(Topic ID: 162946)

Flash Gordon - target causes TILT


By Pafff

2 years ago



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  • 34 posts
  • 10 Pinsiders participating
  • Latest reply 2 years ago by Quench
  • Topic is favorited by 4 Pinsiders

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#1 2 years ago

Hello Pinball World!
I own a Bally Flash Gordon (1980, with AS 2518 35 MPU) and I have a problem which causes headache… I try to describe it here.

Hitting a certain round target causes TILT. In test mode, the target usually causes a "07" display (TILT) instead of "15" (the correct code), but sometimes it is displayed correctly one time, then wrong again:

  • the target is linked to the MPU together with all playfield switches on connector J2, pins ST1 and I6
  • the TILT assembly which is involved in the problem is on connector J3, pins ST0 and I6.

At the beginning, it happened only when the pinball was "warm", now it happens all the time.

The capacitors and the diodes on both switches as well as the 6820 are OK (I replaced them). Using freeze spray on all the components on the MPU board as well as C and CR of the switches does not help (but sometimes causes a "reset" of the pinball).

I have tried many many things, to make it shorter I describe the "minimal" configuration under which the problem happens:

  • all microswichtches are OFF
  • I have disconnected everything on J2 and J3, except:
  • on J2, I put a diode directly between ST1 and I6, to simulate the target without being influenced by the playfield switch matrix
  • on J3, I put two wires on ST0 and I6 and link the with the tilt assembly (the big one on the left side with the hanging knob and the ball in the cage, which I disconnect from the "original" wires (but using the normal wires makes no difference).

Then, in this configuration:

  1. the target is still detected as "tilt" but
  2. if I replace the tilt assembly with the capacitor and the diode alone, the target is detected correctly
  3. in configuration 2, if a pin of the capacitor or of the diode touches a "big" metal mass, like the cage around the hanging knob, the target is detected again as "tilt" even if the metal mass is not fixed to the cabine.

Look how ST1, ST0 and I6 (here named "PB6" and measured separately on J2 and J3) look like:
FG_ST1_PB6_ST0_PB6.jpg
When the problem occurs, I6does not go down to 0V after ST0 goes low (and ST1 high), but stays at about 1-1.5V instead. When the problem does not occur, I6 goes down to 0V. Playing with the value of the capacitor on the tilt assembly has an influence on the level at which PB6 goes low.

I notice that the ST0 and ST1 signales (PA output of the 6820) are a total mess and look totally chaotic. I would expect all STx to come cyclically, one after another but I observe that sometimes, they are high together. I don't know it this is "by design" (which I don't understand), or if it is the symptom of a problem.

I don't understand what happens. The problem must be on the MPU board, but there are so many diodes, capacitors and resistances that I have no idea where to search. Replacing all on the MPU board will only cause more problems because of all the soldering works required. I wouldn't like to replace my MPU board with a modern universal MPU because I like the idea that my pinball is "original".

Help!!!
Thanks and regards
Patrick

#2 2 years ago

Well that's one hell of a problem report.

#3 2 years ago

... it just fits to the number of hours I have been investigating and the number of hair I have lost

#4 2 years ago

you have a switch problem.

Check every switch in the matrix to see if there are other switch troubles.
Report back with results.

Inspect all switches, to see if a diode is shorted across the switch leg.

#5 2 years ago

Kinda related maybe. My Fireball II would not recognize when a ball drained. The switches checked ok. Wound up being a loose wire on one of the button targets. When fixed, game played normal again. I suggest looking over the game for loose wires/broken connections.

#6 2 years ago
Quoted from Pafff:

I notice that the ST0 and ST1 signales (PA output of the 6820) are a total mess and look totally chaotic. I would expect all STx to come cyclically, one after another but I observe that sometimes, they are high together.

Programatically, the strobes are cycled but they're switched at the same time, i.e. the moment ST1 is released, ST0 is activated. There's a single write to the PIA that changes the state of both signals simultaneously.

What happens if you swap the I6 and I5 wires on the J3 connector, so the tilt mechanism now becomes the start button (Sw06)? Do you get this shadowing when you activate the left flipper return lane switch (Sw14)? And now what happens when you hit the target switch (Sw15)?

#7 2 years ago

Thanks to all of you for your advice. Individually:

@Quench
If you look at the right of the oscilloscope screen dump, you can see that ST1 and ST0 are both high exactly simultaneously (i.e. switched high and then low at the same time, not one after the other). At other times, one can be high for a long time with the other pulsed. It really looks like havoc and not a proper cycle for reading a matrix. Do you think it is normal? I would be very interested in knowing how the CPU uses the 6820, in which sequence it activates the STrobes on PA, at what time it reads the inputs on PB and so on. Any idea where I could find this information without disassembling the ROM?

Then I wanted to try you idea of linking the tilt assembly to I5 instead of I6, and connected again the tilt assembly's capacitor. Then I noticed that now Sw07 is active about 50% of the time (in test mode, one second 07 is displayed, the next second 0) even with both J2 and J3 disconnected (except ST0 and I6 on J3), I don't need to depress Sw15 (or simulate it) any more!

I then connected the tilt assembly to other combinations of ST / I, on J2 and J3. Result: it is always detected as closed and displayed as the corresponding switch.

A look at ST0 and I6 when the capacitor is connected, yellow=ST0, blue=I6, the cycle in the middle is probably the "right" one, i.e. when ST0 is high I6 is read, notice the intermediate level of I6 (about 1-1.5V):IMG_0442.JPGBy the way, what I call "havoc" on ST0 can be seen on the left side of the picture. It doesn't look like a clean strobe for a switch matrix.

And the same without the capacitor, I6 is correct at low level:image1.JPG

So I'm more and more convinced that the problem is on the MPU board... but where?

@scott_freeman and pinkid
The problem occurs even when the switch matrix is disconnected and only the shot target is simulated with a diode connected to the MPU and the tilt assembly connected directly to the MPU board with two wires. So it seems to me that the switch matrix is definitely not guilty.
Furthermore, the problem is not hat a switch is not detected, but that it is detected as the wrong one

Regards
Patrick

#8 2 years ago
Quoted from Pafff:If you look at the right of the oscilloscope screen dump, you can see that ST1 and ST0 are both high exactly simultaneously (i.e. switched high and then low at the same time, not one after the other).

This could be happening during the Display Interrupt Service routine.
The PIA pins used as the Switch Strobe outputs are also used to select Displays (active low) when the displays are being updated.

Quoted from Pafff:

Then I noticed that now Sw07 is active about 50% of the time

So what happens now if you completely disconnect J3 off the MPU board (including ST0 and I6)?

Note down your DipSW settings and switch them all off. They are essentially also on the "I" inputs of the switch matrix.

Quoted from Pafff:

Then I wanted to try you idea of linking the tilt assembly to I5 instead of I6

Did you eventually try this?

Quoted from Pafff:

I would be very interested in knowing how the CPU uses the 6820, in which sequence it activates the STrobes on PA, at what time it reads the inputs on PB and so on.

Ok, here goes, I hope it's not too confusing and the timing calculations are thereabouts:

Start by selecting #$10 (10 hex) which is Strobe 4 as the first Strobe to activate.
- Write the Strobe data to the U10 PIA Port A output to activate a new Strobe.
- From RAM, get the previous reading of inputs on Strobe X from 2 Zero Crossings ago.
- Merge it with data from RAM of the previous reading of inputs on Strobe X from 1 Zero Crossing ago.
- Move the RAM "inputs on Strobe X reading from 1 Zero Crossing ago", to RAM "inputs on Strobe X reading from 2 Zero Crossings ago".
- Wait about 180us.
- Read U10 PIA Port B which is the state of the "I" inputs on Strobe X.
- Save it to RAM, it will become the reading of "inputs on Strobe X reading from 1 Zero Crossing ago" next time we check the state of the inputs on Strobe X (which will be at the next Zero Crossing).
- Any inputs on Strobe X from 2 Zero Crossings ago that were still active 1 Zero Crossing ago, mark them as a valid switch inputs to service later.
- Loop and move to the next Strobe (X-1) set of inputs

The strobe to activate that's written to the U10 PIA Port A at the start of each loop is in order:
#$10 (ST4), then #$08 (ST3), then #$04 (ST2), then #$02 (ST1), then #$01 (ST0), then #$00 (no active strobes).

It takes about 330us to read the U10 PIA Port B "I" switch inputs after the Strobe has been activated.
One "loop" to process each Strobe takes about 390us

#9 2 years ago
Quoted from Quench:

...The PIA pins used as the Switch Strobe outputs are also used to select Displays (active low) when the displays are being updated.

OK, I didn't notice it on the schematics. I think this is a good explanation for what I called "havoc".

Quoted from Quench:

So what happens now if you completely disconnect J3 off the MPU board (including ST0 and I6)?
Note down your DipSW settings and switch them all off. They are essentially also on the "I" inputs of the switch matrix.

All microswitches are "off" all the time, so they should have no influence.
With J2 and J3 disconnected, nothing is detected.

Quoted from Quench:

Did you eventually try this?

I have tried to connect the tilt assembly to other combinations of ST / I, on J2 and J3. Result: it is always detected as closed and displayed as the corresponding switch. On J2 as well as on J3.

Quoted from Quench:

Ok, here goes, I hope it's not too confusing and the timing calculations are thereabouts:

It's not confusing at all, many many thanks for the explanation!

________________
What I know now

  • The ST strobes seem correct

  • I come to the conclusion that something must be wrong on the I (PB)lines. Some interaction between the C and the metal mass of the tilt assembly and something which hangs to the I lines on the MPU board (as it happens even when only the tilt assembly is connected to J2 or J3). But what hangs to the I lines, except the microswitches (all off), J2 and J3 (both disconnected, except for the tilt assembly)?

An interesting feeling of understanding always more, but still having no idea what the problem could be...

Regards, Patrick

#10 2 years ago
Quoted from Pafff:

I have tried to connect the tilt assembly to other combinations of ST / I, on J2 and J3. Result: it is always detected as closed and displayed as the corresponding switch. On J2 as well as on J3.

"always detected as closed" even when the tilt switch is open?
I presume you're just using a standard 0.05uf/0.047uf capacitor across the tilt mechanism?

If what you're getting is expected and correct behavior with the wires changed, from your point of view you have done everything to isolate this problem to the MPU board only?

  • What brand of U10 PIA do you have? Is it a 6820 or 6821?

  • Have you tried swapping U10 and U11?
  • Has someone changed the U10 socket in the past (maybe accidentally shorted PB6 pin to something else)?

There's only a handful of components on the MPU board connected to the PB6/I6 signal. I would start by temporarily disconnecting C39 and C20 to see if the problem still occurs. Then measure the resistance of R40=56k, R53=1k2, R68=1k2. There's nothing else left apart from the U10 PIA.

BTW, C39 is at the bottom right of the MPU board - has your board been repaired for excessive battery acid damage in the past?

My machine has a related problem that tilt is always detected as closed during Winter. I have to disconnect the tilt capacitor/wire from the metal "cage" around the plumb-bob to stop it. I measure about 1.5Mohm resistance between the tilt Plumb bob and the cage (through the wood!) even when the cage is totally disconnected from the matrix! It's something to do with the extra moisture in the wood that the tilt mechanisms are screwed to. It doesn't happen in warmer climate.

#11 2 years ago
Quoted from Quench:

"always detected as closed" even when the tilt switch is open?

Yes, that's the problem...

Quoted from Quench:

I presume you're just using a standard 0.05uf/0.047uf capacitor across the tilt mechanism?

Yes. It has really 50nF. No difference between old and new.

Quoted from Quench:

If what you're getting is expected and correct behavior with the wires changed, from your point of view you have done everything to isolate this problem to the MPU board only?

Yes, that's what I think. The last possibility I see (as the tilt assembly is the only thing connected to J2/J3 and that not over the "official" wiring, but over two separate wires. I measured today R and C of the (open) tilt as seen from the output of the MPU board: infinite ohm, 50nF (with the cap), 1nF (without the cap).

Quoted from Quench:

What brand of U10 PIA do you have? Is it a 6820 or 6821? Have you tried swapping U10 and U11?

I Forgot to check if 6820 or 6821. My first try was to replace U10 with a czech generic chip (CM602P). The I swapped U10 and U11. Always the same.

Quoted from Quench:

Has someone changed the U10 socket in the past (maybe accidentally shorted PB6 pin to something else)?

I don't know, but as the problem exists also with the tilt on any PAx/PBy combination, this shouldn't be significant.

Quoted from Quench:

There's only a handful of components on the MPU board connected to the PB6/I6 signal. I would start by temporarily disconnecting C39 and C20 to see if the problem still occurs. Then measure the resistance of R40=56k, R53=1k2, R68=1k2.

Same as above, I don't think C39 and C20 are guilty, as the problem is exactly the same with the tilt on any PAx/PBy

Quoted from Quench:

BTW, C39 is at the bottom right of the MPU board - has your board been repaired for excessive battery acid damage in the past?

No acid damage. 3xAA NiMH batteries in a separate box (I modified it).

Quoted from Quench:

My machine has a related problem that tilt is always detected as closed during Winter. ... It's something to do with the extra moisture in the wood.

Not the problem on mine (R infinite, C OK). Furthermore, it even happens when only the cage around the tilt know is connected to the wire, but not fixed on the wood.
________________
The fact that the problem occurs with all combinations of PAx/PBy is really confusing. I will probably try to measure the PA and PB signals at several places in the MPU board to try to find out where the 1-1.5V on PB come from. Or disconnect STx.

What I really don't understand is that it really looks like if the PB lines where driven from somewhere else - as nothing is connected to them on J2/J3, except the tilt (C, CR and the metal mass) - but from where?!?

#12 2 years ago
Quoted from Pafff:

Quoted from Quench:

"always detected as closed" even when the tilt switch is open?

Yes, that's the problem...

Ah, ok so the problem is not specific to PB6/I6 then..

Can you post pictures of the MPU board? Front and back? Also pictures of the plumb-bob tilt and the ball roll tilt?

Have you thought about just simply removing the tilt switch capacitor so the game is playable? At least until you can try another MPU board some day?

#13 2 years ago

I have already removed the cap, so playing is not a problem.

Actually, as the problem began about 6 months ago, I solved it be replacing the cap with a new one. Then after some months, it happened again, and I put two caps in series. Again some months later, problem again even with 10nF instead of 50. Only when "warm". And now, it happens all the time, even in "cold" state and even without needing to hit the target.

Anyways, looking at the MPU board I don't see anything suspicious (as an electrical engineer I have already seen many PCBs to compare), but I never looked at the backside.

I will take pictures, but I'm a little afraid of removing the MPU board from the backbox... 35 year old big PCBs can be rather fragile!

#14 2 years ago

Did you ever try freeze spray when the problem was only happening when warm?
BTW, measure the frequency on the output of the 555 timer on the MPU board. If it's drifted high, it can cause weird things.

Those MPU PCBs are pretty sturdy. You'll have to learn to get over this fear of removing the board I'm sure you will take proper care.

#15 2 years ago

Using freeze spray on all the components on the MPU board as well as capacitors and diodes of the concerned switches did not help (but sometimes caused a "reset" of the pinball).

I will check the 555, do you know how fast it should tick (I'm too lazy to compute it myself...).

#16 2 years ago

The 555 should output 320Hz on pin 3.

Do you have the Bally Theory of Operation FO601-2 book? Someone like you might find it interesting/useful. Google is your friend

#17 2 years ago

Great, this book, 1000 thanks! Unfortunately, my friend google doesn't know a version with a good resolution but anyways, it's still readable.

Newest results:

  • U10 is a 6821, U11 is a 6820 at the moment
  • the 555 outputs 348 Hz. Doesn't seem far away from 320 Hz, isn't it?

The pictures:
MPU board front
IMG_0447.JPG

MPU board back
IMG_0450.JPG

MPU board back, bad solder under CR47 and CR48 (on D6 and D7 of the processor's data bus, funny circuit, what's the use of it?)
IMG_0452.JPG

Tilt assembly
IMG_0456.JPG

Regards
Patrick

IMG_0456.JPG

IMG_0456.JPG

#18 2 years ago
Quoted from Pafff:

Great, this book, 1000 thanks! Unfortunately, my friend google doesn't know a version with a good resolution but anyways, it's still readable.

I'm currently OCRing it, but because of the low res it's requiring a lot of manual correction. I can give you a copy when done.

Quoted from Pafff:

the 555 outputs 348 Hz. Doesn't seem far away from 320 Hz, isn't it?

Agreed, 348Hz should be ok.

Quoted from Pafff:

MPU board back, bad solder under CR47 and CR48 (on D6 and D7 of the processor's data bus, funny circuit, what's the use of it?)

No sure, since the NVRAM is only connected on the upper 4 bits, I can only assume it's somehow related.

Nothing obvious stands out in the pictures.
Is it worth unscrewing all the tilt related switch wiring from the chipboard as a test (both ball cage and plumb-bob)?

#19 2 years ago

@Quench:
I already have removed everything from the cabinet. Even with only the brass cage around the knob linked to the wires, the problem occurs. Looks like it would create a resonance.

I go to south Italy for 3 weeks tomorrow, so I won't be able to debug my Flash Gordon in that time, but thank you very much for your invaluable support!

Regards
Patrick

Magical_circuit_(resized).jpg

#20 2 years ago

I wish you the best of luck resolving it when you get back!
Have fun in Italy, and keep an eye out for a spare MPU board . I hear there's a healthy Pinball market there.

#21 2 years ago

image_(resized).jpg
That leg on the cap looks broken off

4 weeks later
#22 2 years ago

Good observation Raff!

The broken off leg is the workaround to be able to play normally: without cap, everything (except the TILT function) works perfectly.

When testing, i connect it again.

#23 2 years ago

what happens if you cut every cap on the switches on the switch matrix the tilt and target are in? do one at a time.
my bally kiss did the same exact thing. ended up being a ceramic disc cap somewhere on the matrix.

#24 2 years ago

also, why not just buy an altek mpu for giggles and put it in and see if problem goes away. . you can keep the board for a spare or sell it after you are done.

#25 2 years ago

Let me sum up what we have so far, and tell me if I'm wrong with anything:
1) Hitting the target causes the machine to detect the tilt is closed
2) If you hook up a different switch to the relevant pins, it works as intended
3) It only does this when the target switch, 15, is closed, but not any other time.

I'll have to test tomorrow, but I thought the bally switch mode on the early mpus only shows the lowest number currently activated. So if, say, row three is stuck on, it will only display the number for where row three meets column 1, and not any others. This makes me think that the whole row for 15 (of which the lowest is 07) is being activated at once. Which would be I6 in this case. And if it only does it when the tilt is connected, then you've got a short on the row wire going to the cabinet, right? Is the late hour making me stupid?
I'm curious, when it does this, what happens if you hit the right outlane switch? Does it say 07 then, or 31?

P.S. Also, why do you say the solder is bad on those two spots? I see burning around the pad, from someone using a desoldering iron inexpertly, but the solder itself looks ok....

#26 2 years ago

Thanks to all of you for your support!

Quoted from CadillacMusic:

1) Hitting the target causes the machine to detect the tilt is closed
2) If you hook up a different switch to the relevant pins, it works as intended
3) It only does this when the target switch, 15, is closed, but not any other time.

Not correct any more. Now the tilt is detected closed even with nothing else closed. If I connect the tilt to any other combination of STx/Iy, both on J2 and J3, exactly the same happens - tilt detected closed.

Quoted from CadillacMusic:

I'll have to test tomorrow... then you've got a short on the row wire going to the cabinet, right?
I'm curious, when it does this, what happens if you hit the right outlane switch? Does it say 07 then, or 31?

It also happens with J2 and J3 disconnected (i.e. no switch matrix connected), so there should be no short in the wiring. And also when the tilt assembly is taken out of the cabinet, so no short or creeping current there. I will try the "right outlane test" but I expect 07 to be displayed as 07 is less than 31.

Quoted from CadillacMusic:

P.S. Also, why do you say the solder is bad on those two spots? I see burning around the pad, from someone using a desoldering iron inexpertly, but the solder itself looks ok....

I just wanted to say that it does not look very good there.

Quoted from silver_spinner:

what happens if you cut every cap on the switches on the switch matrix the tilt and target are in? do one at a time.

Nothing. The problem occurs also with J2 and J3 disconnected (i.e. no switch matrix connected) and the tilt assembly connected directly with two separate wires to J2 or J3.

Quoted from silver_spinner:

why not just buy an altek mpu for giggles and put it in and see if problem goes away

Mainly because I love the idea that almost all components in my pinball are original. These alltek boards are not so cheap that I would like to buy one just to try - even if I would bet that the MPU board is guilty, considering the symptoms.

#27 2 years ago

you can SELL the alltek board after you confirm if its a board issue or ? you may lose $20 bucks selling it used. But isnt it worth the time wasted? idk, time is precious to me. that's what i would do. get a quick answer then fix what's needed.
all this guessing is getting you nowhere my friend.

#28 2 years ago

@Pafff:
Do you have an EPROM programmer?

#29 2 years ago
Quoted from Quench:

@Pafff:
Do you have an EPROM programmer?

Unfortunately not. I know somebody who knows somebody who has one, but it takes weeks or months to get an EPROM programmed and the first (and last) time I asked, it was not correctly programmed (it was the french speech EPROM of my Flash Gordon...)

#30 2 years ago

may be dumb question, did you check you switch's in the coin door? I had a 6million dollar man, that when hitting a certain target would cause a tilt, problem ended up being one of the coin slot switch was closed. its easy to forget them.

#31 2 years ago

I'm coming into this late and to be honest didn't read every single post, but both of the following solder joints look very suspect.

cold-solder-joints_(resized).jpg

#32 2 years ago
Quoted from wdennie:

...did you check you switch's in the coin door?

The problem happens even if J2 and J3 are completely disconnected, i.e. no switch matrix (and no coin door switch) at all connected to the MPU. It is enough to connect the tilt assembly with two wires and nothing else. So I guess nobody else than the tilt assembly (with all components tested and OK) or the MPU board should be involved...

Quoted from terryb:

...both of the following solder joints look very suspect.

Yes they do, but I tested them yesterday with a multimeter and found no short circuit and all connections were OK.

4 weeks later
#33 2 years ago
Quoted from Quench:

My machine has a related problem that tilt is always detected as closed during Winter. I have to disconnect the tilt capacitor/wire from the metal "cage" around the plumb-bob to stop it. I measure about 1.5Mohm resistance between the tilt Plumb bob and the cage (through the wood!) even when the cage is totally disconnected from the matrix! It's something to do with the extra moisture in the wood that the tilt mechanisms are screwed to. It doesn't happen in warmer climate.

THAT WAS IT!

I have known for months that there is a parasitic resistance of a few Mohm in parallel to the tilt assembly's capacitor of 50nF (precisely between the plump bob and the lower tilt bracket). But my mistake was to never think that such a big resistance could disturb the function of a simple switch like the tilt assembly!

By simulating the circuit (6820 PIA output according the the MPU's program, some resistors, diodes, capacitors, switch and back to the 6820 as input) using LTSpice, I understood why even such a big parasitic resistance can disturb: during the switch matrix reading cycle, the capacitor is charged by a 6820's PA pin during about 300us through resistors of a few kohm, then unloaded during about 10'000us through the parasitic resistance of a few Mohm. The capacitor is loaded only about 3% of the time, so the parasitic resistance, even if big, is able to prevent the charge from reaching a level which would allow the tilt to be detected as "open"!

I guess the problem is caused by the high humidity level in my basement. With time, it went into the wood, which now doesn't isolate as well as originally. Unfortunately, my wife doesn't want a pinball in the living room, so I had to look for a technical solution...

I have solved the problem (I hope at least...) by replacing the two rusty screws which fasten the lower bracket to the wood plate with nylon screws (M3 x 8mm) and putting nylon washers behind the bracket so that it doesn't touch the wood plate any more. That way, I interrupt the circuit from plumb bob to lower bracket through the humid wood and the capacitor doesn't discharge spontaneously any more.
IMG_1333 (resized).JPGIMG_1337 (resized).JPG
The nylon screws do not hold as well as the orignal metal ones, but it should be sufficient as the bracket is not really sollicited mechanically during play.

Many, many thanks to the community and especially to Quench for the invaluable help!

Patrick

#34 2 years ago

Excellent! It's been an interesting journey and glad you got to the source of the problem that cost you next to nothing - even to the point where you simulated the circuit/condition with LTSpice, impressive!

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