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800XL U20 IC shows most pins low


KLund1

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I'm about as far as my ability can go in finding why this rev D board shows a black screen. I have tested all the socketed IC's in another working rev D board I have. They are all good. Other symptoms are the CPU gets hot, no ram gets warm, GTIA gets warm not hot like cpu, other ic large/small get a bit warm like I think they should. U7,8 not socketed, but test right with probe, and do not get warm.

Using SAMS, I have been checking IC's with a logic probe. U20 dose match SAMS. all the pins that should pulse, are showing low. The IC is good in the other machine, and in an IC tester. So I think it must be some discreet component near by.

I am unsure how to proceed in further testing with just my probe and multimeter. (no scope)

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U20 dose match SAMS. all the pins that should pulse, are showing low. The IC is good in the other machine, and in an IC tester.

 

Per Sam's U20 is the 4050, which is in the video circuit. It makes sense that it's not showing any activity if the system isn't booting.

 

(Please, take this in the spirit with which it is intended: don't refer to IC's in common Atari machines by socket number - those numbers can vary between models, but *most* Atari computers have *most* of the same basic IC's. So if you'd said, "the 4050 isn't showing activity," people would be able to respond quickly without needing to refer to a schematic of parts list to try to help).

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If the CPU crashes you should still get a video signal (even if black, you should get csync) so it's possible the CD4050 is bad.

 

Well, he said the chip was good in his other machine, so … ?

 

In fact, I thought he said all the socketed chips work when tested in the other machine.

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(Please, take this in the spirit with which it is intended: don't refer to IC's in common Atari machines by socket number - those numbers can vary between models, but *most* Atari computers have *most* of the same basic IC's. So if you'd said, "the 4050 isn't showing activity," people would be able to respond quickly without needing to refer to a schematic of parts list to try to help).

Noted. Thanks, will do better in the future. I appreciate such advice.

 

The 4050 is in working order, as is the GTIA. I suspect other near by parts, transistors, diodes, resistors, etc may be bad. what and how to test.

 

I get a black screen on power up, no sound.

 

Good idea on the SIO test. No drive access on power up. Basic and OS chips known to be good.

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My suspicion is ANTIC U7, so I wonder if you get any

pulsing from pin 29 /REF signal. The MMU is using that

as a clock input and thus the entire machine is

dependent on it. ANTIC is actually the only chip that

doesn't need a chip select signal (derived from MMU)

because it's the one running the show. Check also for

pulse on pin 7 ANTIC U7 another very important clock.

Although ANTIC isn't the source for that one, it does

create the /REF signal however. If that is absent and

pin 7 has steady pulsing, it doesn't look good for ANTIC

in my opinion.

 

I've been wrong before though, and you can expect many

other opinions as well, each of us looking for the

quickest way to prove their hunch in turn is all this

amounts to now until someone stumbles upon something

significant that clearly gives a direction to proceed

in.

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I much appreciate everyone's input! Thanks!

 

The clock shows high on one pin and low on the other. Compared to working machine and it was the same.

Antic pin 27 pulses, pin 7 only high.

Again all IC's, large and most small, have all tested good in another working machine. (U24, U25 far right on board, not socketed, but logic probe correct. U26,27 (LS158's) near RAM not socketed, but probe correct. Ram not socketed, but none get warm/hot after 5 minuets, Ram probe correct. I can put any of these into sockets if warranted)

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CPU is getting hot? How hot is hot? compared to other Ataris?

check around it.

 

no video from rf as well as luma chroma or composite?

 

power is clean and no dead capacitors? power supply works with other Atari's? Without distortion, humm, or hum bars.

 

if all moveable chips have been cleaned and reseated you only have a couple soldered items to check...

 

Memory is all good? you've swapped or moved them?

 

does a disk drive connected to this try to boot?

 

any audio of any kind, pops or clicks during what should be boot time?

 

did you try a star raiders or other cart that takes over the computer early (so called diagnostic carts)

Edited by _The Doctor__
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Thanks for all this .... :)

 

CPU is getting hot? How hot is hot? compared to other Ataris?

check around it.

Just about too hot to touch after about 5 minutes. But I only left it on a couple times that long. Did not want to damage parts.

 

no video from rf as well as luma chroma or composite?

Nothing RF just same black, My cable has only chroma, composite, nothing from each.

 

power is clean and no dead capacitors? power supply works with other Atari's? Without distortion, humm, or hum bars.

Clean? How to test caps on the board? PSU works great on other XL/XE machines

 

if all moveable chips have been cleaned and reseated you only have a couple soldered items to check...

Check solder points? How? Which ones? Do you mean to reflow some connections?

 

Memory is all good? you've swapped or moved them?

Not tested fully, not socketed. Logic probe tests match SAMS, none get hot

 

does a disk drive connected to this try to boot?

No disk response at boot

 

any audio of any kind, pops or clicks during what should be boot time?

None

 

did you try a star raiders or other cart that takes over the computer early (so called diagnostic carts)

Yes, same black screen. Used SR. fmi, what other carts do this?

 

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My bad, I've misread the schematic and confused ANTIC

pin 7 with pin 29 in prior posts. Which of course

causes a cascade effect on other pins called out.

 

Pin 8 ANTIC is /REF signal, it needs to pulse as a clock

signal output to MMU for chip select use.

 

Pin 29 ANTIC is phase two clock, it also needs to pulse

as a clock signal input to ANTIC. Ram chips clock

signals would be derived from this and since they are

cool, /RAS, /CAS might not exist to work ram into a

warm agitated state of readiness.

 

The clock shows high on one pin and low on the other. Compared to working machine and it was the same.

Can't know which clock you speak of here, but it does

appear to be a benign issue with testing results lining

up with a good machine.

 

Antic pin 27 pulses, pin 7 only high.

Sorry, pins 8 and 29 now please. Pin 7 high is fine.
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Antic pins 8, 29 pulse

 

The clock I checked was Y1 (crystal?)

 

Recheck SR test. I forgot to plug in the monitor cable. my bad, sorry

I get many colored vertical bars for about 1/2 sec, then gray/blue vertical bars on the left. Also get one normal first bump sound,

 

post-15598-0-45586100-1540094026_thumb.jpg

post-15598-0-48790100-1540094054_thumb.jpg

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cpu to hot to touch is either a bad cpu or something pulling current through the cpu.....

check that ram! swap C060472 and its support chips

visual inspect solder joint no bridges or cold joints (magnifying glass helps)

you've got patterns so something lives...

no bulged or leaky caps?

then move on to through hole stuff

Edited by _The Doctor__
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Thanks,

I just went over a lot of the board with a soldering iron, added a bit to some joints that looked like could use more.

Now, on boot the screen is bright like pic 1 above, then fades of a darker shade, then jumps to pic 2. So I might have fixed something, or made it worse.

Recheck CPU in other machine, and it is still good.

 

You think I should unsolder all the ram and put in sockets, even though none of it gets warm/hot?

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if you have an extra ram you could try tension of chip legs and stack on ram chip one at a time.. if ram is not shorted but open.. it might identify bad chip... but the hot cpu says something might be shorted.. can't hurt to try...

I would think sockets are always a good thing just use double swipe sockets... :) but yes I always try to socket a broken machine...

 

Make sure no chips have folded under or shorted broken pins before during and after all work...

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IT WORKS !!!

 

I replaced all the ram with sockets. put in some known good ram it it boots right up.

ADD one more to the working list.

Thanks again everyone! I learned a LOT on this project.

Now it is of to the several dead 600's .....

I'm sure I will have more questions later, but on another tread.

Again, many thanks!!

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