Black Knight Sound issue

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What is the common point to begin troubleshooting this problem:
I'm sure it all ties together so I'll list chain of events.

1- Sound was not working for a day. player 1 through 3 commas had a hazy glow on displays.
2- game was moved in back of truck
3- boards reseated, sound came back on. displays were perfect. assumed this was fix
4- been working fine, and not moved or bumped since.
5- starting about a week ago, when turned on you hear a loud hum that diminishes within seconds in a tapered volume way. hum like the sound when a speaker doesn't have input on an old stereo/turn table
6- today the game was left on for a few hours. played a couple games and noticed the black knight was missing some random words here and there in sentences. speech was fine other than occasional missed word every 5 or 6 phrases.
7- after a few hours, played a game and discovered no sound. also noticed the comma's were glowing hazy again on player 1 through 3
8- turned it off a few hours and tried again. game plays fine, no sound, commas glowing hazy

This almost sounds like a bad ground somewhere but the game hasn't been moved at all to jiggle anything like the board seating. Although on one game I did give it a gentle side to side bump to unstick a ball which caused a tilt.

This was a home use game with minimal playtime and has almost zero repairs on it over the years. so the connectors are perfect, uncorroded and untouched. I don't see any obvious problems anywhere.

Any ideas where to start troubleshooting?
I'm hoping the display comma's help ring a bell and tie in an obvious problem to the sound with one of the experts around here.
Thanks a bunch.
 
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First thing that comes to mind is that re-seating the boards temporarily solved the problem. Re-seating connectors generally just identifies the problem, but bad connectors come back usually. Could also be cracked solder joints at the connectors, very common on those boards.

It's possible there is insufficient voltages coming in from the transformer or something, but unfortunately those are probably separate problems. Sound board has it's own CPU, power supply and everything on its board.

Have you tried running the two sound tests? One is that you can run it through the CPU board diagnostics, similar to the lamp and solenoid tests. The other is the dedicated test button on the sound board.

For the displays, I'd start with checking the +/- 100v voltages coming from the main power supply. or possibly the connector between the main display card and the CPU board.

-Hans
 
Thanks.
I'll get the schematics out and start confirming voltages.

The sound doesn't work in the diagnostic test where it counts up.
But the sound works when I use the onboard test button.

The key here is probably going to be- Need to figure out how the sound relates to the comma's hazy lighting on p1 through p3.
 
Hmmmm. Could also be a bad 6821 on the CPU board at IC36 or bad pull-up resistors at the resistor pack IC37. That PIA chip handles the sound outputs, and I forgot it also does the comma's in the display.

-Hans
 
+1 to all of Hans suggestions, it could also be the header pins on the MPU where the connector sends the data to the sound board. Reflow the header pins on the MPU and the sound board and repin the connectors for good measure as well.

It doesn't matter that the game was home use, the boards and connectors still degrade over time.
 
gotcha. I'll pull stuff tonight and look. Thanks again!

I've already been asked today by the grandson AND my wife why the pinball isn't working. (the backglass being off constitutes...not working.) he he he.
 
ahhh, I see.
pulled out the schematic for the CPU and clearly see what you are talking about with IC 36 PIA and IC37 resistor pack that go directly to IJ8 connector.

problem about has to be right in that vicinity doesn't it, and with the past intermittent nature I'm going to hope/wish/desire it to be a simple solder or connection problem. knock on wood.

Let's say IC36 or 37 end up being questionable. Is there a best source for finding those chips?

The Williams design is becoming more clear to me with every t-shooting experience. Feels much different than the Bally I was familiar with.....

UPDATE- just pulled the CPU and a lot of the pins all around the board look like they have solder pulled away from the pins. Especially IJ8. pins are non-corroded and shiny. Man, I hope reflowing all that stuff fixes it!

Hmmmm. Could also be a bad 6821 on the CPU board at IC36 or bad pull-up resistors at the resistor pack IC37. That PIA chip handles the sound outputs, and I forgot it also does the comma's in the display.

-Hans
 
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Cleaned up the pins with sandpaper.
Reflowed the pins including adding solder and removing with desolder braid twice.
They are clean and solid.

NO GO. hmmmph!

What is the best source for
--IC36, MC6821 PIA (part #5430-08972-00)
--IC37, 15 Dip Res/pack 10K ohm (part# 5019-09223-00)

I presume while I've got the shotgun loaded might as well replace the capacitors between ic37 and ground, too?

I don't intend on pulling this board out repeatedly in the future for flakey sound issues, wanna get everything possible in one fell swoop.
Thank you.
 
You should be able to test that resistor pack with an ohmmeter no problem at all.

The 6821's from Jameco are labled as Pull's, but last batch I got in were all brand new. 68A21's and 68B21's also work just fine. You can also use Jameco part number 2143558 which is a 65C21. Just plug in and go.

-Hans
 
Excellent.

--Found the ceramic capacitors at Great Plains.
--Ordered the PIA from Jameco. Got a couple 6821's as a pull and the one you recommended since they required a minimum $10 order. guess I'll see which one is brand new.
--Found the resistor pack Beckman 661-1-r10k at American microsemiconductor.

The joy of paying 3 times the shipping for what would otherwise be a very small package. :)

As far as measuring the resistor pack, i'm not confident in any results since this problem has gone and come twice now. i don't trust any of the components in that area..... and i'm concerned about the wear and tear on the cpu/driver connector every time i pull those boards. the flex has to be causing microscopic damage.... crappy ass design on that long connector...what were they thinking..... I did put some light silicone on the pins, the stuff that comes with car lightbulbs. seems like the connectors have less friction when installing/removing.
 
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HURRAY!

Replaced everything in that circuit on the CPU.
The Black Knight doth speaketh of challenges again!

Shouldn't have to work on that for another long while.
Thanks a ton for focusing my efforts in the schematics.
Cheers!

ps-the silicone lube on the connectors really takes the stress off the circuit board when messing with connectors. I'm sticking with that plan from now on....
 
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