Tandy 1000SX Video Output Issue

[Benson86]

Hi all, newbie here. Been collecting/working on vintage game systems for a couple years now but am brand new to vintage computing. I recently got a Tandy 1000SX with a matching VM4 composite monitor that's starting to have some video issues. Google brought me here to this thread: https://forum.vcfed.org/index.php?threads/dim-composite-video-on-tandy-1000-hx.73102/ But I didn't want to bump a 3 year old thread so starting a new discussion here.

My issue sounds somewhat similar, everything had been working great since I got it about week ago. Ive been spending allot of time on it playing and doing upgrades. I did notice the brightness being a little inconsistent and a little occasional shimmer in the display but just assumed it was a tired monitor. Yesterday while using it the picture dimmed by about 50% and almost immediately completely lost sync on the VM4. If I turn the computer off and back on the picture holds for about a second and then desyncs again. I tested the monitor with an NES and its completely fine no issues so I now its not the monitor. I then plugged the computer into a CRT TV and the TV does a much better job holding the sync with the signal its getting but you can tell the signal is subpar. There is a little shimmer in the image and the longer the computer is on the worse it gets, I have to assume a part is failing somewhere.

In the thread above a transitor is mentioned that would be part of creating the composite signal and if that's failing that could cause the signal strength to degrade? I dont have an RGBI monitor yet to test that output, but I did order one and should have it by Friday. Any suggestions on where to start? I don't have a scope but do have a multi-meter. I am comfortable soldering and can replace caps or transistors. Thanks!

 

[Benson86]

Couple updates, I wanted to share a a quick video of what its doing :

Also saw on another similar thread member Eudimorphodon said that Transistor Q3 is the main component in the amplification circuit and could be causing issues like this. I took a couple photos of the board and the transistor labeled Q3. I ordered a little multipack pack of them that included this one. Its easy enough to swap a transistor I figure ill start with that. The RGBI monitor should be here soon as well and I can test that.

PS. Mods I realized after making the post that this might belong in the IBM Clone forum and not the Tandy forum? If so feel free to move, thanks!

 

[Benson86]

Update. I replaced the transistor, it did not fix the issue. The composite output still acts exactly the same. BUT I did get my RGBI monitor in today and that is working excellent. So the issue is with the composite output only as far as I can tell. I would still like to get it fixed so I can use the monochrome monitor when I want to. Does anyone have any suggestions on where to go from here? My google info has run dry... thanks!

 

[Great Hierophant]

Update. I replaced the transistor, it did not fix the issue. The composite output still acts exactly the same. BUT I did get my RGBI monitor in today and that is working excellent. So the issue is with the composite output only as far as I can tell. I would still like to get it fixed so I can use the monochrome monitor when I want to. Does anyone have any suggestions on where to go from here? My google info has run dry... thanks!

I would consider testing the 78L05 at VR1 with your multimeter to see if it is outputting 5v. The weakness may not be in the transistor but the power source driving it. Also consider replacing C175 and C176, which are capacitors in the path of the voltage to the amplification circuit.

 

[Eudimorphodon]

The composite signal in those Tandy 1000s is created by summing together the same four R-G-B-I lines that go to the RGB port via a resistor ladder (for the grayscale luma portion of the signal) along with two unique signals from the "Big Blue" video chip; a composite sync and composite color line. (These also go through resistors to the same common output at the RGBI lines, and then the whole mess goes through the output transistor.) I'm a little concerned that you might actually have an issue specifically with the composite sync output from the Big Blue. When you have the monitor hooked up to a color TV do the colors generally look correct, it's just the sync/stability that's a problem? (IE, white is white, etc, and the brightness is roughly consistent?)

The mixing resistor for the composite sync signal is R61, 680 ohms, according to the schematic I'm looking at. It might be worth checking if that looks correct, and perhaps replacing it. All the signals from the Big Blue are buffered through a 74LS244, U57, if you have an oscilloscope it might worth checking to see if you're getting a clean sync signal from the output from that. If the output from the Big Blue itself were bad... it could technically be a reparable situation, you could construct a new composite sync signal by combining the H/Vsyncs going to the RGB port, those are apparently fine, but hopefully it doesn't come to that.

 

[Benson86]

Thank you guys, ill check all of those points and see what I can see. I took a couple more videos of it hooked up to a color composite TV. As well as side by side with the CM-11. The TV really tries to hold sync but it just cant. When the sync drops it drops color also, but I dont know if this is just from loosing sync or if the color signal is affected as well? Also I dont have a scope, just a meter, but ill test 78L05. Thanks!

 

[Benson86]

The composite signal in those Tandy 1000s is created by summing together the same four R-G-B-I lines that go to the RGB port via a resistor ladder (for the grayscale luma portion of the signal) along with two unique signals from the "Big Blue" video chip; a composite sync and composite color line. (These also go through resistors to the same common output at the RGBI lines, and then the whole mess goes through the output transistor.) I'm a little concerned that you might actually have an issue specifically with the composite sync output from the Big Blue. When you have the monitor hooked up to a color TV do the colors generally look correct, it's just the sync/stability that's a problem? (IE, white is white, etc, and the brightness is roughly consistent?)

The mixing resistor for the composite sync signal is R61, 680 ohms, according to the schematic I'm looking at. It might be worth checking if that looks correct, and perhaps replacing it. All the signals from the Big Blue are buffered through a 74LS244, U57, if you have an oscilloscope it might worth checking to see if you're getting a clean sync signal from the output from that. If the output from the Big Blue itself were bad... it could technically be a reparable situation, you could construct a new composite sync signal by combining the H/Vsyncs going to the RGB port, those are apparently fine, but hopefully it doesn't come to that.

Follow up question, if I didnt feel like trying to keep hunting this at the moment, is there any reason not to pickup something like this? https://www.ebay.com/itm/123598621473?hash=item1cc70d4b21:g:I-UAAOSwxx1cP~w-

Its a Tandy part that can do CGA/EGA and has a composite output. Seems like it would just avoid the onboard video all together, I doubt it has the Tandy display mode, but with EGA that seems unnecessary? Would also free up all my 640K of RAM.

 

[Eudimorphodon]

Its a Tandy part that can do CGA/EGA and has a composite output.

I can't say this with 100% certainty, but I don't think those RCA jacks are composite out, or at least they aren't on that particular card. Original IBM EGA cards have a couple phono jacks on the back, but they aren't actually connected to anything; they loop around and come out on the internal "Feature" connector, and unless something is connected to that connector and driving them they don't do anything. Here's a "VGA Museum" page about that card that has some links to old reviews, and the InfoWorld review says that the phono plugs and feature connector on it are implemented identically to the original IBM version. So if composite is really what you want this card isn't going to do it for you.

Is your goal specifically to drive the VM-4 monitor? If you really can't get the onboard composite port to behave it would be fairly easy to build a resistor-ladder DAC and sync combiner circuit to plug into the DB-9 port and re-create a monochrome-only composite out.

 

[Great Hierophant]

Follow up question, if I didnt feel like trying to keep hunting this at the moment, is there any reason not to pickup something like this? https://www.ebay.com/itm/123598621473?hash=item1cc70d4b21:g:I-UAAOSwxx1cP~w-

Its a Tandy part that can do CGA/EGA and has a composite output. Seems like it would just avoid the onboard video all together, I doubt it has the Tandy display mode, but with EGA that seems unnecessary? Would also free up all my 640K of RAM.

As Eduimorphodon said, those RCA jacks are useless without a Feature Board. Using an EGA card would avoid the on-board video, but you have the issue of programs that expect Tandy video in a Tandy 1000, EGA is slower than Tandy video in many programs and some programs only use Tandy sound when using Tandy video.

 

[Benson86]

I can't say this with 100% certainty, but I don't think those RCA jacks are composite out, or at least they aren't on that particular card. Original IBM EGA cards have a couple phono jacks on the back, but they aren't actually connected to anything; they loop around and come out on the internal "Feature" connector, and unless something is connected to that connector and driving them they don't do anything. Here's a "VGA Museum" page about that card that has some links to old reviews, and the InfoWorld review says that the phono plugs and feature connector on it are implemented identically to the original IBM version. So if composite is really what you want this card isn't going to do it for you.

Is your goal specifically to drive the VM-4 monitor? If you really can't get the onboard composite port to behave it would be fairly easy to build a resistor-ladder DAC and sync combiner circuit to plug into the DB-9 port and re-create a monochrome-only composite out.

Thanks! You're right its mainly just to get the composite working to drive the monochrome monitor. For the most part the RGBI monitor works great and ill be using that most of the time, and I would like to get the internal composite working. Is there anyway really to diagnose without getting a scope, or would I pretty much just be replacing parts blind at that point?

 

[Eudimorphodon]

Is there anyway really to diagnose without getting a scope, or would I pretty much just be replacing parts blind at that point?

Without a scope you really are mostly shooting in the dark. You can go ahead and replace the rest of the analog final drive components north of the 74LS244 digital buffer (I mentioned the level resistor specifically on the composite sync signal, R61, as maybe your best shot in the dark), and then the buffer itself, but a scope would let you directly see if you're getting a good sync signal out of the big blue and through the buffer to get messed up in the first place.

Looking at the schematic again I guess another thing I could suggest is this? It looks like the +5v power for the composite video output transistor is derived from an onboard regulator, VR1, drawing from +12v instead of the +5v feed from the power supply. (Not sure why? To prevent noise/feedback, maybe?) Try sticking a voltmeter on both the output of that and on the +12v going into it and see if there are glitches that correspond with the video problems. There isn't much that uses the +12v feed in a Tandy 1000, and if you're not using the floppy drives in your machine (IE, you have an XT-CF installed and you mostly run off of that) it's possible for it to get flaky without immediately obvious showstopper symptoms.

 

[Benson86]

Thank you I appreciate it. Gives me a good place to start. I doubt its the 12v rail, I do use the floppy drives allot including copying disks and haven't any issues there. Ill keep diagnosing and see what I can find, I should really just get a scope lol. Thanks for all the help!