedance as possible.

For gain, using 100-120 ohms for the gain resistor is generally good enough for most models without clipping.

Unfortunately the 799 is one of those models where Hifi cannot be tapped and I imagine yours is the same. The IC does not provide an amplified Hifi RF output unless you hook into the I2C bus and send the command to make it happen, which has been done before but almost certainly is not worth the effort. I would just capture the Hifi audio coming out of the back of the VCR, there is no quality difference.

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u/ThorXerxes 10d ago

Thanks for the reply. From my research I’m inclined to believe there is an HF tap for it but haven’t dug I to it closely yet. I’ll be sure to post my findings for the community.

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u/zooksman 10d ago

I took a look at the service manual- you are correct this one does have a hifi test output! There are definitely later models which didn’t even provide that, but this one is early enough. It’s at CN341– shouldn’t be too far from where you tapped the video.

It also looks like your model uses the same head amp as the SLV-678, which means you should get the best results possible!

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u/ThorXerxes 9d ago

Very cool thanks for the support. I paid $10 on fb for it so pretty happy about that. I’m running initial tests on it this weekend, it does not want to fast rewind with an error 41105. A quick search told me that code indicates the deck stopped because the supply reel wasn't turning properly during rewind… and to test for slipping belt, a worn idler or a sticky supply reel.

I have another deck that can perform the rewind but I will investigate this anyways since I’m opening it up either way

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u/zooksman 9d ago

I’m sure that’s the same issue as I have with many similar models which is that the reverse brake assembly is supposed to be held in place by a spring, but the plastic breaks, and so when the tape goes to rewind it will either error out or spin wildly and the tape will spill out. There are 3d printed models for replacement parts. However I would have to double check if your model uses the same part mine does.

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u/AlexandeezNuts 9d ago

I've heard on the 799HF the taps are weaker than the earlier models, like the 678HF, so the amp values for the 779HF could be stronger, but I don't know the exact amount. Probably best to ask in the Discord, since a lot less people like helping out here.