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Fixing 800 video--where to look after GTIA?

800 video gtia

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#1 vputz OFFLINE  

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Posted Wed Oct 4, 2017 11:38 PM

Wonder if you folks can help... I'm trying to restore a friend's 800, which is giving... well, just awful video.  Using the RF mod, when powering on the 800, the screen definitely CHANGES, but the screen is close to static (you can almost see a distorted frame as the display cycles around).  We attempted composite video from the 5-pin DIN, but didn't get much of a picture, although trying the O-scope on the composite port yielded the below picture; it's trying, but not giving a picture.

 

We found the repair manual and followed the flowchart for grey/black screen, tracing all the clock pins (which all seemed to look OK).  Pin 25 of the GTIA gives a nice clean (composite sync?) signal.

 

Since the flowchart recommended replacing the GTIA, just in case, I bought a "known tested" CPU board from eBay, which gives the same results.  So I don't think it's the GTIA, but maybe something in between it and the RF mod/DIN port.  But I'm not sure what to check next.  I found this schematic https://atariage.com...-1338873000.jpg, but it'll be mildly painful to trace the whole thing through and I'm not 100% sure what I'm looking for.

 

Any suggestions for the next step?  Thanks--it'd be quite a thrill to get this working again.

 

Oh!  One more clue (maybe?)--I haven't owned an 800, so I don't know; the speaker never seems to make any sound, either when powered on or when trying control-2 (which I hear buzzes).  Is it supposed to?  Not sure if that helps.

 

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#2 russg OFFLINE  

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Posted Thu Oct 5, 2017 6:14 AM

Wonder if you folks can help... I'm trying to restore a friend's 800, which is giving... well, just awful video.  Using the RF mod, when powering on the 800, the screen definitely CHANGES, but the screen is close to static (you can almost see a distorted frame as the display cycles around).  We attempted composite video from the 5-pin DIN, but didn't get much of a picture, although trying the O-scope on the composite port yielded the below picture; it's trying, but not giving a picture.

 

We found the repair manual and followed the flowchart for grey/black screen, tracing all the clock pins (which all seemed to look OK).  Pin 25 of the GTIA gives a nice clean (composite sync?) signal.

 

Since the flowchart recommended replacing the GTIA, just in case, I bought a "known tested" CPU board from eBay, which gives the same results.  So I don't think it's the GTIA, but maybe something in between it and the RF mod/DIN port.  But I'm not sure what to check next.  I found this schematic https://atariage.com...-1338873000.jpg, but it'll be mildly painful to trace the whole thing through and I'm not 100% sure what I'm looking for.

 

Any suggestions for the next step?  Thanks--it'd be quite a thrill to get this working again.

 

Oh!  One more clue (maybe?)--I haven't owned an 800, so I don't know; the speaker never seems to make any sound, either when powered on or when trying control-2 (which I hear buzzes).  Is it supposed to?  Not sure if that helps.

 

You say you've never owned an 800, so I'll belabor some obvious.

You have the CPU in the shielded rear slot.  That has be correct since it won't fit in any of the

front four slots.   Then you have to have the ROM in the front slot and RAM in any of the remaining

three slots behind the ROM.  If any of the cards aren't encased, the chips go to the back.

Sorry if this is obvious.


Edited by russg, Thu Oct 5, 2017 6:34 AM.


#3 DrVenkman OFFLINE  

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Posted Thu Oct 5, 2017 6:59 AM

If you don’t hear the “click” of the machine trying to boot, and you’re not getting the common solid color screen prior to OS startup, I’d suggest looking at ANTIC and/or 6502 as your likely issue, with an outside shot at bad OS roms, though I doubt that - if ANTIC is running you’d get a solid reddish/brown screen at first.

#4 vputz OFFLINE  

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Posted Thu Oct 5, 2017 5:16 PM

Russg: It's "obvious", but I missed it at least once.  We totally tried powering it up without the ROM/RAM before kicking ourselves.

 

DrVenkman: Hmmm, the problem is we swapped out the entire CPU board (6502, ANTIC, GTIA, everything) and got exactly the same results.  All power pins on the power supply board tested with good voltages, too.  I sorta feel like it's a problem somewhere between the CPU board and the video out.  I will say I don't trust our RF mod results, because the TV in question has a digital tuner and those old RF mods were never THAT precise, but the composite should have given us better results, and the lack of sound is still distressing.  Hmm.



#5 DrVenkman OFFLINE  

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Posted Thu Oct 5, 2017 5:41 PM

Here's something to try: remove all the RAM but the first board (right behind the OS board). Will the machine boot with just the one board? If not, swap all the other two boards into that first slot and see. It might be you have one or more bad RAM boards in the system. If it boots with one, then try two, swapping boards around to ID which one(s) might be bad. If RAM board swapping gets you nowhere, you've got work to do.

 

If you're lucky(*), it'll be something easy to fix like a bad POKEY chip or broken socket; that's on the main board, along with the 6502 PIA. If you're not lucky, you're going to have to dig out those schematics and start looking at stuff on all the boards - main board, power supply board, RAM boards, the support logic, etc. On the other hand, it might be something incredibly basic like a bad capacitor on the power supply board. Anyway, good luck. An 800 is worth resurrecting!

 

(*) "Lucky" being relative, of course - 800's are all fully-socketed but POKEY is the single most expensive IC in the machine these days, at about $20 apiece. 



#6 vputz OFFLINE  

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Posted Sat Oct 7, 2017 11:22 AM

Hmm, progress of a sort.  I've been using a jury-rigged composite cable (literally a jack with two wires jammed into the bottom (ground) port on the DIN connector and the composite (immediately to the right of bottom as looking into the port?) holes on the DIN plug, but it turned out the problem was on the TV side; jiggling that gave me more solid results, but they aren't that illuminating:

 

- With no cartridge, solid blue screen with minor artifacts (thick bands slowly scrolling up)

 

- With "Submarine Commander" cartridge in LEFT slot, more off-white screen that looks pale blue in pic.  Interestingly, pulling the cartridge and rebooting gets this same white screen, but if I leave it powered off for a while and boot without cartridge, I get the blue screen again.

 

- With cartridge in RIGHT slot, "broken video" as if sync was strange (both horizontal and vertical).  Why would left/right make a difference?

 

Anyway, see attached and let me know if it rings any bells.  I will say that the field manual says under the "Solid Blue Screen" flowchart, "Are the words "MEMO PAD" on the screen?  No?  Swap out the POKEY chip" so that's the likely next step, but I wanted to check before ordering one; as you say, about $20, not going to break any banks, but hey.

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#7 Bryan ONLINE  

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Posted Sat Oct 7, 2017 11:27 AM

Thick bands moving on the screen typically means there's a power supply problem (you're seeing excessive AC noise in the DC output).



#8 vputz OFFLINE  

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Posted Sat Oct 7, 2017 11:38 AM

Hmm... although all the voltages at the power supply-motherboard connector  looked pretty clean when I probed them with the O-scope.  There was one large cap that looked sliiiiightly domed, but I can't convince myself fully of that.



#9 shoestring OFFLINE  

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Posted Sat Oct 7, 2017 12:22 PM

Bryan is right. You may have good DC voltages but still have poor AC filtering which causes banding on the screen.

A bad PS can hide many faults.

#10 vputz OFFLINE  

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Posted Sat Oct 7, 2017 12:38 PM

Curiouser and curiouser.... disassembled it again, still don't see any obvious caps, but connected it to a different monitor with composite video... no banding whatsoever on the blue screen on this monitor, or white screen with cartridge in left slot, but cartridge in right slot gives "no video" message on this monitor.

 

That blue screen is nice and solid--and interestingly appears regardless of the ROM/RAM cards being in place.  I reseated every chip on the motherboard and rom/ram cards to no avail: blue screen.

 

What do you think... replace POKEY?  Any known suppliers besides ebay vendors I should patronize?

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#11 DrVenkman OFFLINE  

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Posted Sat Oct 7, 2017 2:02 PM

Only a bare handful of A8 cartridges use the Right slot. Don't test the system using anything plugged in there except one of those rarities - it definitely won't boot properly that way, so you'll only be masking problems by trying it.



#12 vputz OFFLINE  

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Posted Thu Oct 12, 2017 8:16 PM

Ok, I'm at wit's end. By now we've replaced the whole CPU board and POKEY, and still a solid blue screen, no speaker sounds. One cartridge, submarine commander, gives a light blue screen (and some buzzing from the speaker once and never again).

The next step on the flowchart is "call Atari". But that's probably not a great option.

Any ideas?
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#13 vputz OFFLINE  

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Posted Thu Oct 12, 2017 8:43 PM

Also--same blue screen with no ram, which is interesting (and we've tried several boards). And the colour is changeable with the potentiometer on the CPU board, should we wish to look at a depressing green screen instead.

#14 DrVenkman OFFLINE  

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Posted Fri Oct 13, 2017 5:38 AM

Have you checked the discrete logic chips on the main board and RAM boards? Have you checked the power output of the power supply daughter board?

#15 vputz OFFLINE  

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Posted Fri Oct 13, 2017 11:15 AM

The power supply, yes--every pin on the power supply board connector seems to give a nice clean signal on the oscope with almost no ripple at the voltages described in the service manual. The two big capacitors have tops that don't look 100% flat but are not notably bulged and I can't fault the oscope measurements.

Haven't tested any chips in the ram boards, but have tested the ram boards themselves by cycling them (we have three). Have not tested the personality board or discrete logic on the motherboard mostly because I'm not sure how; suggestions welcome!

By now my friend is considering buying a whole new 800 just to have swappable parts, and I can't say he's wrong, but it's be nice to have a less clumsy plan...

Edited by vputz, Fri Oct 13, 2017 11:16 AM.


#16 ClausB OFFLINE  

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Posted Fri Oct 13, 2017 1:26 PM

Have you tried reseating all the ICs in their sockets? Pins corrode over the decades and prying them half way out and pushing them back in scrapes it off. Also the 4050 CMOS chip on the motherboard is involved with video and it's fragile. Try a new one of those.

#17 vputz OFFLINE  

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Posted Fri Oct 13, 2017 5:10 PM

Yep, all things are reseated on all boards :)  We may just get another 800, part it over until we isolate the defect, repair that, have two 800s, then sell one... but I'll flail a bit longer first.



#18 DrVenkman OFFLINE  

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Posted Fri Oct 13, 2017 6:32 PM

Have you tried reseating all the ICs in their sockets? Pins corrode over the decades and prying them half way out and pushing them back in scrapes it off. Also the 4050 CMOS chip on the motherboard is involved with video and it's fragile. Try a new one of those.


It’ll be a lot cheaper to just order a slew of replacement 74xxxx and 4xxxx logic chips from Digi-Key or your supplier of choice than to track down and buy a whole second 800.

Frankly, I’m surprised this one is so troublesome - it really sounds to me like something basic on the main board (like one of those discrete logic chips) is causing the issue. Hell, it may just be the reset circuit capacitor too (another ridiculously cheap thing to replace).

Having said that, I’ve had a 4050 hex buffer in the video circuit fail (as pointed out by ClausB above) but in my case, it was an 800XL and the failure didn’t cause a failure to boot - merely screwed up the video so that white text was black. Essentially the failure fubar’d the video output, not the basic operation of the machine.

#19 vputz OFFLINE  

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Posted Sat Oct 14, 2017 11:08 AM

Yeah, I'm puzzled myself to be honest.  Part of it is my lack of understanding of how the thing works together (I'm happy with basic circuitry, microprocessors, discrete logic chips, etc... but how they really work together so that I can understand the system and its boot process is a stretch for me, which is good and educational but I haven't found a nice description anywhere).  We actually have a pretty good "junk electronics" store in town that tends to have bins full of old ICs, so I may take my friend's son downtown and go diving just for fun; that's the direction I would prefer to take, but I think they're losing interest :)  I'll see if I can convince them to try that, though.



#20 vputz OFFLINE  

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Posted Sun Oct 15, 2017 9:04 PM

It's cursed.  Tried replacing every 74LSXX chip on the motherboard, the CMOS hex buffer, and the small chips on the personality board, all to no avail (the one exception being the "Transistor Array" on the motherboard since I wasn't sure I got the right chip.  Solid blue screen every time.  Sigh.



#21 1050 OFFLINE  

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Posted Yesterday, 4:50 AM

Boot process was never really covered by Atari so
reading the OS code assembly printout to step thru
it mentally at the very least is the only help
Atari offered.

One item of note for me at least is that out of
three 800 I got in a batch project, 2 had defective
6520 PIA chips. Today's replacement chip is 68B21
made by Hitachi/Motorola and since I have a couple
sticks of same it was no problem. But that's
pretty high odds for boot failure if you ask me.
And it was failing to boot a disk drive in my case.
Other details are a bit foggy since that was about
20 years ago.

Your situation may be that the 6520 is not letting
the boot process proceed past that point in the boot
process? Without a 2nd 800 you'll have to source
each chip recommendation out of pocket which takes
time and really is an open ended proposition since
you don't have conformation that it's a good
replacement chip in the first place.

Not the scientific approach here at all but I would
place Atari BASIC cartridge in the left socket so
that you might see a READY prompt on the screen if
it ever did complete the boot. You should be seeing
the memo screen, so I'm just covering my hairy rear
end with a doily here but it would make me feel less
naked so that's what I would do.

I'm not responsible for your mental images as a
disclaimer.

So now I'll have to look up what transistor array
you speak of, the 3086 is a known failure point
in 1050 disk drives.

#22 1050 OFFLINE  

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Posted Yesterday, 6:18 AM

Two of them, A104 and A203 or A204 depending on if
it's tuesday or not. CA3086, LM3086, LM3045 or
NTE 912 and ECG912. LM3045 is simply military
temperature range version of 3086 offered by
National Semi. CA is of course short for RCA,
sniffle, sniffle.

A203 or A204 is the RF modulator entirely, other
one is heavily involved in color system, wouldn't
hurt to change them for new but you would have
nothing if they were failed outright as is. And
you have something so they are working to some
extent. I wouldn't call it a critical mission
just yet.

#23 vputz OFFLINE  

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Posted Yesterday, 3:17 PM

Thanks a bunch!  The LM3086 is what I picked up hoping it would work, so I could still try that; thanks for the verification.  I'm using composite video out so I'm not suspecting anything from the RF mod (but I no longer suspect video since I'm not getting any chirps on the speaker either).  PIA seems doable too; can grab a 68B21 for around $10 including shipping, so if that's a drop-in replacement it may be worth a try (but that'd be the last chip on the motherboard we'd replace...).  Hmm.  We'll also watch the sales and see what we can learn about working 800s :)



#24 1050 OFFLINE  

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Posted Today, 6:32 AM

Yes, the 68B21 is straight drop in. A104 is running
the speaker too. Often Q106 is found bad which will
silence the RF audio, 2N3904 works there IIRC.

68B21 are quite common in the used parts stores as
you mentioned too. My local one looks up the price
on eBay while you watch and then quotes you the
highest price found there. Nobody buys those ones.
And telling them that just seals your fate.

Just sorry I didn't get to you before you changed
out everything else...





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