CurtPalme.com Home Theater Forum

[mcpherv]

Hey, I ended up using one of the Vishay DG611/612/613 series to replace the sd401. Took some jerry-rigging into place, but it worked flawlessly. Not too sure how much difference it really made to be honest, but it DOES work - just need to replace it with the appropriate variant (can't remember if a normally open or normally closed configuration is required of the switch). High voltage video pathway section of the neckboards are really the limiting area in terms of bandwidth... and its hard to do much about that. The rest of the path can be improved though, and does make a nice improvement.

Glad to see people are still pursuing this.

[Nashou66]

I have three of those chips in house, never got around to doing the mod.

Athanasios

[1031]

I have also used dg612 in place of sd5401 for some time now.. I never got it working 100% perfect, so i´m about to test Mikes "mini board" that replaces video section of that sd5401..
But someone who likes to test dg612.. Here is how i did that mod
Most problems what i had with that dg612 board was related to onscreens.

[1031]

I found some more pictures about that mod.
Here is risetime compair between sd5401 and dg612.
At some point when i tested that mod, i was able to get really good sharpness out of it (picture is from my testbench ,M8000 with hd-8´s) But when i tested that vim with my main projector, picture was useless, sharp yes, but too noisy

[dvh99]

i have some different inductor values now to play with and some varicaps so i can undo the limiting factor on the neckboards.
i will report later in another thread.
hope to get this done this week, will try the different inductors on green first.

[mcpherv]

[mcpherv]

[Nashou66]

I probably would have done that mod as you say McPherve. Its possible the board itself is picking up noise , but since you have not scoped it its hard to say if your had the noise as well.

The peaking circuit on the vim that Scott at VDC is something you should see. it brings the pre amp up to about 240 mhz pixel clock.

Athanasios

[mcpherv]

Your point is well taken, and I agree regarding the no scope issue.

Hey - got a link for the peaking circuit? I found the following thread which shows the schematic for some new gen neck boards, which has a peaking circuit in the feedback loop for the video signal opamp:
http://www.avsforum.com/avs-vb/showthread.php?t=1127826

Peaking is important, I think, for exactly what TSE mentioned in that thread - to overdrive the transistors in the latter stages of the video path at high frequencies (ie. the third order harmonic of the video signal which makes up the sqaured edge of the square waveform). The signal attenuation should reduce a lot of the ringing of the waveform in the earlier stages down, but you end up with a closer-to-square wave at the tube itself, which is obviously desireable. Obviously, the tradeoff is that this is variable at different frequencies, and could add artifact at lower frequencies. Note, this attentuation would also be happening to high frequency noise introduced into the video path at the preamp stage... Just sayin (re: tradeoff/benefit of the switch replacement - faster rise time, some noise on scope).

Also, a couple years after I stopped doing all of this, I found this new analog multiplier:
http://www.analog.com/static/imported-files/data_sheets/ADL5391.pdf

Would be a PAIN to install into the circuit due to its form factor (although an adapter board would work - seems like some around here are into custom pcbs), which is a 3mm x 3mm size, but if you could get it to work, one of the main bandwidth limitations on the preamp section of the video path would be eliminated. Also as is mentioned in that thread, OPA695s rock. And multiple parallel caps for decoupling are good too...

[Nashou66]

McPherve, here is the thread where we talk about the Peaking circuit. Scot posts some Spice models of the circuit.

Your right about the resolutions being specific. This circuit is now good for anyone who wants to use say 1080p and 720p. You have to tune it in for the res your going to mostly use.

I have a blend set up so i have all inputs scaled to the same res.

https://www.curtpalme.com/forum_archived/viewtopic.php@t=15194.html

Athanasios

[mcpherv]

Impressive result there! Dramatic difference pre/post peaking circuit. Still could definitely be done with another amp with a higher slew rate, resulting in a shorter rise time, but that is a pretty good result!

[mcpherv]

[Revox]

That is very interesting! Seems for me like for one resolution the mod can be a bigger improvement than a few other mods.

I learn for my control engineering exam in the moment, and that is realy a good transfair of this knowledge!

The problem of this big thread is, that you can only can guess what to mod if you read carfully throught the whole post.

So is it possible for you collect a few info?
To snapshot the part in the schematics of this VIM (might be possible for all VIMs or only one special?).
It would be very nice to, if you can add the mod parts via mspaint (but this is only if you have the time!).

Greetings from Berlin, Stefan

[dvh99]

mcpherv, scott explained a bit about this in another thread.

https://www.curtpalme.com/forum_archived/viewtopic.php@t=24037.html

i haven`t played with it sorry .

the muratas have high q at 100mhz so i expect much of them.

[mcpherv]

Here's the patent for the mirror cascode amplifier used by the neckboards as the high voltage amp stage - describes the circuit in detail.

[dvh99]

hey thanks for that

[Nashou66]

Here is the idea for adding a peaking circuit to the VNB's along with his notes on changing the Inductor values like McPherve just mentioned.

[mcpherv]

So another member of the original mods thread played around with this with a scope a number of years ago - the rise time adjustment can cause overshoot of the signal, which is what I meant by "ringing". At the time when I was talking with them about this, we were discussing the merits of exaggerating this overshoot to improve the shape of the waveform as it goes through the latter stages of the amp.

Oh, also - there may have been another document on the mirror cascode, but I can't seem to find it right now.

[dvh99]

that member is the designer of the neckboards, the godfather of the marquee on this forum.

scott i hope its ok with you i called you the godfather, if not please dont put concrete shoes on me and dump me in the ocean.

[Nashou66]