CurtPalme.com Home Theater Forum

[9kids]

Well I am no EE nor am I a tech with years of experience, but I have been doing a lot of reading since I got my Marquee. I have spent a lot of time reading old maintenance and mod threads here and over at the other site. When I read some old post about AC coupling vs. DC coupling, they made prefect since about the dangers of DC coupling with external sources. Then for some reason a little light bulb went off in my head. It dawned on me that if you were using a Moome VIM-HD internal card, that the output of the card would be DC referenced to the rest of the projector, since it is using the DC supply from the projector. From what I have read you can DC couple from a DAC to the opamps as long as they are in the same device and using the same DC and ground reference. I could be totally wrong but I thought I would throw out the idea and see what people say.

You could just take out the capacitor and resister and jumper it all but then you could never use the RGBHV input 1 again, but if you never plan on using any thing but the VIM-HD input 2, it would be quite easy. Then I thought about using a DPDT relay to switch the capacitor and resister in and out of the signal path based on input 1 vs. input 2, you could use the I-RELAY signal to trigger it. I am really in way over my head with this stuff, but it just seemed pretty simple (on paper). Finding room for the relay and the right relay would be the hardest part. Here is a quick schematic I did to show what I was thinking about (based on green video chain).

Feel free to tell me, I have no idea what I am talking about and all the reasons this is a bad idea.

[Nashou66]

I understand where your comming from since the Moome card isn't an analog input although it does output analog to the vim, so your saying that any stray unwanted dc signal from the source would be left in the moome card with out harm of it getting into the signal. or is it the other way around no ac in the signal? I havnt read those posts in few weeks. it should work. And i remember someone actually took out that 470nf cap and said the image was way better. maybe it was Bill Blue?

TSE would know best about this idea. Scott any thouhgts?

Athansios

[mp20748]

[mp20748]

[Nashou66]

[quote="mp20748"]

[Nashou66]

I only use the moome card so i might try taking out the .47uh cap i have in there and see what happens.

Athanasios

[9kids]

[9kids]

[Nashou66]

I think D34 wouldnt matter? right? on the 2005-03p vim its d2 and its before the 470nf cap in the schematics.



Athansios

[Nashou66]

Looking at the schematics is it possible to use the relay already on the board. we could use the unused part of relay RL2 that the decoder board uses. route the input 2 signal that comes from JB2 into Relay RL1 into pin 6 which then goes out from pin 9 to RL2 pin 6 but instead we reroute it to pin 17 on RL2 . Then from RL2 we would route the BNC signal which would come out of pin9 to the de coupling cap C33 the normal way and then by pas that cap for the signal comming out of pin 14 directly to the op amp.?
Does this make sense?

this is all with the red channel:

So JB2>pin 6 of RL1 >pin 9 of RL1>pin 17 of RL2> pin 14 of RL2 directly to op amp.
or some variation of that mabe have the input 2 signal totaly by pass the first relay and go directly to pin 17 on RL2?

Athanasios

[1031]

Very nice idea If that direct coupling works, then it can be possible remove also dc-restoration. I must look also those schematics closer. I think if that is possible to remove dc-restoration, it will reduce noisefloor because that op-amp what is used for dc-restoration is powered from +-15volt supply and if i remember correct those voltages was quite noisy. We must first scope outputs from moome card to see is blacklevel at 0v if it is then its possible to remove dc-restoration.
Damm you 9kids.. you gived too much to think. I got allready too many modprojects going.

[PaulB]

Praise indeed 9Kids, praise indeed!

Gratuitous snapshot more for my benefit than for anyone else on this thread

[Nashou66]

[1031]

Athanasios I havent got time look that thing right now. But maybe later.
But first thing is that someone must look is moomes card output at 0v(black level), if it is then direct coupling is possible.

[9kids]

This would be the simple never use Input 1 solution. Thanks for the Pic PAULB

[Nashou66]

Yes if you never want to use input 1 ever agian. But if you still wanted the functioality of input 1 for hooking up test pattern generators and just having it incase for some reason the moome card faults you would have input 1 to test if the vim is good or if its the moome. I gues we could always just add jumper pins in strategic locations to enable input 1 again. Also couldnt we just go from the start of the signal before R125 to the op amp?

Athanasios

[9kids]

Athanasios

R125 & R137 form the 75ohm termination, so they need to stay in. I thought about using RL2 but it is physically a long ways away and the traces are all under it. You would have to remove it to get to the traces. But here is how it could be used.

[Nashou66]

But that way still allows input 1 to by pass the 470nf cap. Ohh ...does realy RL1 control RL2 so when RL1 switches it would also switch RL2? it just seems alot more reworking the path than my idea


yours


Mine







But not sure if it will work either...the easiest is to just eliminate input 1 and Jarmo's idea also could work as well. wonder if moomes black level is 0, i think i read somewhere it is.

Athanasios

[9kids]

OK here is a pick of the board with how a new relay would work

[Nashou66]