Troubleshooting 800XL, working but no video

[wildstar87]

I know this machine's video worked before, but I'm not getting anything out of the 5-pin DIN, except audio.  The machine is definitely working, I can load something from Dos, typing in a memorized command, it's happily making SIO noises, and self test audio works as well.  However, I put a UGV in thinking it might be an issue with the monitor port for some reason, maybe a bad 4050.  UGV is connected directly to an S-video jack, and is not going through the DIN5.  I know the UGV is working, as I have tested in in 3 other machines.  I still get no video even with UGV, so thought it might be the Antic, and swapped that for a known working good, but still no joy.  I'll start tracing down signals later, but was hoping maybe someone ran into something like this before?

[_The Doctor__]

leaves you with GTIA and transistors

[wildstar87]

Oh, forgot to mention that, the GTIA has stayed with the UGV as a module when swapped into other machines, so it definitely works.  I'll give the transistors a check, luckily I have a bag of 2N3904s if necessary around here somewhere..

[Peri Noid]

For me it sounds like the 4050 buffer is dead. 

[_The Doctor__]

thought he already did the 4050 and was using a UAV/UGV

[Beeblebrox]

Yup I'd imagine the 4050 was removed as part of the OP's UGV install. (I think you can leave it in place alongside UGV if you wish as it has been mentioned having a 4050 in place alongside can help other things). AFAIK Mytek advocates removing it completly. I did a UGV in a 65XE the other day and removed the 4050witn no problems. Amazing image quality with the UGV and so cheap to by preassembled here in the UK. Worth every penny.

 

So I think it sounds more like the issue the OP is having is going to be transistor related. If I thought this issue had occured as a result of the UGV upgrade I'd assume the UGV to Din5 jack wiring was a potential cause. But it seems the issue was present beforehand. Worth double checking the latter wiring just in case there as an additional layer of issue introduced with said wiring I guess.

[Peri Noid]

Then the next question is obvious - is the UGV properly installed.

[+DrVenkman]

Is you UGV installed inside the 4050 socket? Verify that you’re getting good power/ground. Years ago, I had the small electrolytic cap fail on an 800XL near the 4050 socket fail and killed powered to the socket. 

[Beeblebrox]

22 minutes ago, Peri Noid said:

Then the next question is obvious - is the UGV properly installed.

OP said video issue was present before the UGV was installed so an issue with the stock machine and what sounds like the video circuit from the direction of investigation this thread has taken.

I just thought aside checking the stock video (transistors next on list to check), it' also worth checking the UGV wiring anyway - just in case there is additional UGV wiring issues as well a the original stock video.

 

@wildstar87 Here is one of my UGV install threads where I am sure you have referenced this from Mytek:

EDIT: I just clocked you have UGV hooked up to an Svideo jack rather than the Din5.

 

Can you post some hi res images of the whole board with UGV in place, etc?

 

Also in installing the Svideo jack can you explain what you have done step by step? Just in case in doing so something has been adversely affected during the installation of said svideo jack?

 

OR was Svideo working off the Svideo jack at one stage and now it isn't?

 

Either way mods have been made to several areas of the stock machine so it makes sense to check the svideo jack install as well.

 

Finally is the Svideo jack Mytek's pcb design or a standard jack?

Edited June 27, 2023 by Beeblebrox

[wildstar87]

4050 hasn't been removed on any machines so far, as I'm only using an s-video jack temporarily, until I finish mounting everything.  UGV isn't mounted in 4050 slot, it goes under the GTIA.  Literally, I have been popping the UGV/GTIA out as a module, and just pulling the GTIA on machines, and swapping.  S-video jack is not mounted anywhere, just on the UGV S-video partner (Mytek designed) board, on the end of the cables coming out of the UGV, just kind of hanging out of the machine.  The machine had been modified to bring out the Chroma signal to the Din5, and had worked just fine in the past.  When I plugged it in recently to test it, didn't get anything out of the Din5, though I haven't tested the Composite connection yet, but since the UGV isn't working, my guess is that Composite won't work either.  RF Modulator has been removed in the past as well.  Initially thought the machine might be dead, but happened to have S-Drive Max plugged in, and it started loading Dos, so I suspected that the machine was working, and after hooking up audio, verified that it was.

 

I can take pictures, when I get home later, but not sure it will tell you anything, other than the Chroma line on the bottom of the board, everything else is stock, other than the UGV, which has the S-video jack hanging off it. 

 

Can a machine boot w/o GTIA?  I haven't had a chance to probe the board yet for voltages/signals.

 

[Beeblebrox]

Hi. 

Quick reply as on a train. 

 

Images of the setup and pcb would help still I reckon. 

 

Also because you can hear it working it sounds very much like a wiring or transistor issue in the video circuit. The machine wouldn't boot without any of the 40pin ic's in place no. The gtia can fail and you can get glitching with colours or worse as a result of a failing gtia chip, but if you are hearing the machine booting to basic, or loading via the sdrive max it is a good thing. 

 

Can I check this is a stock 800xl and not an 800XLF?

 

So my advice would be take out the UGV and get the stock video out to work first. 

[+DrVenkman]

26 minutes ago, wildstar87 said:

Can a machine boot w/o GTIA?

Nope. The clock circuit feeds the OSC signal to GTIA, which then generates a signal called Fast PHI0 to ANTIC. ANTIC then generates the PHI0 clock which feeds SALLY. SALLY in turn generates the PHI1 and PHI2 clock signals which sync up everything - it all creates a feedback loop. 

[wildstar87]

Yes, stock 800XL, fully socketed, had bad ram which had been replaced a while ago.  I'm looking at 800XL schematics, and since UGV pulls signals directly off GTIA, wondering which transistors I should look at, since the 4050 isn't coming into play, unless possibly a failure in that section, is causing a feedback to GTIA, corrupting signals to UGV (just completely spitballing here..)? 

[Overange]

As you have mentioned you have tried the main ICs and they appear ok and the UGV is good.

 

The next thing I would do is start to check for continuity between all pins to and from  the GTIA and possibly Antic, using the schematic below as a reference.

 

As well as checking continuity from say GTIA Pin 20 to Antic PIN 5, just check the ajoining pins are not shorting, so as per schematic, with the probe on GTIA pin 20 to GTIA Pin 19 should not have any continuity.

Then just another sanity check, with the probe on the pin you are testing, just check it to ground/0v or the +5v rails.

 

It would also be best to ensure ICs are removed, before buzzing out the board. 

 

The only other common component is this section is U18, have you checked this IC?

 

 

[_The Doctor__]

Don't forget to check continuity through the sockets. As has been the case, deformed and damaged sockets have been an issue. The traces all read good but the sockets not so much. Damage to traces under and around sockets, and of course flakes and solder specks can do some fun things as well.

Edited June 27, 2023 by _The Doctor__

[wildstar87]

Didn't have a lot of time to do stuff last night, forgot to take pictures.  Tested continuity between Antic and GTIA, except for Address, and D# lines.  Replaced LS08 with new F08, pulled the 4050, checked continuity between chip leg and back traces on GTIA and Antic, checked power/gnd.  Everything looks to be ok, nothing immediately obvious is poking out.  I'll have more time this weekend to look into it in a bit more depth.  I'll be looking at signal lines for patterns/voltages out of Sams Computerfacts, any others you might look for?

[wildstar87]

Well, I feel kind of stupid, but it wasn't exactly obvious.  I didn't have a CRT handy, so was using S-video input on an LCD.  Turns out there is a Sony PVM at work, so I plugged in my problem machine into that CRT, low and behold, I got a picture, only a B/W picture though.  The LCD apparently doesn't display a picture, if it doesn't get both Luma AND Chroma signals, I verified this by removing the Chroma lead from the UGV (on a working machine) and it no longer displayed on my LCD.  This machine still has a problem with the Chroma, it's definitely got an issue, since I also just got a Retrotink 5X Pro yesterday, which will function without a Chroma signal, and hooking up to that, with the UGV on the problem machine, the Chroma is mostly non-functional, but it is flickering on and off, so have to figure out why it's doing that, but at least I know the video isn't completely dead, just that my monitor has to have both signals.

[wildstar87]

It was the god damn color pot..  Checked the color signal on a oscope, looked very different from the working machines, went through the Sams troubleshoting for color, which had me look at voltage while adjusting the pot, went back to the oscope signal, and looked very different from before, more like the signals from the working computers.  Plugged it in, and damned if it wasn't coming up green.  Sprayed some deoxit in it, worked it around, then adjusted it back to the standard blue background.  That's all it was..