Space Invaders Deluxe (Midway) - Garbled graphics

ccie38296

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Donor 3 years: 2019, 2024-2025
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Hi! I recently acquired a Midway SID Cocktail. It boots OK, but sometimes - maybe half the time - either during the attract mode or during gameplay, horizontal lines will appear across the Invaders and clutter up the screen. They actually *seem* to interact with the missles as well.

IMG_4264.jpegIMG_4244.jpegIMG_4262.jpeg

I installed a Braze multi kit in part to get the diagnostics and to see whether the ROMs may be suspect, and according to that:
  • RAMs test OK
  • Sometimes on boot (maybe 20% of the time), the BIT test fails. When this occurs, there's a continuous tone that is one of the "thumps". Using the diag menu to play the tones, it appears to the "Thump BIT2" tone.
  • On one boot, I got an INP error as well. Assuming that is "input," maybe indicating a stuck input. I think the leaf switches on the paddles controls need adjusting, they're super touchy.

IMG_4268.jpeg

I've only observed the graphic glitches in SID but haven't spent a lot of time trying other games from the kit. To be clear, the graphic glitches occurred BEFORE installing the Braze kit, and they persist AFTER. I'm assuming that clears the ROMs as being the culprit.

Sometimes the glitches appear during attract and remain, sometimes they clear after the next attract cycle starts. When playing, sometimes I can play several levels without a problem, other times they appear mid-level early on.

I checked voltages at the motherboard edge connector and got:

+5V = 5.19V
-5V = -5.1V
+12V = 12.5V

But hey, it looks like Earl identified this problem way back in 1981!


IMG_4270.jpeg

I read that as:
Pastime Pub [maybe?]
Scrambled Picture
What else….
3-31-81 Earl


Any pointers on what to check next? I read a thread that seemed similar where apparently the fix was the chip at A5. Not sure what I should be checking on there. I have a logic probe and a very entry level oscilloscope, but I'm not good enough understanding the schematics to be able to figure out what to be looking for.

How about for the BIT failure? I saw the Joe's Video Games video where they found the 556 timer chip that drives the Thumps was bad. Any tips on testing/verifying that chip or something else in that chain to see why I'm having a problem there?

Would love to take a crack at repairing myself rather than send the boards out, I just need some pointers on what to investigate.

Thanks!
 
notice how the static images are good. only the moving images that get passed through the shifter are effected. I would make sure the shifter legs are cleaned and the socket is good. it appears control circuitry is working for the shifter. Another thing to consider is the data out from the shifter goes out to ABCD3 four separate 74153 chips where it is combined with all the external inputs from the control panel into the mx data stream back to the 8080 board. In summary I am saying the mb14241 might be giving you problems or the 153 chips might be a problem. When you look at inputs to mb1421 in your diagnostics give a quick look at the data signals going into it because the interconnect between the 2 boards might be hinky.


ps just for information. space invaders video is integral to the operation of the program. it is how the game knows where objects are by seeing that there is data at a position it wants to write too unlike newer raster games where the game runs independently of the video and objects are kept track of in the game ram and the video ram access the video rom to display certain blocks of data from the video roms. video in space invaders reads out ram data in synchronization with the scan rate of the monitor which as the ram has data in it it turns the electron beam of the black and white monitor on and off as it is scanning the screen
 
Awesome, thank you all! I'll start investigating around the shifter and related chips.
 
Picking back up on this topic, I was hoping someone here could help me understand the SID schematic relevant to the shifter and the 74153 chips. As I investigate this issue, I want to know what I should be seeing on the pins of the MB14241 shifter and the 74153s. Like, I see that pins 1-8 of D5 are Data lines (I assume from the "D" designation), so I know what I should be expecting there. But pins 16-23 that head to the 74153s in ABCD3 are labeled on the schematic as like P32/P33, P34/P35, etc. What does the "P" designate, and what should I be expecting on those pins when poking them with a logic probe (or oscope)? I am guessing those are the outputs from the shifter to those 74153 chips, so I would imagine I would expect "data" type behavior on those?

When looking at ABCD3, it looks like 2 & 14 are address lines (A8, A9) so again, I think I know what the logic probe should show me there. The chip datasheet calls them "A" and "B" so I'm guessing they are a selector or address register of come kind? Pins 7 & 9 all go to lines labeled as "MX" lines on the schematic. What does MX mean? On the chip datasheet they are marked "1Y" and "2Y." What should I expect to see with the logic probe or an oscope?
Really just trying to get better at understanding the schematics here and seeing labels I'm not familiar with.

Lastly, the ABCD3 chips are labeled 74153. I see Mouser has SN74LS153N chips in stock. Are these compatible replacements? The datasheet for those (https://www.ti.com/lit/ds/symlink/sn54ls153.pdf) appears to be originally from 1974 and references SN74153, saying it's obsolete but groups the SN74LS153 with them and shows them with the same pinout. Any reason I can't use them, assuming I may need to replace one or more of those chips?

Sorry, a thousand questions, I know. I'm really trying to get better at understanding this stuff but I am hoping the collective wisdom of KLOV can give me a couple pointers. Or point me to some resources I can learn from. Thank you!

IMG_3170.jpeg
 
I know what MX means but for the life of me, I cannot remember. Yes the 74LS153 are compatible. I'm unsure of the other answers.
 
I know what MX means but for the life of me, I cannot remember. Yes the 74LS153 are compatible. I'm unsure of the other answers.
Thank you! I asked a zillion questions, so even one or two answers helps!
 
I've only worked in a few of those boards. There's other people here that are much more knowledgeable on these than I am.
 
\I know what MX means but for the life of me, I cannot remember. Yes the 74LS153 are compatible. I'm unsure of the other answers.

Multiplexed

It's only driving to another bank of 153s, so LS153 should have enough drive strength.

It's still more likely the MB14241 itself than one (or more) of the 153s.
 
the mb14241 should have activity on all pins but the bar reset the gnd and the vcc pins. the 153 should be recieving data from the mb14241 you should see control signal on the 153 that switches from sending video data to the main boar down the mx data stream and also the input from the controls down the same stream. you could be missing some data from the shifter or maybe a whole bit is not making it through the 153. I am telling you in the most general terms that might give you results. maybe broken trace in that data path
 
the mb14241 should have activity on all pins but the bar reset the gnd and the vcc pins. the 153 should be recieving data from the mb14241 you should see control signal on the 153 that switches from sending video data to the main boar down the mx data stream and also the input from the controls down the same stream. you could be missing some data from the shifter or maybe a whole bit is not making it through the 153. I am telling you in the most general terms that might give you results. maybe broken trace in that data path
Thank you! This is extremely helpful. I will probe out those chips as soon as I get a chance.
 
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