VOGONS


First post, by Tiido

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Allo !

Some of you know that the VGA passthrough of most Voodoo cards is not quite perfect and one has definitely noticed that through the passthrough, higher resolutions are noticably softer or even outright blurry and image is a little bit dimmer too compared to direct connection to the host video card.

I hooked up an oscilloscope not so long ago to the input and output and I could see that about 0.1V was lost on the already low 0.7V video signal, which explains the dimming and loss of resolution and the cause is the CMOS switch used for video switching. It has fairly high On-resistance and while the cards connect two sections per signal in parallel the result is still far too high... but what if there was 2 more ...? And that is what lead me to the fix ~

The switch chip, PI5C3384QE(X) is still made and can easily be bought from Mouser and other places for very low cost and you'll need one piggybacked to the existing one on your card to provide good enough result. One could go overkill and add 3rd more but there's nothing really to be gained from there unless you try to pass 2048 x 1538 (or something higher 🤣) at high refresh rate through the Voodoo... 2 is enough for most uses cases 🤣

All of the photos of the process are found here : http://www.tmeeco.eu/BitShit/PCschit/VoodooFix/
I can only attach limited amount of photos to a post so only some of the important ones are shown here :

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Old chip vs new chip
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The general process consists of straightening the pins of the new chip, I have used a screwdriver against something hard to push the pins straight. You need all of the straightness possible because the pins barely reach the original chip even when maximally straightened.

You also want to tin up the pins of the new and old chip or you'll have hard time getting the two connected. Use as much flux as necessary and don't worry about solder bridges, they will contain enough solder to fill in all the cracks and make good connections.

DO NOT TRY THIS IF YOU HAVE NO FLUX AT HAND ! Stuff in your solder wire does not count, you will need a can or tube of some rosin paste or a syringe of no-clean flux or a flux pen. You're only gonna make your life very very hard without and most probably ruin a card in the process. Get some flux before proceeding or let someone else do it for you.

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Tinned up pins, with a blob
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Once all the pins are tinned you'll line up the new chip on top of old and run the iron over the pins (when you have fluxed them up more). The solder in those bridges will drag with the iron and fill in all the gaps between the chip pins, making good connections. You can wick away excess solder with desoldering braid or simply get it transferred to the iron's tip and remove it that way. I generally do it without the braid as it can suck away too much solder and you must apply some more to get connectivity between the chips...

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All connected up
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There's also a bonus feature possible, many cards have inductors and capacitors on the RGB lines to reduce EMI but they are tuned to low resolutions and will muck up the image slightly. Getting rid of the capacitors and bypassing the inductors will let you have cleanest possible signal :

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No more output filter !
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Here's the mod done on a Voodoo2 :

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Voodoo2 mod
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Happy enhancing ~

Last edited by Tiido on 2020-11-30, 14:07. Edited 1 time in total.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
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Reply 1 of 19, by keropi

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Thanks for the guide Tiido! Finally a solution for the voodoo blur plague!
I will mod my cards soon-ish 😀

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Reply 4 of 19, by chrismeyer6

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Excellent job figuring out this solution to the pass through imagine quality reduction. After you did the mod is the image quality on par with a direct connection to the monitor?

Reply 5 of 19, by Tiido

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Yeah, it pretty much is. There's no more brightness drop and all the resolutions I tried looked proper again. LIke mentioned before it would be nice to see some comparison with a capture card but I lack such fancy toys.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 6 of 19, by Deksor

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Does that also fix the "bleeding" effect (I'm not sure it's the right term but what I experience is the mouse cursor always bleeding to the right, same for windows in windows 98, etc) ?

It's really easy to spot on a CRT

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Reply 7 of 19, by Tiido

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Yeah, it does. It lowers the impedance from something like 100ohm back to very close to 75ohm which should fix all the ringing and reflections related problems.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 9 of 19, by Cyberdyne

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Well if you have an old Orchid Righteous 3D then it will make a relay click, but the picture is not hindered with a semiconductor voltage drop.

I am aroused about any X86 motherboard that has full functional ISA slot. I think i have problem. Not really into that original (Turbo) XT,286,386 and CGA/EGA stuff. So just a DOS nut.

Reply 10 of 19, by Tiido

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I imagine that particular card is not exactly common and probably on the expensive side too, but yes, a relay can bypass this problem and shouldn't introduce new ones if it is a sealed type.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 11 of 19, by kaputnik

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Finally got around to attempt this fix on my Voodoo 1 card 😀

Ordered 10 bus switches, both to keep as spares for future fixes, and as a safeguard in case of the screwups I was certain I'd do. Those things are small and my eyesight isn't getting better with age...

Began with replacing the original bus switch. The package was slightly higher than the new ones, and as implied in the OP, every fraction of a millimeter you can get those pins closer to each other counts. Somehow it also feels better to use two chips of exactly the same make/model, even though they're equal at all relevant specs according to the datasheets.

Used the "wrong" end of a pair of pincers to straighten the pins on the top chip. A picture says more than a thousand words:

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Just clamp the pins and push down on the chip. When the chip meets the edge of the pincers, axial force will be applied to the pins aswell, tightening the upper bend, further extending the pins downwards. No need for after adjustments with this method either, the chip came out with the pins still perfectly parallel 😀

Tinned the pins generously as recommended and soldered the chip on. Removed the excess solder by "combing" it to the top of the upper chip's pins with my pointiest conical tip, and wicking it off.

Checked the joints for continuity, and also for unintentional bridges/shorts. My homemade mechanical pencil/sewing needle precision probes were really helpful:

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Cleaned flux residues off, plugged the card in, and fired it up. Success!

The resulting image might not be perfect yet, but there's a major improvement. Very well worth the effort 😀 Considering trying to piggyback a third bus switch at some point, right now I need to rest my eyes though 😁

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Last edited by kaputnik on 2024-02-06, 11:17. Edited 1 time in total.

Reply 12 of 19, by Tiido

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Excellent ~

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 13 of 19, by kaputnik

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Tiido wrote on 2024-02-06, 08:19:

Excellent ~

Yeah, I'm very happy with the results, thanks for the inspiration 😀

Might try bypassing the output filters tonight and see if that improves the picture further. Sometimes I wonder if they're there only to meet EMI regulations rather than improving the output quality, wouldn't be the first time filters actually have a detrimental effect to the output.

Need to do some measurements first though and figure out how the filters are designed, it's impossible to determine what's what visually on my card. At a glance, it looks more like band pass filters than the low pass filters on your card.

Reply 14 of 19, by Tiido

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Yours probably have a peaking low-pass filters, that's what the CLC things do normally. They will boost things at a specific resolution but are detrimental to everything else (except EMC stuff). In any case bypassing these will improve higher resolutions a lot, I get rid of them on every card I have and there's always improvement at 1600x1200 and beyond, or at high refresh rates.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 15 of 19, by kaputnik

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Tiido wrote on 2024-02-06, 12:28:

Yours probably have a peaking low-pass filters, that's what the CLC things do normally. They will boost things at a specific resolution but are detrimental to everything else (except EMC stuff). In any case bypassing these will improve higher resolutions a lot, I get rid of them on every card I have and there's always improvement at 1600x1200 and beyond, or at high refresh rates.

Yeah, that's probably what it is then, counted two caps per inductor close to the VGA port. Will get rid of them later tonight 😀

Reply 16 of 19, by AndrettiGTO

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kolderman wrote on 2020-11-30, 12:20:

Has anyone tried to retrofit a mechanical switch? I think one brand used them.

Prior to reading this thread, I never realized the amount of signal degradation the pass through introduced. I checked my Win98 system today and yes, far dimmer and not as sharp with pass through.

Reluctant to modify my cards so I also wonder if a two port VGA switch would provide a clean signal or introduce some other issues.

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It's all fun and games 'till someone loses an eyeball

Reply 17 of 19, by Tiido

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External switch will help, then only the voodoo's own image will be a bit softer and dimmer than ideal.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 18 of 19, by kaputnik

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Removing/bypassing the output filters improved the image a bit further. It's generally sharper now, even in 640x480 and 800x600 resolutions, one of which the filters plausibly were tuned for.

There's still some diagonal moving lines left, especially in GLide mode, but that seems to be a thing with Diamond voodoo cards. Probably a design problem demanding further investigation. The bus switch mod helped a lot with that though, it's not as pronounced anymore. Might be worth a try adding a third bus switch.

Thinking of butchering a VGA cable and experimenting with using the coax wire from it for signal transfer at least from the bus switch to the VGA output port. Should be doable.

Reply 19 of 19, by y2k se

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AndrettiGTO wrote on 2024-02-07, 00:15:

Reluctant to modify my cards so I also wonder if a two port VGA switch would provide a clean signal or introduce some other issues.

My solution was to find a GPU that has a working DVI output and then use a monitor with DVI and VGA inputs.

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