X68000 VGA adapter frying stuff and blowing fuses

[cr4zymanz0r]

I've got an issue that has me completely stumped. I bought a X68000 Expert that has had the PSU completely replaced and everything is suppose to work fine. He even said he tested several games and played a while again before shipping it out to me. He has an actual X68000 monitor he uses, but he did build a VGA adapter to use it with newer monitors, but he just said when he tried it it just said no signal or something. I assumed the LCD monitor he tried it on just didn't support the resolution the X68000 was outputting. Anyway, he sent that converter along with it. When I first got it I just plugged in power only and loaded the only game I have for it (Akumajo Dracula) to make sure I heard the loading music. It did so I shut it back down all the way and hooked up the video converter.

When I flipped the power on the X68000 I heard a loud zap and immediately turned it off......it fried my 26" LCD monitor  . I even opened it up an a trace was lifted and and some tiny surface mount components were visibly burnt. Also, I'm assuming when this happened that it also blew a transistor on the X68000 video board because there was a blown one when i opened it. Here's pics of it http://imgur.com/a/J70Id. After this I immediately assumed he had screwed up and routed the 5V in the VGA adapter. However, I checked the pinouts myself and 5V was not connected. The only things "wrong" i found was the ground on pin 12 of the X68000 side was not wired up and blue and green on the VGA side were switched. Everything looked fine.

Next I got a working CRT monitor from work that was going to get tossed so if another monitor got fried it wouldn't be anything I valued. I replaced the transistor on the video board, triple checked the VGA adapter to make sure it wasn't wired wrong by getting the pinout wrong from confusing the gender layout or anything. I still saw absolutely nothing wrong except one missing ground (which shouldn't matter) and the swapped blue and green on the VGA side. I decided to be brave and test again seeing if last time was some horrible fluke. The good news is nothing died......the bad news it blew the fuse in the X68000 PSU, so apparently something was 'suppose' to fry but that just saved it. Being completely baffled and even talking to an online friend with a X68000 that had successfully made his own adapter I decided to completely rewire the adapter myself and do it cleaner and completely by the book with all grounds connected and all RGB connections matching correctly. I followed this guide EXACTLY http://translate.google.com/translate?hl=en&sl=es&tl=en&u=http%3A%2F%2Fwww.zonadepruebas.com%2Fmodules%2Fsmartsection%2Fitem.php%3Fitemid%3D1232. That was how I was going to do it anyway, but it helps show how I wired mine up if anyone has any questions.

So, after rewiring the adapter myself I decided to test again. I turned on my step down converter that the X68000 is plugged into, then I flipped the power switch on the back of the X68000 (with no video adapter plugged in). That was fine like it should be, so I turned everything off again then plugged the VGA adapter into the X68000 but didn't attach it to the monitor cable yet. I did the same thing.....I turned everything on, everything was good. I shut everything completely down again then plugged in the monitor cable to the other end of the VGA cord. I turned on the step down converter and that was good. Then with the monitor still off (but plugged into power) I flipped the back power switch on the X68000......and the fuse in my step down converter blew. I'm clueless.

I know everyone is probably going to tell me to make sure 5V isn't wired up but I've already checked that dozens of times, which makes me even more confused how this is blowing fuses and such and only when the VGA adapter is fully hooked up to a monitor. I wired mine EXACTLY like the guide above, and before I rewired the thing myself it was the same as the guide other than 1 missing ground pin and the swapped blue and green I mentioned before. It's just straight up wires, no components or anything. I even used some heat shrink tubing to make sure pins weren't touching each other.

[lydux]

Wow !

First of all, stop trying to power on your x68k now or you will just kill it forever !
Videos signals are TTL level and so, should transport 5v max, and such voltages should not explode components like this or even blown PSU fuses ! You probably got a short with higher voltage somewhere else.
If I remember right, this transistor is related to the invertion of horizontal sync. It's probable that the overload came from the ground of your x68k, enter your lcd screen and revert back to the hsync signal.

You should have 2 boards inside your lcd :  the inverter and logic board. On which one dead components are located ?
Also, you said the PSU were changed ? Remove it now from your x68k, and test volages outputs.

That's all I can say for now.

[cr4zymanz0r]

Thanks for your reply. Just to note, it powers on "fine" as long as the VGA adapter isn't hooked up.

As for my LCD monitor, the dead components are on the logic board and not the power board. It's not even very far at all from the VGA plug.

When I get home from work I'm going to test the resistance and voltage on the X68000's video port to see if maybe some pin has a far higher voltage than it should. As far as testing the actual PSU, I'd be a little scared to do that but I might be able to if someone held my hand, lol. I'll check the video port first though when I get off work in a few hours and I'll report in on those findings.

[133MHz]

Where are you from? What's the mains voltage in your country? Are the plugs polarized? It could be a grounding issue, as in 100-120V components seeing an effective 220V potential even behind a step-down autotransformer because of the earth ground system, or something more mundane like reversed Ground & Neutral at the wall outlet.

Try to measure AC & DC voltage from the outer metal shield of your X68000 and the metal shield of the VGA plug on the monitor with both plugged in but not connected together, either with a multimeter or a neon test lamp, sounds like there's a significant voltage potential between the two. Worst case scenario you might need an electrician to inspect your house wiring, something's clearly amiss if electronic components blow up like this.

[cr4zymanz0r]

You lost me with some of that, but I'm in the US which is 120V on the wall outlets I believe. However, I am running it on a stepdown converter to bring it down to 100V. I live in an apartment and don't have issues with any other electrical stuff plus I assume the building would have to be up to code or whatever.

Try to measure AC & DC voltage from the outer metal shield of your X68000 and the metal shield of the VGA plug on the monitor with both plugged in but not connected together, either with a multimeter or a neon test lamp, sounds like there's a significant voltage potential between the two.

How exactly do I do that with a multimeter? Just put one lead on the metal shield then the other on a known voltage pin? If that's what you mean then I can do that for DC voltage, but I've never tried to measure AC voltage before.

[lydux]

Where are you from? What's the mains voltage in your country? Are the plugs polarized? It could be a grounding issue, as in 100-120V components seeing an effective 220V potential even behind a step-down autotransformer because of the earth ground system, or something more mundane like reversed Ground & Neutral at the wall outlet.

Try to measure AC & DC voltage from the outer metal shield of your X68000 and the metal shield of the VGA plug on the monitor with both plugged in but not connected together, either with a multimeter or a neon test lamp, sounds like there's a significant voltage potential between the two. Worst case scenario you might need an electrician to inspect your house wiring, something's clearly amiss if electronic components blow up like this.

.

Yeah why not !? Could be the cause.

Don't connect the x68k and monitor together.
Set your multimeter in AC mode, with something around 20~100v. Then measure voltage by putting one lead on your vga connector shield, and the other lead to the shield of the x68k video plug. Do as well in DC mode. Both measurement should indicate 0v.

[133MHz]

Oh, OK. It's just that when I read 'stepdown converter' I immediately assumed Europe (220-240V), so forget the first thing I said. You might still have a mis-wired outlet or something along those lines, though.

How exactly do I do that with a multimeter?  [...]  If that's what you mean then I can do that for DC voltage, but I've never tried to measure AC voltage before.

Like this:



Shielding to shielding, with both equipment plugged into the wall (not necessarily powered on, but at least on standby just to be sure). AC or DC depends on the function the multimeter is set to, so do both just in case. Any voltage over ~55 volts which produces more than just a slight tingle if you happen to touch both sources is a safety hazard to both you and your equipment and must be rectified immediately.

[NFG]

I have had a similar problem, because I used a 2-prong power cord (the X68000 original) connected to the ATX power supply inside (which required a third ground wire).  What happens is, because of the way they're built, the ATX PSU has nowhere to drain the excess power that it normally and always has running through its own shell.  Normally this isn't a problem, but it does create extra voltage, and whenever I plug a video cable into my X68000, I get sparks.

I had a similar problem with a regular PC 'cause, in the Japanese house where I was using it, there were no ground pins in the wall sockets, and an entire metal shelf became electrified and shocked me every time I touched it and something that was grounded at the same time.

[133MHz]

I have had a similar problem, because I used a 2-prong power cord (the X68000 original) connected to the ATX power supply inside (which required a third ground wire).  What happens is, because of the way they're built, the ATX PSU has nowhere to drain the excess power that it normally and always has running through its own shell.  Normally this isn't a problem, but it does create extra voltage, and whenever I plug a video cable into my X68000, I get sparks.

Yes that is normal, to a certain extent. The way switching power supplies are made, line noise is derived to ground through a pair of capacitors placed between Line-Ground and Neutral-Ground. If there's no earth ground connection the capacitors will form a sort of voltage divider which will electrify the case to about half-mains potential (about 55V for 110V countries and 110V for 220V countries). The current leakage is very small so it shouldn't give you any more than a small tingle if you happen to touch it or very small sparks when connecting equipment together, and once you add a proper earth ground into the mix, it immediately goes away.

What cr4zymanz0r is experiencing is definitely not normal, blowing up fuses and electronic components tells me that there's some serious current flowing from device to device, I'm afraid it could be enough to get a good electric shock by touching the two devices. I'm thinking more of a wiring fault instead of a simple lack of ground.

[cr4zymanz0r]

Well, I'm home now and I haven't done much testing yet.....but I did one test and I don't exactly know what it means but it sounds bad  .

I did the voltage test where I put one end of the multimeter to the X68000 video port's metal casing, and the other end to the VGA plug's metal casing while they're not plugged into each other. The monitor is plugged in but off. Measuring DC voltage, if the power switch on the back of the X68000 is off then i register 0V. If i flip it on so that it's in standby power.....i get 46V. Now I'm not terribly knowledgeable with electrical stuff......but that sounds bad. I tried measuring the AC voltage, but either there is none or I don't know what I'm doing with the multimeter because it wasn't really registering anything.

Anyone wanna interpret this for me while I check out some stuff? :\

Edit: With it off I measured the resistance between pin 7 on the video port (voltage) to ground. It tells me 620 ohm resistance. Like i said, i'm not terribly knowledgeable on electrical stuff but that sounds VERY fishy.
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Edit again: I couldn't find out exactly what was causing it.....but I unplugged all power from the mainboards and was still getting 46V over ground. After that I took the entire PSU out and laid it on the table next to the X68000. I powered the PSU on and then I got 0V over ground (obviously), so then with the PSU still laying on the table I hooked all the connections back up to the X68000 and I still don't get any voltage on ground with it powered on.

I connected it to the monitor and nothing exploded.....but the monitor light just blinked like it was in standby mode even when the Drive 0 light is blinking waiting for a disk. I guess I'm not entirely sure I replaced the blown video board transistor with an appropriate replacement since I was having to rely solely on someone else's advice, or maybe another video component fried that's not visually obvious :\

[lydux]

I guess I'm not entirely sure I replaced the blown video board transistor with an appropriate replacement since

What's its reference ? Should normally be a DTC114.

[cr4zymanz0r]

I guess I'm not entirely sure I replaced the blown video board transistor with an appropriate replacement since

What's its reference ? Should normally be a DTC114.

Yes, the original transistor says C114 on it but I was having a hard time finding an exact match replacement for it since all I had to go on was the "C114" on it and me not knowing hardly anything about transistors. I asked different people that were more knowledgeable than me and eventually found the answer I went with here http://redd.it/snk6f.

In that, the user markrages basically gave me two options. He told me a UNR4210 would be a drop in replacement, or I could do some mild rigging with a 2N3904. I chose the 2N3904 option since I could grab that at Radioshack and test that same day. Essentially the (supposed) deal is the original transistor is ECB for the pinout, but the 2N3904 is EBC so I had to bend the B and C legs around to match the pinout. He also told me I had to put a 47k resistor between the B leg and the board to make it match the correct specs. I basically just had to have faith that he knew what he was talking about.

As much as I wanted to get this thing working (and expected it to just be working when I first got it) It looks like I'll be going the refund route. Feel free to still try to help me on the crazy chance that someone might figure out how to fix it in the next few days while I'm arranging getting the thing shipped back. I'm curious what's wrong myself and I always like increasing my knowledge, but if this thing can be repaired I would probably need extensive long term help from someone else.

[lydux]

Ok, I've looked to all 3 datasheets.

First, the exact reference for the original one is DTC114E. Which is hard to find nowadays.

Datasheet ==> here

Pinouts of DTC114E is as follows :
1 : Emitter
2 : Base
3 : Collector

So, EBC as well.

A correct replacement could be UNR4211, but not UNR4210 ! UNR4210 have only one builtin resistor of 47Kohm at Base, while the DTC114E have two 10Kohms resistors : 1 at Base, and 1 between Base and Emitter. (see schematics representation on first page of the datasheet)

And about 2N3904, I'm not sure... It has not the same purpose, not same power consumption, double collector current...
Not an ideal replacement, you will probably have sync issues.

So actually, you can try to reverse your 2N3904 to the original order, and put your 47kohm between pin 2 and the pcb. It will probably not be enough, I can see a 1Kohm resistor next to the base and the ground or vcc, bias voltage might be wrong.
I have to draw some schematics, and do some calculation to get a correct resistors values to fit with this transistor.


Don't give up ! The x68000 is a wonderfull machine ! 

[cr4zymanz0r]

I'll try switching the pins back around after work today, but I don't really have any time to order new parts, but if Radioshack has something in stock then I might would gamble a couple of bucks if you're sure about it. If I somehow make some significant progress in the next few days then maybe I'd delay returning it for a refund, but the video isn't the only issue. I'll just go ahead and list all the issues while I'm talking about it I guess:

1.) No video. This is the most important issue currently. If this was working or even giving me some sort of display other than a blank monitor screen then that would restore some of my faith.
2.) Music starts getting static and then eventually locks up. Here' s a video I made of it X68000 sound messing up. Just skip about halfway or so. However, the issue seems to pop up much much faster now than it did in the video. I don't think it even takes a minute now from a fresh boot.
3.) The state of the floppy drives is unknown. They were stated as working before it was shipped to me, but the only game I have is Akumajo Dracula. It was sold to me as working, but I guess I can't confirm for sure since the X68000 is acting up. I ran the X68000 one time when I got it before connecting it to a monitor and frying stuff. I heard the loading music for the game, but it never got past the loading music and looked/sounded like it wasn't reading from the floppy drives anymore either. Post-frying, it doesn't always start the loading music but does a decent bit, but it looks like it barely tries reading from Drive 1. I was working on a way to write some games to some blank 5.25" floppies so I could test other games, but I got side tracked working on the X68000.

Those are just the issues I know of. I have no idea if I might have other problems such as garbled graphics or whatever if I did happen to get the video output working. Feel free to speculate on the music issue, but I don't really want to go replacing components on the sound circuit unless I get the video working.
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Edit:

First, the exact reference for the original one is DTC114E. Which is hard to find nowadays.

Is this one? http://www.ebay.com/itm/220767176744
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Edit again:
Well, looks like I can't do anymore anyway. The seller informed me that for a refund I must send it as it is currently and that I don't tinker with it anymore. I'd still be interested in hearing theories about the issues, but unfortunately I can't act on them anymore or I risk voiding my refund apparently.

[BlueBMW]

I've made my own VGA adapters and done PSU swaps on systems and havent had an issue yet.   There is something really funky going on.  I know when I mounted a replacement PSU into the system I was very careful how I screwed things to it.  I know the heat sinks inside the PSU were connected to the live side of things (meaning they shocked the crap outta me when I brushed up on them during some tinkering)  So I made double sure I didnt accidentally drive a screw in and make contact.  I wonder if they might have done that.  If so, then you might get some AC voltage going to the metal casing of the PSU and then the system shell.  Being grounded I guess maybe it would loop back somehow.  (not as familiar with how AC behaves)  If you have current going through the shell and getting back into things, that could cause no end of issues.  The sound cutting out, no video, freeze ups, all that could be related.

Just curious, what did you spend on that 68000?  and where are you located, if by chance I'm not far away?

[cr4zymanz0r]

When I was taking the PSU out of the case I noticed that one of the heatsinks was actually attached to the original PSU's metal shell by being screwed in to it. I wondered if that had anything to do with it, but the thought of power flowing through a heatsink sounded crazy. I just knew that once I had the PSU outside of the X68000 that there wasn't power over ground anymore so then I figured I'd test the video, then if it worked I was going to get to the bottom of the grounding problem. Since it didn't work, I just put it back together as per the seller's request so I can send it back.

After the cost of the system, expensive good shipping from Japan to the US, paypal currency conversion fees, etc. it came out to roughly $440. I really don't know how good or bad of a deal that was (assuming it had worked correctly and not fried stuff). I just know I was having an extremely hard time finding one to buy at all that wasn't on Yahoo Japan auctions. I didn't really want to go that route since I can't read any Japanese and I wanted to certain of the condition of the X68000 I was buying (oh the irony now).

Anyway, I live in Arkansas  . Why do you ask? Do you live in the US and have a X68000 to sell me  ? This has definitely dampened my mood on getting one. If I tried again I'd prefer it be from someone in the US (or possibly Canada) so shipping wouldn't be crazy if I needed to send it back for an issue, but I bet about anyone in the US with one isn't willing to let it go.

[BlueBMW]

Ahh, if they had the PSU shell screwed into a heat sink, that could definitely create issues.  If their replacement PSU is like the ones I use and has the heat sinks tied to the live side of things, that could definitely be a problem.   At this point, I'd probably just return it for the refund and be on the hunt for another unit.  Though how is the seller going to handle return shipping if they're in Japan?  Hopefully they dont intend to make you pay return shipping (which will be quite pricey considering the weight of the unit)  Depending on what they say regarding return shipping and refund etc, maybe offer to take a partial refund and keep it as a parts unit.  As far as pricing, $440 for a refurbished unit is not too bad.  I know people have paid as much as $600 for a fully refurbished system.

I asked where you lived just in case by some luck chance you were somewhat nearby.  I'm in east Tennessee, so a good days drive away  I've got an almost working expert here (bad floppy drives) that we could have used to do some parts swapping / testing with.

I've been buying junk X68000's through a friend in Japan off of Yahoo Japan.  I get them fairly cheap and have them shipped by surface mail, so it takes a while to get them in.  So far they've been hit or miss on repairability.   Right now I don't have any systems that aren't already claimed, but in time I should have an extra one.  When I get them I refurb them as best I can.  (mainboard recap, psu refresh / replace, floppy recap/lube/clean)  So if you are patient, we will find you one eventually!

If you do get to keep that unit for parts, let me know as between that unit and a system I have here, we could probably make a working system no problem.  I've got an Expert HD here that just needs floppy drives to be good.

Best of luck and don't give up the x68000 dream!   Once you've got a good working system its a lot of fun.   I love games from the 8/16 bit era, so the 68000 is a gold mine of good stuff.

When I was taking the PSU out of the case I noticed that one of the heatsinks was actually attached to the original PSU's metal shell by being screwed in to it. I wondered if that had anything to do with it, but the thought of power flowing through a heatsink sounded crazy. I just knew that once I had the PSU outside of the X68000 that there wasn't power over ground anymore so then I figured I'd test the video, then if it worked I was going to get to the bottom of the grounding problem. Since it didn't work, I just put it back together as per the seller's request so I can send it back.

After the cost of the system, expensive good shipping from Japan to the US, paypal currency conversion fees, etc. it came out to roughly $440. I really don't know how good or bad of a deal that was (assuming it had worked correctly and not fried stuff). I just know I was having an extremely hard time finding one to buy at all that wasn't on Yahoo Japan auctions. I didn't really want to go that route since I can't read any Japanese and I wanted to certain of the condition of the X68000 I was buying (oh the irony now).

Anyway, I live in Arkansas  . Why do you ask? Do you live in the US and have a X68000 to sell me  ? This has definitely dampened my mood on getting one. If I tried again I'd prefer it be from someone in the US (or possibly Canada) so shipping wouldn't be crazy if I needed to send it back for an issue, but I bet about anyone in the US with one isn't willing to let it go.

[cr4zymanz0r]

Oh wow, you are close.....yet still far away .

As far the refund, the seller said he would pay the return shipping. However, he did send me an email saying if I just wanted a partial refund and keep the X68000 that he could do that too. He didn't explicitly state how much the partial refund would be yet, but it sounded like he would refund half of what I paid. I'm waiting for him to respond to some of my additional questions about that, but do you think that's reasonable? I don't know what buying one in this condition would go for normally.

Anyway, to try to help me come to a decision I'd like to talk to you some more about possible ways you could help me if I do end up keeping it. It'd be nice if you could help since shipping something to troubleshoot wouldn't cost a fortune within the US. Do you have any instant messengers you use such as AIM, Google Talk, etc. that you could PM me so we could discuss this more in depth?

[BlueBMW]

Sent you a pm.  We will get this sorted out.

[BlueBMW]

I've been messing with this machine some more.  The power supply swap is a total mess... a wonder the thing didnt catch fire or shock someone.

I traced back the whole circuit of that blown transistor in the video board.  No dice on it yet.  Video board is probably blown, but the rest of the system may still work ok.

[RobIvy64]

Just curious, who did the PSU swap?

[BlueBMW]

Just curious, who did the PSU swap?

Not sure, crazymanzor got it from someone in japan who did the refurb work.

[lydux]

So, what were done with this DTC114 ?


And silly questions : don't you have another of this video ouput board to test with ? And did you try the RF out ?

[RobIvy64]

I just wonder if it's Lukemorse from youtube.

I have had a few people contact me on youtube after buying "refurbed" systems from him, and they were having all kinds of issues.

He even shipped a Macintosh Apple video to VGA adapter with one of them...

[eidis]

.... and he even rescued x68k units from being thrown in the trash and fixed various problems. Are you saying that it would have been better if he did not bother resurrecting them and just let them go to the dumpster ?

Eidis

[RobIvy64]

lol, calm down

All i'm saying is that if you are going to repair it with the intention to sell it, repair it correctly.

I'm not saying that the system in this thread was "repaired" by him, I'm just telling you about the individuals who contacted me after buying a system from him.

Tossing a Macintosh video adapter in the box was pretty crappy.  It's obviously not going to work.

Poor guy was flicking the adapter's DIP switches in hopes he could get his new X68000 working...

[BlueBMW]

So, what were done with this DTC114 ?


And silly questions : don't you have another of this video ouput board to test with ? And did you try the RF out ?

Apparently this unit did originally come from LM.  Whether or not the problems were intentional, I cant say.  It was a sloppy job, but I'm not impervious to mistakes myself, so I wont do any judging.

I dont have another video board at the moment.  I traded my other expertHD unit to a friend so I dont have it here.  I traded the other Expert unit I did have to the guy who started this thread.  I've got one more Expert unit incoming so I'll test then.

The other thing, this system hangs on boot.  Its like it freezes partway through booting anything, so there's still other problems beyond just the video issue.

Another electronics guy looked at the circuit and suggested I try this:



I tried it, but no change in results....

You said RF out?  Excuse my ignorance, but do these systems have an RF out?!  Where is it!

[RobIvy64]

Apparently this unit did originally come from LM.

Yikes! I was afraid of that.

I'm glad it didn't catch fire. BlueBMW will get the mess straightened out  

[lydux]

Lol ! I feel stupid... Because I know that in 90's years japaneses love to building systems with the good old RF antenna, I were sure the plug next to the analog RGB was an RF out !  
Forgive me...

Hum, well !
I have something different in my video output board, here is a quick drawing :


Seems you have going to get back your nice logic probe !  
Check for pulse on HSYNC and VSYNC !


Edit : you can test for this directly on the main board on pin 7 and 8.

[lydux]

One more thing :

The way it hangs up doesn't look like a floppy or sound issue. On a system bus crash, the OPM will not get its data anymore, the music will stop like this. As well, the poweroff sequence is also software controlled ! System bus crash = no data flow = no poweroff.

The CRTC (Vicon) give the system bus a direct access to its VRAM. This memory area could be access by either the cpu or the dmac. And the floppy controller can perform a dma transfert, that's a probably the way used by Akumajo Dracula on startup to speedup the loading (transfert of some kana from the CGROM or graphical elements).

Finally, to me, that's smell more a faulty CRTC, VRAM bank, or DMAC. Any one of these 3 components could hang the system like this !

Now that you have a keyboard, you can access the remote debugger instead of the normal startup. So try to get a null modem cable if you don't have one. This debugger could really be a nice tool for diagnostic !
From this point, we could give it some kind of life with a minimal system bus access, and perform some tests of the whole system.

[BlueBMW]

One more thing :

The way it hangs up doesn't look like a floppy or sound issue. On a system bus crash, the OPM will not get its data anymore, the music will stop like this. As well, the poweroff sequence is also software controlled ! System bus crash = no data flow = no poweroff.

The CRTC (Vicon) give the system bus a direct access to its VRAM. This memory area could be access by either the cpu or the dmac. And the floppy controller can perform a dma transfert, that's a probably the way used by Akumajo Dracula on startup to speedup the loading (transfert of some kana from the CGROM or graphical elements).

Finally, to me, that's smell more a faulty CRTC, VRAM bank, or DMAC. Any one of these 3 components could hang the system like this !

Now that you have a keyboard, you can access the remote debugger instead of the normal startup. So try to get a null modem cable if you don't have one. This debugger could really be a nice tool for diagnostic !
From this point, we could give it some kind of life with a minimal system bus access, and perform some tests of the whole system.

Definitely time to get the probe back out!  Once this other expert system arrives, I'll do some swapping to first figure out the video output issue and second maybe narrow down the system hang issue.

Thanks for the great leads and the better video board schematic.  I just drew up the one I posted based on what I was able to trace with a meter.  I've no doubt I may have made a mistake in it somewhere.