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  #1  
Old 06-24-2013, 03:55 PM
Philly_pa Philly_pa is offline
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Default Comet - All Displays Blank!

Williams Comet, June 1985 machine. Replaced Master display board (MDB) because all displays went black/blank. All displays lit up for the first 3-4 power-on times. Now displays are all black/or blank again! I have the correct voltages getting to the MDB, however no digits or signs of power/activity appear on any of the displays. I've tried reading the schematics, and there doesn't appear to be any 1 component on the MDB that would prevent ALL of the displays to be all blank if the correct voltages are appled to the Master Display board. Could it be the Zener (ZR1) diode? But why would this component go bad on both the original & newly installed MDB's. If thats the case then the problem is up stream (on the Main CPU board). Any direction on resolving this issue will be greatly appreciated.
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Old 06-26-2013, 11:08 PM
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are both the -100volts and the +100volts at the power supply output? if one of these is missing there will be no displays
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Old 06-27-2013, 11:48 AM
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I had checked the voltage levels at connecter/input J11 of the Master Display Board On the 1st/original MDB and all three voltages +100 DC/-100 DC/+5 DC, checked good, yet nothing showed on the displays. Then I tested all the ribbon cables for continunity and they all checked fine. I have not re-checked the voltages since installing the 2nd MDB because everything seemed to work for the subsequent 3-4 power-up's/turning on of the game. After that the displays went dark again. I will re-check the voltages again later this evening. I am thinking it has to be related to U5 (the DIsplay PIA Chip) or its related circuitry on the CPU board.
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Old 06-28-2013, 10:56 AM
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Applied power & turned game on. The following information appeared while the game was in attract mode. I re-checked voltages @ MDB input J11. All voltages are as follows, +99.9 DC/-100.1 DC/+5.2 DC. Game has been unplugged & not powered up since Monday evening. However i turned on the game to check the voltages this morning & the player displays fired up normally. The credits/Ball display took about 3-5 minutes to illuminate. For some reason the last two digits on all of the player's displays are dark all of the time. I didn't put the game in play mode to see if that made any difference this morning. I am sure while I work on the game this weekend the displays will go dark/once again.
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Old 08-08-2013, 12:14 PM
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Ok, Game has been Fully Shopped & everything is in place Execpt the PF & Translite glass, and cabinet tilt bob/ball. Game is 95% playable. Here are the latest findings concerning the Displays.

Turned the game on & played about 10-12 Full games. Some times the displays lit & worked well, minus the last two digits on all four Player displays (must be a wiring issue here). Other times, the displays were blank with only the comma's showing on the 4 player displays. I've also noticed when only the comma's are showing the Credit/Ball display is blank as well.

It didn't seem that the ball count, amount of credits or player score had an effect on this occurance.

Game play functions normally whether the displays are lit or blank.

I would consider adding new LED displays, however if this is a wiring or board issue, I'd rather resolve it prior to paying for a upgrade that may have the same problems.

I need to check with DMM to see if all the correct voltages are STILL present at the times when the display's go blank (comma's only).

Suggestions anyone?

Last edited by Philly_pa; 08-08-2013 at 12:33 PM. Reason: Changed MDM to DMM
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Old 08-09-2013, 11:06 AM
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sometimes 1 bad display can prevent the other displays from working correctly. with the machine in display test mode dis-connect each display 1 at a time and see if the missing digits come back. also check idc's on mdb J8,J9,& J10 and mpu 1J3 for loose or bad connections. since all displays go blank and mdb has been changed , U5 PIA chip on mpu is bad or may have had battery corrosion damage. i install sockets on these so the new chip doesnt have to be soldered in , and makes changing easy.
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Old 08-14-2013, 11:28 AM
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Things have gone from Bad to WORSE!!!

First, I validated continuity on all connectors & wires. no issues found there. Tested all displays individually, no issues found here either.

So I ordered a new PIA 6821(U5, the Display PIA) chip from Marcospec along with the correct DIP socket.

New parts arrived yesterday. Removed the old PIA 6820 from CPU board, soldered in the new socket & carefully placed the new chip into the socket, remounted the CPU board into the back box, wired it all in and........!!!!

NOW the game IS DOA!!!! I get some type of 2540 error in the Player 1 display & the game sits quietly flashing this number. The Main CPU board has a 0 on its diagnostic display.

Ok I must admit, I didn't do any type of testing after soldering in the Dip socket. However I did make sure all the pins on the chip entered into the socket correctly & the chip is properly seated.

This is freaking killing me here...5 steps backwards to make 2 steps of progress.
Sorry I digressed for a moment!

Ok, I will remove the main board & new chip tonight, TEST all my solder connections & hope for some luck there.

Meanwhile I am accepting all suggestions again. Thanks Everyone.

A Special shout out to Pinball DAVE H who's always willing to rollup his sleeves & help. Thank You Brother!

Last edited by Philly_pa; 08-14-2013 at 11:31 AM. Reason: Forgot to add a line.
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Old 08-19-2013, 11:17 AM
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Making Baby Steps.....I guess.

Ok I totally don't know what has happened to my MPU board, but here's the latest weekend update.

I have removed the New 6821 chip & unsoldered the new socket. Cleaned off excess solder & then VERY CAREFULLY, Resoldered the Socket back into place. Re-inserted the PIA chip & still received the 2540 error in Player 1 Display.

So then I removed a Known GOOD PIA chip from my working System 11 Cyclone machine & placed it in the System 9, Comet MPU board. I still received the 2540 error in the Player 1 Display.

Then I placed the Comet's PIA chip into the Cyclone's MPU board & the Cyclone played as normal.

So that tells me I didn't destroy the new Comet PIA chip with a faulty/bad solder job & the new Comet chip is still functioning properly.

So my Comet problem has to be located elswhere on the MPU board. I've ordered a new U9/74154 chip & socket from Marco Spec. I'll see if this along with a fresh set of batteries will do the trick. If not, I'll give up & just have the professionals take a look at the board.

****Note**** The 2540 Error in the player 1 Display might have 2 additional digits/characters after the "0". The last two digits on all my player displays aren't working. This is the reason the New PIA (U5) & 74154 (U9) chips were suspect/ordered.
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Old 08-20-2013, 11:33 AM
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The Good, The Bad & the WHAT Now!!!

The Good:
I've changed the 74154/U9 chip & installed a fresh set of batteries. Something has to work now...Right????

The Bad:
I still received the 2540 error or code in the player 1 display, and the game was not active..So I thought.

Out of frustration I tried to push the manual up/down & the advance switches. Well I was totally shocked when the player 1 display changed to something different than the 2540 code I have been seeing. I scrolled the display via the switches, not really knowing what mode the game was in or displaying. So I just powered the game off & back on!

Wah-Lah!!! The game came on in Attract mode! I was able to add a few credits & play a game or three without any problems.

The "WHAT Now".... After replacing the two major components of the displays circuit on the MPU board, the Display's don't seem to just blank out/off any more, however the last two digits on all of the displays are still NOT working!

Can you say "Yeah!" followed by an "UUUUGGGHHH!!!!"
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Old 09-05-2013, 11:07 AM
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Still trying to figure out why the last two digits of all the Player displays aren't working. After replacing the PIA (U5) & 74154 (U9) chips, I am totally shooting in the dark here, so I've ordered a new 74LS245 (U23) chip & socket. If that doesn't resolve the problem, then I will be totally lost as to what transistor, resistor, diodes, electrode, etc. would only have an effect on the last two digits of the player displays.
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Old 09-13-2013, 06:20 PM
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if your machine has IDC female connectors that go to 1J1,1J2,and 1J3 you might consider changing them over to trifurcon connectors to eliminate possible wiring connection problems.
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Old 09-23-2013, 10:49 AM
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Ok so finally the new 74LS245 (U23) chip & sockets arrived. Took my time & soldered the new socket in place, Placed a new chip in to the socket & placed the Main board back into the backbox. Powered the Pin up & STILL the last two digits of all the player displays are NOT working!!! And to make matters worse, the overall intermittent disappearance of All the displays seemed to have resurfaced. I am totally at a loss here. I fear trying to replace any other component on my main board as it will give the appearance of hacking the board. Perhaps it’s just time to go with the new Digital displays.
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Old 11-03-2013, 12:00 AM
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Finally sent CPU board & Both of my Master Display boards out to a Preferred Pinball Repair shop . The D-10749 master display board I was using was setup for a Shuffle Alley and was set to run 6 players( 5 digit) and a credit/ball (4 digit) displays. It was missing the jumpers W1 - W8. Once the jumpers were installed, All 4 player display digits appeared as normal.

Although the CPU board & 2 Master Display boards were sent out, repaired, tested & returned ready for play, upon installation back into my Comet, NONE of my displays turned on until I was on the 3rd ball of the second game I played. Yes, I managed to add a few credits, and start the machine without any displays.

With assurance my Master Display boards & Main CPU board are all working properly, there is either something wrong/going bad on the Power Supply board or the wiring harness that prevents the displays from lighting when I first power up the Comet. However after several games yesterday, once the displays lit, they didn't blank out again during 2hrs of constant game play.

Confident a move to LED's will now be successful (no missing display digits).
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