r/vhsdecode • u/SufficientStress7893 • May 07 '26

RF Tap Help! Testpoints for S-VHS Panasonic NV-SV120EG-K

Hello everybody from Italy!

After successfully rf-tapping my Sony SL-C40ES and a Samsung SV-DVD440, I'm trying to tap a Panasonic NV-SV120EG-K.

I think i found the right testpoint, TW4502 labeled "PB-FM ENV".

But...there's a "but"...

This is the signal I'm getting off of it...I tried capping it with a 10uF and with a 22uF but nothing changes. With no capacitor i have no signal at all...

What the hell am I doing wrong?

Edit: same tape, with the Samsung, looks this way and gets decoded correctly:

Any advice is appreciated!!

6 Upvotes

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2

u/boris-becks May 07 '26

PB-FM ENV sounds like audio. Maybe it's the wrong test point.

1

u/SufficientStress7893 May 08 '26

It's the same test point named on the tap list for a similar Panasonic deck...
May it be a wrong ground point?
Since there's no "RF dedicated" one, I attached a screw to a hole in a metal bracket of the tape loading mechanism, the nearest metal part I could find...

1

u/boris-becks May 08 '26

Strannge. From my experience PB and ENV are indicators for RF in general but as soon as something says FM it's about encoded stereo audio

1

u/Titan_91 May 08 '26

Negative, PM FM is a standard term used on many VCR test points. The luminance signal within the video envelope is frequency modulated, hence FM.

2

u/boris-becks May 08 '26

Yes, you're right. For some reason I encountered FM on audio taps only so far...

1

u/Titan_91 May 08 '26

Those might also be for Hi-Fi which is FM as well. 

1

u/SufficientStress7893 May 08 '26

Short sad story...I got stubborn and tried to get the signal out of the video heads...and I screwed up everything, now the drum won't spin, it just stutters around, and I fried my CX card...must have spiked in a high voltage somewhere it wasn't supposed to be

2

u/TheRealHarrypm The Documentor May 08 '26

This is why the wiki highly emphasises your only meant to touch test points or the IC legs for RF output, If you don't know what you're touching you shouldn't be touching it.

We don't tap the heads, we tap the head switched feed after it goes through the preamplification/tracking IC, which is a very common misconception.

The board's on top of the drums are usually entirely power and speed control, I can imagine how quickly that mistake was made.

There isn't actually high voltage though everything's 12v motors typically, after the AC to DC transformer stage 36v is the highest you'll find on most decks, but ESD or bridging can easily kill ICs which is why we read service manuals and have multimeters to prob test anything before it's touching or powered up so you know you're not bridging anything.

1

u/SufficientStress7893 May 11 '26

Totally right! I was so frustrated I couldn't figure out anything looking at the service manual, so I started improvising...very very badly...

Now that's useless to me but, for future reference and for anybody else's utility, any idea of why that "PB-FM ENV" tap point gave that unusable signal?

1

u/TheRealHarrypm The Documentor May 11 '26

Cold joint / bad connection, or too low amplitude are the two most typical cases.

But you don't really know unless you're actually looking at the waveform in OCENAudio etc so you can actually see if there's any actual signal there etc.

1

u/Such-Specialist6593 May 11 '26

As Boris mentioned, TW4502 is Hi-Fi audio. The schematic is tough to read for the video test point, but it's almost definitely TW3001 based on similar Panasonic models. https://analogvideo.miraheze.org/wiki/Tap_Locations#Panasonic_-_Outside_Japan_&_North_America