Help Needed Troubleshooting ISA SVGA Cards

[SkydivinGirl]

Hello everyone!

I've been working on an 8088 project and I'm having trouble with the ISA SVGA Card developed by Sergey. As with most kits, I bought more than one. In fact, I purchased 3 PCBs (in case I goofed up on the SMD Trident chip) and, initially, parts for two of the boards. To make a long story short, I was able to assemble all three boards, including the Trident, without issue but I cannot get any of them to work.

I've emailed Sergey once, but haven't heard back from him. His web site says that he doesn't have time to help people troubleshoot their projects so that's probably a dead end.

So, I'm hoping one or more of the electronics gurus on AmiBay can either give me tips on troubleshooting the boards or perhaps be willing to receive one of them to troubleshoot. Two of the boards won't let the computer POST and the monitor does not detect a signal. The third board allows the computer to POST (I can hear it beep) and the monitor detects a signal but the screen is black.

I've tried writing the BIOS files on multiple EPROMs so I'm sure it's not an issue there. I know the soldering is good and there are no shorts. No magic smoke has escaped from any of the components. They should all work, but they don't.

I'm a bit frustrated because I've only ever had one kit that I assembled not work. I've even done some cards with 0603 resistor networks that are much smaller pitch than those on the Trident chips and those cards work perfectly.

Any help would be appreciated!

Heather

 

[SkydivinGirl]

Sergey wrote back!

The two SVGA boards I built (one with current PCB, one with previous/beta version PCB) worked the first time I powered them. I also have several other people build these boards, and I most of them there successful.

I should note that for unknown reason this card does not work on the original IBM PC, but works on IBM XT and various XT clones. It doesn't seem to be CPU related, as it is working with either 8088 or NEC V20. It might be that the chip is sensitive to a minor timing differences between XT and PC.

As far as debugging: First make sure you have no issues related to soldering the TVGA chip. As you noticed it has a fine pin pitch, and any solder bridges might lead to a failure. I normally use solder wick to extract extra solder, and then wash out the flux with alcohol. Next I check resistance (or short circuits) between adjacent pins of TVGA chip.
If you have an oscilloscope or a logic analyzer you can try to see if system indeed tries to fetch data from VGA BIOS EPROM. It should be reading and check-summing the EPROM at boot. You can also check timing / control signals on DRAM chips, as well as signals on VGA outputs. You might also see the TVGA9000i databook for more detailed information about its signals, perhaps there is something in there that could help with debugging.

So, I'll triple check the Trident chips with the meter to look for solder bridges, but a visual inspection using a magnifying loupe didn't show any. Can anyone point me to information on how to check the timing/control signals for the DRAM or how to check if the EPROM is being read? I have an oscilloscope, but I haven't learned enough yet to figure it out.

Thanks!

Heather

 

[keropi]

So sad to hear about these troubles Heather... I am pretty sure that it's no soldering issue (since you also double-checked it) but a Trident chip one... I bet they were just DOA.
I don't have the knowledge to help with oscilloscopes , it's no easy matter to check all that Sergey mentions :\
Is there another source where you can find Trident chips? It would be a good test IMHO to replace one and see what happens

 

[SkydivinGirl]

Thanks for the information Leo! I think I've figured out how to test the BIOS so I'll do that today. I think you may be right about the Trident chips so I'll try to find another source. In the meantime, I'll figure out how to test the timing/control settings on the DRAM.

I'd still be happy for any assistance anyone can provide.

Thanks,

Heather

 

[sputnik]

Have you asked anyone @ Vintage Computer Forums?

 

[SkydivinGirl]

Not yet. I'll definitely ask there as well. Thanks for the suggestion!

Heather

 

[SkydivinGirl]

Good news! I've got two of the cards working 100% now.

It's a bit embarrassing because the resolution was simple. I believe that there were some electrical bridges through flux that was underneath the legs of the Trident chip. While troubleshooting one of the cards, it started working after it was on for a while. I figured it had to be either a bad component or a maybe a bridge that stopped conducting once it warmed up.

Since I did a visual inspection multiple times on all of the cards, I decided to use some spray flux remover and a toothbrush to really get in between the legs of the trident chip. After cleaning up all the cards, two of them worked perfectly and one of them works, but has an issue.

The one that is partially working will allow the computer to POST and I can see something on the screen. Unfortunately, the text displayed on the screen is barely visible and certainly not legible. Any ideas on how to troubleshoot this issue?



Thanks everyone! I'll be posting pictures of my entire XT project later so you'll see the output of these cards. It's a dramatic difference from the quality I was getting out of my CGA converter.

Heather

 

[keropi]

Awesome news then!!! glad that 2 are finally working! maybe the 3rd has faulty ram chips?