7800 only working in 2600 mode - expert help needed!

[alortegac]

My new challenge a 7800 that only runs 2600 games. It does loads the 7800 logo, then blinks with vertical lines. I have checked voltage regulator and cleaned cart reader. All looks ok. 2600 games play just fine. The on/off button does not work well, only sometimes turns on or off. I will replace it.

 

Any ideas on where the failure might be are welcome.

 

Thank you all!!

Edited May 6, 2016 by alortegac

[+Stephen Moss]

Have you brought a truck load of dud 7800s?

 

The fact it plays 2600 games means you can probably eliminate most of the systems from you search because if the problem was in any area that is common to playing both 7800 and 2600 games, i.e. power, 6502, RF modulator the problem would not be isolated to issues with 7800 games. Thus my initial thoughts would tend towards either the TIA/MARIA switching logic or MARIA itself.

 

Been a while since I used a 7800, if the boot logo normally appears in all three modes (2600 Cart, 7800 Cart and No cart) is it displayed correctly and for the correct amount of time (until it trys to run a game) in all three modes?

Does the problem occur when running the internal game or just with 7800 carts?

Does it appear to make any attempt to play either the internal game or 7800 cart or does it appear to just get stuck at the 7800 logo.

 

[alortegac]

Hi Stephen, Hope all is well!

Well, this one I bought cheap from Ebay for parts or repair....since I have a bunch of parts now from the other 2 units that died....... Honestly, I did not know I was getting into until I tested it.....

 

Boot logo appears in 7800 mode only; With a 2600 cart, the system goes right to game (no logo screen). If "NO cart" the system goes to a black screen or vertical lines, similar to the "no cart" Atari 2600 .....(there is no built - in game in NTSC OS version). In 7800 mode, system goes to logo screen before running the game. Yes, it appears is trying to switch on to run the game when it gets stuck in the logo screen then it begins to blink and the vertical lines appear as in the "NO cart mode"..

 

I was doing some research, I found the following:

 

"The 7800 runs in three distinct modes: Internal Maria mode, External Maria mode, and 2600 compatibility mode. The system automatically decides which mode to run in, depending on what kind of cartridge is plugged into the slot when power is turned on. Initially the system comes on in the Internal Maria mode, and the cartridge cannot be accessed. The program in the on-board ROM then determines if there is anything plugged into the slot. If there is a 7800 cartridge plugged in, the system switches to External Maria mode and the cartridge runs. If there is a 2600 cartridge plugged in, the system switches to 2600 compatibility mode. If nothing is found in the cartridge slot, the on-board ROM simply executes its built-in game program. The logic of this mode-switching is performed by the 6-bit 74LS174 latch (U11) and the 74LS32 OR gate (U12). The latch is set to all 0's on power-up, disabling the EXTernal cartridge (and enabling the internal ROM), turning Off the Maria ENable, TIAENable, and the latch disable line (pin 2 of U11). The processor may continue to write to this latch, changing modes, as long as the latch disable line is low. When it writes a "1" into that bit, the latch is locked, keeping 2600 cartridges from writing to it. The EXTMEN signal is ANDed with the U11 pin 5 output to form HEN, allowing future devices to control the system through the video expansion port."

 

 

 

In this case, the system is deciding correctly which mode to run in before it crashes/.....Since the bad power button was causing a lot of false resetting (on/off), is it possible that the switching "to EXTERNAL MARIA" process U11/12 is damaged?

 

Thanks for your help!!

 

Al

Edited May 6, 2016 by alortegac

[alortegac]

Update, replace both U11 and U12. No change. thanks!

[alortegac]

Update it was not MARIA.....replaced and no change.... may it be SALLY?? thoughts...

Edited May 7, 2016 by alortegac

[+Stephen Moss]

Difficult as I cannot find enough information on how the 7800 starts and detects cartridges, shame the NTSC 7800s do not have the internal game as that could of helped indicate possible 7800 cartridge read issue - I wonder why they do not have it.

Although you have changed a few IC's that does not completely eliminate the problem from being associated with those parts as there could be a broken track somewhere.

 

Considering the fact that it plays 2600 carts, does not enter "No cartridge" mode with a 7800 cartridge and displays the Boot logo I could be wrong but I personally think with a reasonable amount of certainty that...

Power - OK

Cartridge Detect - OK

TIA - OK

U3 & RF Modulator - OK

RAM - OK

Sally - OK

Primary Clock oscillators - OK

 

If the 7800s start sequence is detect cart type, switch to appropriate cart mode and if 7800 cart switch to External Maria mode, load boot logo from ROM then load 7800 carts is then I would I think with rather less certainty than above that...

All (or majority) of Glue Logic and other support components - OK

Switching between Internal MARIA, External MARIA and TIA mode - probably OK (if Ext MARIA mode required for displaying boot logo).

MARIA - Probably OK (if it uses DMA for Boot logo display) and you say you replaced it with no change.

 

And I have no certainty at all that the system ROM is not defective. You could try swapping the ROM out to see if that is the issue.

 

Other than that I assume you retrieved the information you quoted from the 7800 Atari Theory of Ops documents in which case I suggest having a look at the second paragraph down from that and RepairFlowchart_PG5 incase as it is possible that if MARIA uses DMA to retrieve and display the Boot Logo with Sally Halted that Sally is getting stuck in a halted or wait state for some reason (i.e. broken track is preventing MARIA from unhalting it, Q13) when Maria needs Sally to run a Cartridge (Sally not halted for 2600 carts).

 

Not sure I can help any further as I don't think I know enough about how the 7800 works and the by no means comprehensive 7800 Atari Theory of Ops documents appears to be the closest thing to a service manual available.

 

 

 

[alortegac]

Thank you very much for taking the time to respond, you effort is appreciated. Yes, I retrieved the information from that document. On the parts I replaced I did check continuity for each of the pins to their next connection point, but I know what you are saying the issue could be somewhere else.. I knew it was a long shot trying to fix this console. I will do a more thorough inspection to try to find cold solder or broken tracks. Perhaps trying to swap ROM from the other consoles to see if that helps. Check the document as you suggested.

 

I tested a bit more after replacing Maria and I noticed another issue that I did not see before (not sure if it was present or I caused it). Although 2600 games play well every time, while on TIA screen protection mode with TANK game a buzzing sound and clear thick 1 inch horizontal flow line appears at the top of the screen only sometimes. This buzzing sound and white line stay there until the system moves to the next screen then disappears until the next time. This only happens sometimes, not on every screen change. The game plays perfectly once I press reset, and this issue only occurs in TIA screen protection mode and not in every color screen. I do not know what to make of it..

 

It seems that my collection of parts for 7800 and Atari IC's will keep growing and growing......

 

Many thanks!

Al

Edited May 9, 2016 by alortegac

[alortegac]

Stephen,

 

Update from this project:

 

I replaced the cart reader J1, just in case there was something shorting or not making right contact with 7800 games. (no change)

Socketed 6532 RIOT chip to test it because as I said it is showing a white horizontal line when TIA is in protection screen mode. (NO CHANGE)

I replaced Q13 - (NO CHANGE)

 

Next steps.

1.- ROM socket to test w/replacement.

2.- Sally socket to test w/ replacement

3.- TIA Socket to test w/replacement.

 

.

Edited June 9, 2016 by alortegac

[CZroe]

I read that certain power adapters provide enough power for 2600 mode but will have issue when booting in 7800 mode. Have you tried a better AC adapter?

[Andromeda Stardust]

It is fun to read these threads of people attempting to get busted hardware working. Sometimes I think people take these projects on just for the challenge of it. That said, I would love to hear what you find to get it working...

[alortegac]

It is for the challenge of course, that particular 7800 is KAPUT now.....but I just got another 7800, apparently in good working condition but with a similar failure that the one from this thread: """.Although 2600 games play well every time, while on TIA screen protection mode with TANK game a buzzing sound and clear thick 1 inch horizontal flow line appears at the top of the screen only sometimes""". ( just this issue)

 

 

..I find it interesting...... I think it was a factory problem. This one does run 7800 games well. However, only in 2600, in some games (like COMBAT) and only when in TIA screen protection mode in ON a white horizontal interference line appears making a buzzing sound, then the next screen protection frame comes up and the line and sound disappear......also heavy compatibility issues with IMAGIC carts.

 

NO CLUE on why this is happening, all looks ok, and from my previous attempt of fixing this same problem, I think it may be an unreported factory issue..... like a recall. Have someone heard of this problem? I will take a picture of the screen and the label on this Atari.

 

 

thanks!

Edited August 24, 2016 by alortegac

[Jinks]

What is tia screen protection mode in ON?

[CZroe]

I recently obtained one that I thought would only play games in 2600 mode. 2600 games would boot but, after the Atari logo, 7800 games would either show a blank screen with a couple oddly placed vertical lines or, more often, a distorted rolling screen where you could sometimes tell that the same vertical lines were wrapped into diagonal rolling bars. That rolling screen would also sometimes show as monochrome/B&W.

 

I gave up after tinkering with it for most of the day but then I encountered a 2-port 5200 from the same lot which would also scramble and roll. I left it on for a few minutes while setting up my 4-port for some cross-console troubleshooting and when I returned the game had actually started, though it was in black and white. A few minutes later and color returned. A few more minutes and I couldn't even be a glimpse of B&W even after fully draining the power and leaving it overnight.

 

Remembering that the 7800 issue looked similar and was also sometimes B&W, I tried the same thing with it. It worked! Unlike the 5200, it didn't just boot when warmed up enough (had to cycle the power) and it also didn't seem to stick (allowing it to cool down required another ~15min warm-up).

 

I'm pretty sure this is power-related. Perhaps bad capacitors or something. Considering that low-amperage power adapters have also been known to cause the issue where 2600 games work and 7800 games don't, we were clearly wrong to dismiss the "power" possibility in the first reply and we may have been barking up the wrong tree ever since.

Edited May 5, 2017 by CZroe

[CZroe]

Update: After further review, my "2600-only 7800" that became my "2600-only 7800 without 15+ minute warmup" is now "2600-only 7800 without 15+ minute warmup OR without removing lower RF/EMI shield."

 

https://youtu.be/XNOQk1nQaEQ

 

I definitely see more than a few people saying that their 7800 only plays 2600 games, so I wonder how many other "2600-only 7800" consoles have this particular issue?

[DrVenkman]

Update: After further review, my "2600-only 7800" that became my "2600-only 7800 without 15+ minute warmup" is now "2600-only 7800 without 15+ minute warmup OR without removing lower RF/EMI shield."https://youtu.be/XNOQk1nQaEQ

I definitely see more than a few people saying that their 7800 only plays 2600 games, so I wonder how many other "2600-only 7800" consoles have this particular issue?

Have you tried replacing the MARIA chip?

[CZroe]

Have you tried replacing the MARIA chip?

I guess I could swap with the other console, but where would I get a spare? Also, desoldering the chips is a pretty big undertaking for me, but I'm willing to try if there's a good reason. Is there a working theory for why the MARIA chip would cause this?

 

Also, I suspect that this unit may be from the early 1984 test market... or a "version 1.5," if you will. It came with a Rev A manual that references it being a potential home computer platform (gone from the Rev B and Rev C manuals) and the case has an opening near the expansion port (though still unpopulated) and a smooth area in the textured base (presumably where it would dock with something underneath). Wikipedia says that the 2nd version has an "indentation" in the shell where the expansion port would be, but mine has a straight-through open hole that leads right to the PCB. It actually looks like a good place to mount upgraded video connections without even cutting the case or RF/EMI shield.

 

 

It has a Rev A PCB where my other unit has a Rev B PCB. I'm going to take a peek at the date codes on the chips next time I'm inside, though I imagine that there were some old parts/units when it re-launched in 1987.

 

So what it boils down to is: was the "indentation" Wikipedia describes an actual opening or something else? If it is an earlier model then I'm keen on keeping it as close to stock as possible.

Edited May 10, 2017 by CZroe

[DrVenkman]

To answer your questions sort of in order - the MARIA chip is what makes it a 7800 rather than a 2600. If it's fubar, it won't play 7800 games at all. Another possibility might be the system BIOS or the SRAM chips, since those are also 7800-specific. Best Electronics still sells MARIA chips, I believe.

 

If you had a 1984 production machine, your serial number would begin with either "EP-" or "AT-84." Your machine would have a fully-socketed or nearly so PCB, and would have the Expansion Interface installed on the left side of the board. See below for a photo of my AT-84 board.

 

 

[CZroe]

To answer your questions sort of in order - the MARIA chip is what makes it a 7800 rather than a 2600. If it's fubar, it won't play 7800 games at all. Another possibility might be the system BIOS or the SRAM chips, since those are also 7800-specific. Best Electronics still sells MARIA chips, I believe.

 

If you had a 1984 production machine, your serial number would begin with either "EP-" or "AT-84." Your machine would have a fully-socketed or nearly so PCB, and would have the Expansion Interface installed on the left side of the board. See below for a photo of my AT-84 board.

 

7800board.jpg

 

I did notice that the Rev A serial number was significantly higher than the Rev B unit, but the expansion port definitely isn't populated. My Rev A board's switches are the same style as yours where my Rev B switches are the lower quality switches that crap out like my power button (look completely different). I can see in my video that at least one chip is socketed but it doesn't look like all are (I'll take another look after work).

 

Thanks for your help.

[CZroe]

Can someone upload a quick YouTube video of this? I don't know how long it's going to be before I am equipped to load this.

[Jagosaurus]

Maybe some replies from my previous 2015 thread on the same subject will help. I included multiple pics.

 

http://atariage.com/forums/topic/220365-7800-plays-2600-games-not-7800-games/?fromsearch=1

 

I later had a second 7800 with a different set of issues :/. Sticking to 7800 emulation since.