JVC 30K Bandwidth Fix

For owners of the JVC 30K D-Theater HD VHS Deck only.

I've owned my 30K for well over a year. I've been very pleased with it, and have also used it with the DVE (Digital Video Essentials) test tapes, both 1080I and 720P. Not long after getting the DVE tapes, I've noticed that the 720P tape was much sharper than the 1080I tape, though both were somewhat soft, however the machine was great with the HD tapes that I have.

Once while randomly scoping the output of the 30K thru my transcoder, I noticed a loss in amplitude on the high end of the burst pattern. At first I suspected the transcoder, because I did not expect a HF roll-off be to coming from a brand new deck. Playing and poking around in the transcoder eventually lead me to the Y input, and once there, I noticed that the roll-off was coming out of the deck itself. This puzzled me, because why would this problem exist. I checked the manual for the ability to enable/disable filters or such, but all I could found was a '3DNR' button on the front panel. But this did not effect that roll-off at all . I since searched the web, and found a discussion on this same issue somewhere on the internet, but no cure. I then put the matter on hold, to look at another day. That other day came, and I'm now able to get the deck to pass every test that relates to that roll-off problem. The amplitude is now there, and the unit looks so much better. The colors are even so much better. So much so that it's like having a different machine.

One of my D-Theater tapes is X-men. X-men has always looked good, but somewhat dull in some of the darker scenes. So it was not one of my favorites. After the mod, I've since watched it several times, just to see how much detail was hiding behind that haze. The tape is now enjoyable.

If this is you, and you feel that you can do very fine soldering, here is what you'll need to get started:

- 3 each 4 ohm metal film 1% 1/8 axial lead resistors.

- 3 each FB-2 mini feriite beads - here: http://www.palomar-engineers.com/Fer...ite_beads.html

You're going to need a fine tip soldering iron and some tweazers.

We'll get back to this thread later, I'll allow time for to get the above parts.

If you're not quite skillful with soldering, and don't have the smaller soldering tools - don't try this.

I doubt it. For the HDTV band (that also includes 720P), there's a low pass filter network used after the analog out from the DAC, before the signal to the RCA components on the rear of the unit. The 30K has far too much circuitry (IMOP) before the RCA's. The purpose of the circuitry is very similar to what's on the DAC's of PC cards, but it's much more involved here (a bit of overkill). Plus it has an IC in the line right before the RCA's. And that's not all. There several transistors (a complete circuit) right after the DAC on each of the Y,Pr,Pb lines. However, that circuit is fine.

The purpose of the low pass filters is to remove processor noise (very high switching frequencies) from the analog signal. And in doing this, it only passes freqencies below a pre-determined frequency range. This makes sense on paper (theory), but high performance video needs headroom. It needs the preset limit to be higher, therefore allowing increased headroom and not clipping the upper end of the signals bandwidth.

This is a big problem with consumer electronics, period.

The 30K has very robust filtering. So much so, it's amazing that it works well at all. The over filtering also creates a blockiness (seen by looking directly into the tubes) in the image. This is common from these circuit components (caps and coils) charging and discharging from the various changes in the signal. This is a side effect of these low pass filter networks. The low pass filtering is like a good medicine, it has many side effects. It's a shame that we can't enable or disable this feature. We should have a choice here. My doctor gave me a prescription that had so many side effects that I never took it. The side effects on the label on the prescription, were far worse than my medical issue. That was a no brainer.

Anyway, with a basic change to the circuits (there's actually two levels of change). The unit performs far beyond my best expectations for this unit.

After the change, I did not see any RF interference coming from the RCA's. And that was the REAL purpose of the low pass filtering.

Oops, forgot to link the resistors:

http://www.mouser.com/index.cfm?&han...=2708&crc=true

There's a 1/4" point to point distance for where the resistors will go. You'll also have to place the feritte on one lead of each resistor.

This will go in place of the filter network, once it's removed. The total DC resistance of the coils in the network is 4 ohms. There are two coils in series in the network, with one cap in series across one of the coils (1ohm), and the other coil has a DC resistance of 3 ohms. There's a cap to ground where the two coils connect in series. There's a total of 3 caps in each network. I tried to measure the capacitance to ground in the network. It's too high to mention, so I must be doing something wrong, or there's something that I'm not seeing.

The image is quite sharp with the changes, and I would think it should be considering what's in the networks. There was some hash on each line, and that's where the feritte comes in.

There's a second section, but we'll get to that later, and in due time, I should have my brothers camera.

I've been in a legal deposition for a good part of the day, and then Doug came to help me with the HTPC.

I've been very busy here lately, but will get back to this. I'll need to simply pick the camera up from my brothers, and then we'll get back on track. The improvements are substantial, and really look good on a well tweaked machine.

BTW, in response to emails and PM's. I'm not doing mods on these, I'm only sharing the process for those who are able to do it for themselves.

I'll see what I can do with DVE once I get the camera.

I'm not familiar with a 40K I have the 30K. But If they share the same or similar problem, the end results could be huge on your setup.

It's a simple mod. But requires careful desoldering and soldering in a somewhat tight place. Once done, you'll probably experience the best HD image from any consumer device out there.

I'm getting an improvement in sharpness, background detail and overall image punch.

Gary, I'm working real hard on that MP-5. It's number one on my agenda!