The schematic states D1 or R16 should be installed. R4 should always be installed.

This game isn't a Bi-Phase switched illumination game.
R4 is currently seeing 43 volts (instead of 21.5 volts it should be) which is causing it to dissipate nearly 4 times the power (well beyond what it's rated at) and that's why it's burning. You need to remove D1 and install R16 (2k ohm resistor). I usually replace both of these 2k resistors with 1/2 watt variants because the 1/4 watt rated resistors in use are running close to their max rating. I have seen them go open circuit from fatigue.

It might be possible that the MPU boards 5V supply rail is seeing high voltage spikes/dips when you activate the flippers because of the excessive voltage at R4 from the solenoid power rail. It's potentially a reason for the resets.

You sure you didn't install any of the capacitor on the sound board backwards?

What state is the female connector in that plugs into the sound board?

What voltages are you measuring at test points TP1, TP2 and TP3 on the sound board?

If you have a logic probe, are you getting activity on the four "SND SEL" and "SOUND INTERRUPT" inputs on the sound board during sound playback?

What's the voltages at test points TP5 and TP1 on the solenoid driver board?

Voltages look ok other than the sound board TP1 fluctuating. If you put your black meter lead on another ground point, do you get a stable voltage reading?

BTW, does the flipper reset happen intermittently or every time you first flip in game mode?
Does it require both left and right flippers to be activated at the same time or only one (either/specific) side?
When it resets does the game play the alarm you hear when you slam tilt the game?

??? Check if this is a mechanical problem by you flexing the MPU board a little around TP5.
Where are you hooking up the logic probe to ground when you're doing this? somewhere in the backbox or the Gnd test point TP4 on the MPU board?
How much voltage do you measure at TP5 on the MPU board when you use a common backbox ground point for the black meter lead vs using TP4 ground on the MPU board?
.

How many flashes are you getting on the MPU board LED when the game doesn't boot?

If you disconnect J1, J2 and J3 from the MPU board (leave J4 connected only) does the MPU LED still blink forever?

If you swap the two 6821 chips on the MPU board with each other, does it change anything?

On the sound board are you measuring zero ohms continuity between the negative leg of capacitor C24 and TP3 (Gnd) on the sound board?

On the J1 connector of the sound board, pull out the wires coming from the MPU board one at a time to isolate the connections and see if you can narrow down which signal is causing the boot issue.
Start with the wire at pin 8 on the sound board connector. If no difference then the wire at pin 2, then 1 then 3 and finally 4.

To clarify, unplugging J1 from the MPU board makes the MPU board LED blink 7 times and then the LED goes dim like it should?

With those 5 wires unhooked from the sound board you disconnected all the logic signals from the MPU board to the sound board. That just left the sound board with ground and the unregulated 12V connected.

So it's pointing to a problem with your unregulated 12V supply and a problem here could cause the flipper resets.

Which solenoid driver board (SDB) have you got? Bally factory or some aftermarket SDB?
If factory SDB, is the big capacitor at C23 original?

Can you post some clear high res pictures preferably taken outdoors in shade (so your camera gets maximum natural light without shadows) of this MPU board front and back?

Hmm, I can't see anything abnormal in the pics.

Question: are you hooking up a battery to this MPU board when it's in the machine?

Humor me, please hook up a battery pack with blocking diode.
This board isn't a standard Bally -35 board. The reset section and battery backed RAM rely a little more on a battery being present.

Ok, set your logic probe switches to "TTL" and "Pulse". Hook it up (black lead to ground, red lead to +5V). Do you now get any activity on the four "SND SEL" and "SOUND INTERRUPT" inputs on the sound board during sound test mode?

With your logic probe still hooked up, probe pin 40 (top left pin) of the 6802 CPU. Hold the probe there, it should indicate High. If you tap a flipper button with the other hand a few times, do you notice the yellow "pulse" LED or green Low LED on the logic probe flicker when the game resets?

Yes, and don't forget when you get a NVRAM you require a 5114 compatible replacement, not standard 5101 replacement.

I've gotta bail so will check in later.

Sorry for the delay.

So something is triggering the reset circuitry on the MPU board.

How much voltage do you measure across zener diode D2 on the MPU board (bottom left corner)? Put your black multi-meter lead on the lower diode leg, red meter lead on the upper diode leg to ready the voltage across the zener.

With your bench testing the MPU board and the LED non stop flashing, is the reset line on the CPU (pin 40) steady high or is it pulsing low every now and then? If it's pulsing, how many LED flashes between the reset line pulsing low?

That's too low. It should be over 7.8 volts and preferably closer to 8.2V which is what that zener diode spec is. I don't think it's your problem, but please change it if you have a spare.

With your logic probe, what's happening on pin 13 of U15 (input to the reset pulse chip) during this never ending LED flashing?

Battery still hooked up to the MPU board? What happens if you pull out the 21C14 memory chip at U6?

Are you counting the initial power on LED flicker as a flash?
Without the battery backed RAM at U6, on power up you should get a quick flicker, flash, pause then 2nd flash and that's it.

With the RAM reinstalled at U6, what voltage do you measure on pin 16 of U15? Does it change after every 3 LED flashes?

For now, if you have a spare 1N4148, 1N4004 or 1N4007 diode, can you put it in series with the D2 zener diode to add an extra 0.6V voltage drop?
i.e. unsolder the banded side of the zener and lift that leg out of the board. Solder a generic diode banded leg to the banded side of the zener. Solder the other leg of the generic diode into the board where the banded leg of the zener originally was. This should lift the voltage across the zener points on the board to around 7.7 volts.

Am wondering if the RAM at U6 is suspect. First, please add a generic diode to the zener and lets see what happens.

Power on the board and then jumper a piece of wire from the lower leg of resistor R24 to pin 18 of the RAM chip at U6. This should stop the reset circuit reactivating.
What happens to the LED flashing?

Can you power the board off then back on with the jumper wire still in place? How many LED flashes do you get?

Timing of the reset circuitry is critical to how the CPU starts up. Since that jumper wire is essentially bypassing the reset delay circuit on powerup, depending on when you power the board and how much the caps on the board were still charged from the last power down, startup will be unpredictable as you just saw (sometimes boot success, sometimes the CPU starts before power has stabilised and crashes).

On this revision MPU board, Bally have added some protection circuit to the battery backed RAM at U6 to prevent data corruption on powerup.
What appears to be happening is on powerup when the CPU begins to test the RAM at U6, the signal to enable the RAM is causing the protection circuit that's powered by the "battery voltage" to crumble and is causing the reset circuit at U15 (also powered by the "battery voltage") to reactivate.

At this point I would remove the RAM at U6, carefully bend pin 18 out and reinstall the chip with pin 18 hanging out. Connect pin 18 to 5V power on the MPU board so it's no longer being powered by the "battery voltage".

Another thing you could temporarily try is disabling the RAM protection circuit. Remove resistors R9 and R10 and short out diode D5.

However I have a feeling the problem may be upstream in the "Valid Power Detector" circuit on the MPU board because your other issues with the flipper resets and hooking up the sound board are more likely to be propagated onto the 12V supply line going onto the MPU board where it's used in the valid power detector circuit. Mind you, you have now corrected the voltage across that zener diode.

The Valid Power Detector circuit on the MPU board shown in the schematics has a few voltages listed at various components. Might be worth checking those voltages..

Please try replacing diode D4 (with a 1N4004), and then disconnect pin 18 of the RAM chip from the +5DC supply you hooked up to and connect pin 18 back to any "battery voltage" (aka VAUX on the schematic) point like the top leg of resistor R3.

The sound board static is likely a separate issue.

Your "unreg 11.9VDC" measured 15.2V at TP2 on the MPU board which is about right when you factor in the large capacitor on the solenoid driver board removing the majority of the DC ripple on that supply rail. So 8.0 volts on the anode of D2 is no surprise and has propagated to other voltages you measured.
If it concerns you and your transformer is still wired for 115VAC, change it to 120VAC (with the power cord disconnected from the wall).

BTW, what's the history of this machine/board? Was it working and then started to fail, or did you buy it with these problems?

Before you replied, I modified my post #35 above to change D4 from a 1N4148 to a 1N4004. Which type diode did you use?

I'm wondering if the U6 RAM chip is faulty. When the game accesses it during the poweron self tests, the RAM comes out of standby mode and draws more current from the VAUX supply. Unusually the current surge when it wakes up is causing the reset circuit to trip. Since the game appeared to work when you hooked up the RAM to +5V power, I think it would be safe to presume if you install a suitable 5114 replacement NVRAM it should work. Worst case hookup the NVRAM power at pin 18 to +5V instead of VAUX.

Pity I can't find any datasheets of the specific RAM chip you have to see what its standby and wakeup power/current draw are. An oscilloscope would be handy to see what's going on with VAUX power when RAM is woken up.

Does the sound board play any sound effect when you press the self test button on it?

Hook up pin 18 of the RAM chip to +5V again so the game boots.

Last time you checked, you mentioned that the "SND SEL" signals at Sound board J1 pins 1, and 8, are high, pins 2,3, and 4 are high with pulse.
Can you put the game in solenoid test mode and see if pin 1 at J1 on the sound board ever pulses low. Also recheck the other "SND SEL" signals at J1 pins 2, 3 and 4.
If any of these 4 signals never pulses low, you have a connector issue between the MPU board and the sound board.
Note, the "SND SEL" signals are also the "solenoid select" signals so they go to the solenoid driver board aswell. Solenoid test mode will cause all these four selext signals to pulse low at various times.

Pin 1 should pulse low when the pop bumper, knocker, saucer and right slingshot activate.
Pin 2 should pulse low when the saucer, outhole, drop target reset and the right slingshot activate.
Pin 3 should pulse low when the pop bumper, saucer, drop target reset and the left slingshot activate.
Pin 4 doesn't pulse low in solenoid test mode First Bally I've come across that doesn't use this signal for solenoids.

Can you also check that pin 8 of sound board J1 has good continuity back to the MPU board.

What is the state of the sound board LED in attract mode.
When you start a game, and trigger a bunch of switches to play different sounds, does the sound board LED ever flicker off?

Are you getting good clean voltages at TP1 and TP2 on the sound board?

If you disconnect the output of the digital to analog converter from the preamp section (unsolder one leg of R25 and lift the leg out of the sound board) what happens to the static?

Earth connection on the negative leg of capacitor C10 that you changed measuring zero volts?
TP3 gnd connection measuring zero volts?

Hmm, 1/4 of a volt where there should be zero.. It's a bit much.

The sound board schematic shows the negative of cap C10 goes to earth at pin 13 on the J1 connector. However the wiring diagram shows no wires going to pin 13.
You should probably connect earth to ground somewhere for extra redundancy. I wouldn't mind seeing some clear high res pictures front and back of the sound board to understand where earth and ground are running.

What did you replace C10 with (electrolytic or tantalum) and what voltage rating did you use?

With R25 pulled, if you hold the metal part of a pointy screwdriver and touch pin 6 of U7, do you get any hum from the speaker? Start with the volume low so you don't get a fright from loud hum

Since the noise is being picked up/generated in the preamp/amp section, you can try disconnecting each pre-amp one at a time to see if the scratchy noise goes away. Start with pulling a leg from resistor R26, then the next stage at R33, then the last preamp output at R35. The noise could also be on the output of the 7805 5V regulator which is used by the preamps for mid voltage BIAS.

Did you hook up the NVRAM to +5V on the MPU board? There's no need for it to run off the VAUX voltage anymore.

It looks like earth on the sound board is connected to the unreg return and may be isolated from logic ground (on the board itself).

Hook up R26, R33 and R35 again (leave R25 disconnected), and pull out the CPU and ROM chip from the sound board. Static still there?

If yes:
Can you unsolder pin 14 of the LM3900 (U7) but do it so the pin is disconnected from circuit. Check with your multi-meter to confirm. Pin 14 is the power pin of the LM3900.
Now, one connection spot of R35 connects to the VR1 pot. Unsolder R35 from that spot and connect that PCB spot with a wire to TP2 (+5VDC) on the sound board. Is the static still there?
This will isolate the LM3900 meanwhile allow us to listen to +5V rail on the board.

The static you're getting on the sound board must be noise coming back on the 12V unreg supply (used on the MPU board in the reset section).

At this stage I'm guessing they're related.

The MPU sound select signals go straight to the sound board CPU. There's no "middle man". So it's a little odd that the sound CPU isn't responding to sound commands from the MPU board, yet you're saying will respond to the sound test button.

Can you clarify the status:

With no sound board connected do you get any abnormal power on/resets and what's the exact behavior now?
Did sound work properly when you used a different MPU board in this game before I chimed in on this thread?

It's hardly conclusive but the LM3900 might be bad. With it disconnected, did you still get abnormal behavior from the MPU board? Were the volume pots turned up all the way when you did this test?
Did removing the sound CPU make any difference to the static before doing this test mod?

Hold off hooking up the NVRAM to 5 volts for now. Lets not mask what's going on elsewhere just yet.

Something on the sound board is causing a power supply issue. Without an oscilloscope to see what's happening we're working blind.

You've already disconnected the logic signals from the MPU board to the sound board - all that was left was the unregulated 12V power rail and the two ground lines so it's time to try isolating circuits on the sound board sitting on that 12V rail.

Restore everything on the sound board back to how it should be.

If you look at the sound board schematic, the 12V rail comes in on pin 10 of J1. Start disconnecting the circuit downstream from it one component at a time starting with diode D6. If the MPU board now boots up ok, restore D6 and go downstream to the next component removing diode D8. Test MPU boot and if ok, restore D8 and remove diode D9. Test MPU boot again and if ok restore D9 and disconnect the input to the 7805 regulator at U9.

Hmm, question is whether some of those other caps need replacing: C8, then C18, C23, C17 and C24.
I presume you replaced C9 because it was in the flame firing line from C10 blowing yeah?

Does this still happen (resets) if you disconnect the speaker connector from the sound board?
It's not unusual for those old trimmer volume pots to present static when you're turning them especially if the pot is internally dirty. But the reset is a total different matter.

Hold off for the moment.

BTW, have you tried bench testing the sound board? Does it still produce the intermittent static?

I can't see a reason why C31 would cause a reset unless it's a symptom of a faulty output amplifier. C31 isn't shorting against C32 next to it?
With the sound board back in the game, what DC voltage do you measure on the output of the amplifier chip? For convenience you can measure the output at the + leg of cap C30.

Where did you connect ground from your power-supply to on the sound board?
This might be a good thing because it gives you a solid symptom to work on. Bench testing the board, you could try disconnecting the diodes I mentioned in post #73 to see what stops the power-supply from limping.

Typical DC output voltage of an amplifier is half of its supply - I would have expected something around 6VDC.

The only thing left connected to the 12v unreg on the sound board is the opamp (U7) and the amplifier at U8. You already tried disconnecting power from the opamp which didn't help regarding the static, so do you feel like removing the amplifier at U8 as a test? You could then try removing U7 if removing U8 doesn't stop the resets.

While U8 is out can you measure the resistance across capacitor C31. If your multi-meter isn't auto-ranging, set it to high resistance mode. Make sure your fingers aren't touching the meter leads otherwise the meter will measure resistance through your body, and wait at least 5 or so seconds time as the capacitor charges from the multi-meter before reading the resistance. The capacitor should end up reading open circuit once its charged.

The purpose of that cap is to separate DC voltage between the pin 9 output of the LM3900 op-amp and the pin 1 input of the TDA2002 amp. The DC reference voltage on the inputs/outputs around the amp circuits should be around half of their supply voltage to allow audio signal to swing up and down an equal amount.
The 0.86 volts you read on the output of the TDA2002 amplifier was bad. The resistance check should tell us if it was caused by C31 having a resistive short and not blocking DC on the audio signal path between the LM3900 and the TDA2002.
More likely the TDA2002 itself is bad though.

It should get rid of the static. The TDA2002 at U8 appears to have been straining the 12V unreg supply which also goes to the CPU reset circuitry on the sound board so we hope game play audio will return.

BTW, it seems with C10 blowing up and U8 failing, it's likely that 12V unreg supply rail got hit with a solenoid voltage spike at some point. I would order a LM3900 aswell (they are cheap) since it might be compromised.

If it's not too late, grab a replacement for U15 (74HC4538) on the MPU board. It's loosely connected to the 12V unreg supply so if suspect may be the cause of the MPU board resets. If you replace it and the board still resets then hook up the NVRAM to 5V. I would probably cut the VAUX trace on the back of the board leading to pin 18 of the RAM socket then jumper a wire to 5V on the back of the board. In terms of where to cut the trace if you're able to take some closeup pics of the RAM socket area with the RAM removed front and back, then we can get a better idea where the trace is running from/to.

7 volts DC on the output of the final amplifier is good - the slight speaker hum is also a good sign the amplifier stage is working now.
I presume you reconnected resistor R25 which provides the signal path from the Digital to Analog (D/A) converter onto the pre-amp stages?

I presume pin 6 (/RESET line) of the 6803 CPU (U1) on the sound board is measuring near 5 volts?

With your logic probe, are you getting activity on the address lines (pins 22 to 37) of the 6803 CPU?
When you press the sound test button, do the port pins (13 to 20) on the 6803 pulse as it sends digitized info to the D/A converter?

If you hook up your multi-meter to the output of the D/A chip (pin 4 of U6) and press the sound test button, what AC voltage and DC voltage do you measure when sounds should be playing? And what voltages do you measure when no sound is playing?

I'm curious what sounds you were previously getting when pressing the sound test button especially with the output amp being faulty. When I run Cybernaut in PinMAME, I can only describe the sound test sound effect as a spaceship landing in a very early 80's arcade game.

Did you manage to replace the 74HC4538 at U15 on the MPU board?

It must have sounded badly distorted with the faulty amp.

Seems you're getting some activity on the D/A converter when sound is supposed to play.

Silly question, you have the volume pot on the sound board aswell as the volume pot on the coin door turned up?

Check the inputs of the amplifier and preamp. Grab a pointy screwdriver and hold the metal part with your hand. Touch pin 1 of the amplifier at U8. You should get loud speaker hum. Then go back upstream to the preamp and touch input pin 8 of U7 and listen for hum, then further upstream to input pin 3 of U7 and finally input pin 6 of U7.

Note one or more of these pins might produce LOUD hum so don't be surprised.

Did you hear *any* change when touching those pins with the screwdriver? Hum slightly louder, clicks when you touched and released?

If not it seems like the output amp isn't working - you sure both input pins 1 and 2 aren't shorted with a solder whisker? Whats the chance the replacement is a dud?

You can try and test the amp with a portable (battery powered) audio player if you have one.
Disconnect the leg of resistor R35 that's next to the LM3900 and lift that leg out of the board while the other R35 leg is still connected.
Hookup the headphone ground connection of your audio player to ground on the sound board, and one of the stereo headphone channels to that floating leg of R35. You should hear the audio player through the pinball amp/speaker.

Can you try hooking up the audio player to the other side of R35? (R35 might have attenuated the audio players signal too much so bypass it this time)

The amplifier chip is getting about 14V power right?

Capacitor C31 is soldered on both legs? We didn't lift a leg from a previous test?

With the machine off, if you measure the resistance from the middle leg of VR1 pot to the outer legs with your multi-meter, does the resistance change when you rotate the pot?

Where did you get the TDA2002 amplifier at U8?

Can you post pics front and back of the board around the TDA2002?

Hmm, if you're getting slight hum from the speaker, yet feeding audio into the amplifier chip doesn't produce sound then that TDA2002 seems faulty as though both input pins are shorted.

In terms of other parts, I can't think of anything else worth getting at the moment.

Is the test button sound different than before?

When you put the game in sound test mode, what do pins 8, 9, 10, 11, 12 on the U1 6803 CPU on the sound board indicate are happening with your logic probe?