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Posted (edited)

I've an Atari 400 that I had apart and late one night decided to install a UAV board.  I then connected the power board, inserted the CPU and RAM cards, then powered on.  No video signal.  I checked over my wiring to figure out what I had done wrong, but the wiring was all correct.  Then suddenly I realized something, with the sinking feeling as I would imagine someone might have when they hear the scraping of the belly of their airplane upon a gear up landing...I had installed the cards backwards!  I know what direction they're supposed to go, but had a lapse of attention at some point.  I quickly powered off, turned the cards around, and tried powering on.  This time had a video signal, but a blank screen.  All I can get now is a blank screen, or once in awhile, a green screen.  I've checked power and signals everywhere I can think of, but everything appears to be good.  I eventually figured out that all the ICs are good and it's not the power board.  The problem is definitely a component on the main board.  Power looks good at all cards and ICs (+5V, +12V, -5V), the frequencies are good, the O2 signal looks great at all appropriate locations, the CPU SYNC is going, address lines are all active, CAS and RAS are active on the RAM.  I can't figure what where the problem is, but it's definitely one of the mainboard components.  Anyone have any idea of what I might have fried that can cause this?

 

I forgot to add, there's also no activity on the SIO.  Fortunately, this was my own Atari I was working on.

 

Edited by Panther

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If you have installed the board backwards, it's highly likely you've damaged the UAV itself, also if you did the

mod that lets you put the 4050 into the UAV board so you still have RF, you would have damaged that chip too.

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Thank you for the input, but the UAV, 4050, and all other ICs are fine.  It was the OS and RAM cards I installed backwards by the way.

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1 hour ago, Panther said:

It was the OS and RAM cards I installed backwards by the way.

Oh dear :( , didn't think you could do that, but just looked at me 800 ones and it's the case that keys it,

so I see that if they don't have the case you can put them in the wrong way round, would have been so

simple when they were made to put a keying slot in the card and receptacle.

 

Not really sure what you could have damaged on those cards, looks like a lot of chip replacing or maybe

someone here has some spare cards left over from an upgrade maybe.

 

I would suspect you have damaged the RAM as you have put -5VDC on one of the Data lines (at a guess depending on how their numbering goes)

 

 

 

 

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16 hours ago, Panther said:

Thank you for the input, but the UAV, 4050, and all other ICs are fine.  It was the OS and RAM cards I installed backwards by the way.

400 has no OS card. You mean CPU card.

 

How did you determine that all ICs are good?

 

BTW, I once came inches from a gear-up landing. An airline pilot waiting for the runway called on the radio, "No gear!" We pulled up, got the gear down, and landed. Before he took off he said, "That guy owes me a steak dinner." Later I called the airline to find out who I owed the steak to but they wouldn't say.

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OS, CPU card, what do you want from someone who plugs things in backwards?  Yes, both the CPU and RAM cards are good.

 

Well, after testing all the signals and not being able to locate the issue, I finally just swapped the CPU and RAM cards, power board, and all of the ICs on the mainboard.  No change.  I then stuck the stricken computer's CPU card, RAM card, power board, and mainboard ICs on another mainboard and that one booted up fine.  So yes, I can tell you with absolute certainty that whatever the issue it is with a component on the mianboard.  I've tried going through testing the components in place (at least the ones that I figure might be related to the issue) but have not found any problems.  Yet, it is there, hiding somewhere.

 

Airlines can be so stuffy (no, I never flew for one).  His warning also saved himself from a long delay for the runway to be cleared.

 

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Posted (edited)

Looking at the non-IC components closest to the CPU and RAM slots, I see transistor Q104, crystal X101, polarized cap C192. Did you also check the 4050 IC, particularly the reset signal on pin 15?

Edited by ClausB

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Posted (edited)

The frequencies appear to be perfect, so I don't see it being an issue with the crystal or Q104 (and the transistor tested fine anyway), and I'm not actually using the 4050 since I have the UAV anyway, which did function with the other mainboard.  I will check on C192 after dinner, I might have checked it already, but I'll check again.

Edited by Panther
UAV, not UAC!

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3 hours ago, ClausB said:

Looking at the non-IC components closest to the CPU and RAM slots, I see transistor Q104, crystal X101, polarized cap C192. Did you also check the 4050 IC, particularly the reset signal on pin 15?

I ended up rearranging my work room after dinner, and now I'm planning on replacing all the furniture in there.

 

C192 looks like it could use a replacement, but it's not likely bad enough to cause a problem.  Like I said, the clock signal seems to be good.  I will replace it anyway tomorrow.  Pin 15 on the 4050 socket stays high, whether it's the UAV or a 4050 in it.  Keep in mind, it's not just video that doesn't work though, the system doesn't try to access the drive either.

 

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When you say blank screen do you mean solid black, or blue GR.0 with no text? I had the latter once and it turned out to be a bad CTIA.

 

Plugging the CPU board in backwards would put 12V on GTIA pin 24, LUM2. Are you sure the chip is good?

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Plugging the RAM board in backwards would put 12V and -5V on some data lines as mentioned above. Absolute maximum ratings say that the 4116 RAM chips can handle that briefly, but the LS244 cannot.

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I do have a video signal, but the screen is black, or as I mentioned, once in awhile it comes up green.  It's possible the the RAM I had in there originally was damaged, but I've been using a different RAM card.  If I take the CPU card, RAM card, power board, PIA, POKEY, OS ROMs, 4051s, 74LS10, 74LS42, and 74LS138, and UAV out of the damaged main board and place them in another one, it boots fine.

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Another what, 400?

 

I didn't see the 4050 in your list there. One of its gates drives the CPU reset signal, pin 15, so that could cause a black screen. You said earlier it's always high but it should go low briefly at power up.

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Another 400 main board.  The 4050 is not being used, but the system doesn't boot even with the 4050 installed.  I only see the 4050 pin 15 going high at power on, I do not see it drop low, but this is with the UAV installed, not the 4050.  As soon as I power on, I do see a variable signal from the CPU sync pin, so it does appear to be running.

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If you have an oscilloscope, look for the reset signal to go high around 25 to 50 ms after the +5V supply comes up.

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1 hour ago, ClausB said:

If you have an oscilloscope, look for the reset signal to go high around 25 to 50 ms after the +5V supply comes up.

Both lines went high at the same time and stayed high.

 

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On 3/30/2021 at 3:58 AM, ClausB said:

BTW, I once came inches from a gear-up landing. An airline pilot waiting for the runway called on the radio, "No gear!" We pulled up, got the gear down, and landed. Before he took off he said, "That guy owes me a steak dinner." Later I called the airline to find out who I owed the steak to but they wouldn't say.

Great you heard him over that horn blaring.....;)

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Posted (edited)

 

1 hour ago, slx said:

Great you heard him over that horn blaring.....;)

Piper Arrow. No horn.

Boy you pros have all the gadgets!

 

There is more to the story. I was instructor in the student's plane. We were practicing TOs&Ls when the tower asked us to go around for some reason and to keep the pattern tight because this airliner was waiting (smallish airport, maybe a dozen airline flights a day). We rushed and I didn't notice he put the gear up and we both failed to check it was down before the flare. Classic chain of events, abbreviated procedures, distractions, etc. And this happened during an FAA safety event! Taxiing in, I was sure they would pull my certificate. (And rightly so. BTW the student didn't wait around - he let me out and took right off!) But I got lucky at someone else's expense. There was an off-airport landing nearby which distracted the FAA guy. So I really owe another steak dinner to the poor guy who landed in a field!

 

When the airline refused to give me the pilot's number they did agree to give him mine. He eventually called and I gave him my thanks and my explanation and we made a date at the local steak house. He never showed.

 

It's hard to remember it all correctly after 30+ years and several retellings of the story. Maybe the Arrow did have a horn and I thought it was just a stall warning. Oof. As I recall all this, it really brings on the shame. Hopefully I learned a good lesson there and became a better instructor.

Edited by ClausB
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Posted (edited)
7 hours ago, Panther said:

Both lines went high at the same time and stayed high.

 

There's your problem. Bad 4050. Those old CMOS chips are very prone to static electricity.

 

Or maybe bad components behind it. That's C189, R181, R182, R183, and diode CR103.

Edited by ClausB
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I did check those components already, but wouldn't it just be a problem if the reset line was held low?

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Posted (edited)

No, reset has to stay low for a bit after power comes up, so that the CPU and peripherals can reset their sequential logic properly.

 

So set your scope to trigger on it and look for a delay of a few tens of milliseconds.

Edited by ClausB

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I probably need a better oscilloscope.  Good thing I ordered a new one already, it will be here Friday.  Meanwhile, if this is the cause of the problem, wouldn't it be possible for me to manually trigger the reset and have it boot?

 

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Posted (edited)

Yes, possibly. Just make sure it has a clean edge. If it's a switch, avoid key bounce.

Edited by ClausB

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No luck on that, there was no change to the image.  I will go through the rest of the mainboard components, though some of them I'm not at all sure how to test.

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