This document is Copyright 1999-2003 Nicholas Sayer, all rights are reserved.

Tempest: Detailed theory of operation

n. [ME, fr. OF tempeste, fr. (assumed) VL tempesta, alter. of L tempestas weather, storm, fr. tempus, time -- more at TEMPORAL]
  1. An extensive violent wind esp. when accompanied by rain, hail or snow.
(from Webster's 9th New Collegiate Dictionary)

New! Visit the Video Game Forum to discuss anything tempest related.

According to the Video Arcade Preservation Society, Tempest is the 2nd most collected video game. This is despite the fact that these games are 20 years old now (which makes them officially "antiques" and it is obvious that they were not designed for this sort of longevity.

The purpose of this guide is mainly to help anyone troubleshooting problems on the Tempest main/aux board set. It also hopes to preserve one of the best examples of economy in design and programming of all time.

Note that I have no connection with Atari or anyone else. I am just a collector and have managed to figure out what I have here by reading the schematic and fixing a few broken board sets. If you break your game because of what I say here, don't sick your lawyers on me, please. The stuff in this document may be totally incorrect.

Much of this document is either inadequate due to my (still) limited understanding of the workings of Tempest. Additional information is always welcome.

Note! I don't mind answering question or helping folks via e-mail, but before you write me asking what might be wrong with your Tempest, please at least go through the flowchart first. It probably will tell you just what to do.

You may wish to consult a schematic while viewing this document. They are available here.

Memory map

0000-07FFRAM (P2, R2, P4, R4)
0800-080FColor RAM (low nibble only)
0C00bit mapped register. Low bit to high bit:
  • Right Coin
  • Center Coin
  • Left Coin
  • Slam
  • Test switch
  • Diag step pad
  • HALT (from Vector State Machine)
  • 3 kHz square wave
0D00DIP switch N13
0E00DIP switch L12
2000-2FFFVector RAM (J3-M4)
3000-3FFFVector ROM (N/P3 & R3)
4000bit mapped register. Low bit to high bit:
  • Right coin counter
  • Center coin counter
  • unused (left coin counter)
  • Video invert X
  • Video invert Y
  • bit 5-7 unused
4800Vector State Machine GO
5000Watchdog clear
5800Vector State Machine RESET
6000-7FFFaux board
6000-603FEEPROM Write
6040EEPROM Control
6050EEPROM Read
6040Mathbox status (high bit, read only)
6060Mathbox read
6070Mathbox read
6080-609FMathbox write
60C0-60CFPOKEY #1 (B/C2)
60D0-60DFPOKEY #2 (C/D2)
60E0bit mapped register. Low bit to high bit:
  • 1 player start LED
  • 2 player start LED
  • use 2nd player controls (for cocktail games. See also video inverts)
  • SAEN 2,3 (a debugging signal on the small connector on the aux board)
  • bits 4-7 unused
9000-DFFFProgram ROM (D1-R1)
E000-FFFFCopy of top 8K of ROM (for 6502 reset/interrupt vectors)

Microprocessor and CPU support systems

The best place to start is the brains of the system: The 6502A running at 1.5 MHz.

The top 32K of the memory map belongs to ROM. Not all of that space is used, however. The actual program resides from 9000-DFFF. The 6502, however, requires reset vectors to be present at FFFA-FFFF, so they simply put a 2nd image of the top 8K of ROM from E000-FFFF (A4 gate 2 accomplishes this). If you wanted to hack the board to supply your own program, the way to accomplish it would be to take A0-A11 and D0-D7 from one of the ROM sockets, grab AB12 from A/B1 and A13 and A14 from B3, and apply all of them to a 27C256. You can even just ground the /CS pins on the ROM, since it is E2's job to keep the ROMs off the bus except for when the CPU is reading from them (that isn't really its job, but it can serve that purpose easily enough).

The 6502 treats memory from 0000-01FF specially. 0000-00FF is the "zero page," which is used in the (nn,X) and (nn),Y addressing modes. 0100-01FF is the CPU stack (the stack pointer is only one byte wide). Therefore it is important to put RAM in those locations, and Atari did just that. There is 2K of RAM at the bottom of the memory map that is private to the CPU.

Atari was liberal in their use of bus buffer chips. Most of the address bus is obtained from A/B1 and B/C1. There are no less than 4 data bus buffers: E2 is for the ROMs, F2 is for most of the main board and H2 is for the aux board. In addition, H3 is a bridge between the Vector State Machine and the F2.

The CPU clock is itself derived from a 12 MHz crystal oscilator. This 12 MHz source is divided by the counter at C4 to provide a 6 MHz, 3 MHz and 1.5 MHz clocks. This is divided still further by B4 to provide a 3 kHz clock for the watchdog, the SINP register at 0C00 and the aux board (it also supplies a 24 kHz clock for the main edge connector, aparently for no reason).

The power-on reset circuit is the only part of the system that makes use of the +10.3 v unregulated power input. It does this mainly so K11 can get power before the rest of the circuit. If you're running Tempest off a switching supply, just hook the +10.3 unreg input to a +12 source. The draw will be very, very small, so watch out for supplies that die on under-draw conditions. The watchdog is comprised of counter D4, which is fed from the 3 kHz source. D4 divides the signal by 256, which means that a properly running program must access location 5000 at least once every 80 msec or so in order to avoid being reset.

B3, J2, half of C1, P7 and J5 form most of the address decoding. The first level of address decoding (half of B3) is used to cut the bottom half of the memory map into 4 pieces. The top piece is allocated to the aux board, the next piece down is itself cut into 4 pieces: The coin counter register, VSM GO, Watchdog clear and VSM RESET. The 2nd from the bottom is vector memory, and the bottom piece is again cut into four pieces (with P7): 2 pieces of CPU RAM, the color RAM, and the top piece is cut again into 4 pieces for the read registers for the coin inputs and the 2 banks of main option switches.

Half of B3, J2 and half of C1 are used to select which ROM to read when ROM is selected. One gate from A4 is used to mirror the top 8K of ROM for the reset vectors.

Aux board

The aux board is fed 8 address lines and the aux board address select signal and a copy of the data bus (from H2). It is also fed a 6 MHz, 3 MHz and 3 kHz clock, and a copy of the CPU phase 2 clock.

The two halves of B4 are responsible for the first pass at cutting up the address space. One half of B4 only selects when a write takes place, the other on any access.

The write side cuts the address range into 4 parts, the bottom two go to the EEPROM write latch and the EEPROM control register, respectively.

The other half of B4 divides the bus again into quarters. The top quarter is again cut in 4 by half of B5. The top sixteenth is unused. The next one down selects the start LED and FLIP register. Next is POKEY #2 and POKEY #1, the last sixteenth in the top quarter is unused.

The next quarter down (80-BF) goes to the Mathbox.

The third quarter down is split into quarters again. The top one is labeled YHI, the next one YLO. Those two go to the Mathbox. The third is the EEPROM read register, and the last one (40) is the Mathbox status register.


I haven't got the slightest idea how the Mathbox works. Its purpose is to do the 3D computations necessary for the perspective view of the game.

This little tidbit comes from Gregg Woodcock:

The [...] 4 socketed 40-pin chips on the math box (at locations E2, F/H2, J2 and K/L2 on Tempest) are called "transistor array"s by the manual and the chips themselves carry only the Atari part number 137004-001 on them in an attempt to hide their true identity (to keep people from making illegal copies of the game?) They are really 2901 bit-slice ALUs, which were very popular and are fairly commonly available. They were made by AMD and a number of other vendors. In a technical sense the part really is a transistor array, but calling it that serves no purpose other than obfuscation. These go bad every now and then and will generate an "M" on the self-test screen. NOTE: the "M" is a generic "M"ath box failure indicator and does not necessarily indicate that an ALU is bad; lots of failures cause the "M" indicator to appear but one of the most frequent causes is bad ALUs (Note: The most frequent cause I've seen is the interboard cable failing. Check that first, as it's a lot easier to fix -- Nick). The cheapest place I have found for these is:

B.G. Micro, Inc 800.276.2206 (AMD2901 ALUs $1.50 each)

For reference, B.G. sells two kinds of "2901" chips so be careful not to order the 14 pin chips ($0.55 each) because you can't use them.


The EEPROM is a 64x8 one. It requires a -25 volt erase power supply, which is supplied by K3 and its associated cirtuitry.

Any write to the EEPROM Write area (6000-603F) latches the address pins for the EEPROM. So a do-nothing write must take place before you can read a location within the EEPROM.

Reading the EEPROM read register gates the EEPROM data pins onto the bus. It will also gate the write register if it is not disabled with the correct settings of the control register.

The bottom 4 bits of the control register location are latched by J3. The outputs of J3 go to the control pins of the EEPROM.

Unfortunately, I can find no data on the EEPROM chip they used, so I can't divine how the control signals are used.


I am not all that sure of the internals of the POKEYs, but their wiring is straightforward enough.

The FLIP signal obtained from the coin counter register is used to feed D6, which is used as a player 1/player 2 control arbitrator. If the cocktail signal is grounded (POKEY 1, P4), then the program will assert the X and Y invert signals and FLIP whenever it's player #2's turn in a 2 player game.

The encoder wheel output is a pair of sine waves (square waves will work too) 90 degrees out of phase. The direction of the phase shift indicates which direction the control is being turned. If the knob seems to be operating backwards, swap the two outputs. The 4 inputs (potentially from two knobs) are squared off by E6. D6 then selects which pair to use. C6 is used to convert the two out of phase square waves into a simple clock and direction signal (note that in doing so it introduces an error. The first "click" in the opposite direction from the last movement may register as a click in the wrong direction. Fortunately, the knob is very sensitive and the software reduces the sensitivity so that this error is lost as noise). This is fed into an up/down nibble counter, which forms 4 inputs to POKEY #1. FIRE and ZAP go to POKEY #2 (again, after being chosen with D6). The 1- and 2-player start buttons, DIP switches at D/E2, and the cocktail signal all go to the POKEYs, thusly:

POKEY #1, P0-P7:

POKEY #2, P0-P7:

The audio outputs go to K6, which creates a differential signal after mixing the two together.

Vector State Machine

If the CPU is the brains of Tempest, the Vector State Machine (VSM) is its heart.

In a nutshell, the VSM is itself a microprocessor, except that it is a fairly simple one and it is optimized for its job: generating deflection outputs for vector graphics displays. It has a program counter. This is used to access vector memory and fetch opcodes and execute them. The opcodes even include jump and subroutine call instructions (yes, it has a subroutine stack).

The VSM has a 12 bit address bus. Half of this space is given to the Vector RAM, half to the Vector ROMs. The VSM and CPU interleave control of this address space. The CPU fills in the vector RAM with opcodes to do whatever drawing is called for, then it accesses the VSM GO location. This starts the VSM running. The CPU then polls the HALT bit waiting for the VSM to finish drawing (it is, of course, free to do actual work during this time as well).

There was an excellent article posted to RGVAC giving the opcode list for the vector state machine.

The rest of this section is still under construction.


This section is still under construction.

Power supply & audio amp

This section is still under construction.

Diagnosing Tempest

The rest of this document tries to show how I would go about repairing a Tempest board set. This section presumes that the monitor and power supply are working (it makes this presumption since there already exist lots of stuff in the Wiretap archives about repairing these sections).

Incidently, the most frequent repairs I have seen needed on Tempest machines are:

  1. Replacing the 32,000 uF capacitor in the main DC power supply
  2. Replacing the 6 chassis mounted transistors in the monitor
  3. Fussing about with the interboard cable
  4. Applying a Zannen "fix it" kit to the HV supply in the monitor
  5. Tracking down and replacing blown chips on the main board

Start at the Start field below. Each field you come to will have some action to take. In many cases that action will have an expected result and/or a series of links containing possible outcomes. Select the outcome that seems to apply best. If there is no applicable link (or no link at all), then you have reached the end of the flowchart. If you find and repair a fault, you should start at the begining again. In my experience it is not uncommon to find boards with many faults -- a single fault will take a board set out of service, then it will get substandard storage because it is a "dead" board and get worse and worse.

The procedure


Set all of the DIP switches on the main board on except for N13 #7 and all of the switches on K10/11 (turn them off). Turn all of the DIP switches on the aux board off. Turn the test switch off. Make sure all the harnesses are connected and apply power.

At this point the game should be set for free play and should be in attract mode. Both of the LEDs in the start buttons should be blinking, the LEDs on the main and aux boards should be on and the LED on the monitor deflection board should be off.

Test Mode

Turn the game off, flip the test switch on, then turn the game back on. You should get a big white rectangle that's cut in half. There should be lots of 0s and 1s in the bottom half and nothing in the top half. There should also be a digit in the bottom half that changes with the spinner.

No Video

Close the slam switch a few times. If the logic board is working, you will be cycling (blindly) through the test screens. On one of them the game will test the audio circuitry, hopefully making a series of four decending tones.

Does closing the slam switch do anything?

Disable the watchdog

Find the test loop on the main board that says WDDIS. Take a piece of wire and tie this location to ground. Now power up the game and press RESET a few times.

Disabling the watchdog helped

The watchdog circuit is in question at this point. Using a logic probe or a scope, look for pulses on J5, pin 10. If you see them there, then the address decoder is generating WDCLR correctly. Look for the pulses again on K9 pin 12, K9 pin 11, and D4 pins 2 and 12.

Check the power supply

At this point it would be a good idea to check the power supply. Make sure that the +5 volt supply is ripple-free and within 2% as measured on the main and aux boards. If it isn't, then check the +5 volt supply on the audio/regulator board. If it is higher than 5.5 volts, then check the wiring between the audio/regulator board and the main/aux boards for a voltage drop. Otherwise, adjust the pot on the board until the voltage on the main board is +5.

Ripple in the power supply

The first thing to check is the primary +10.6 unreg supply. Disconnect the primary DC output cable from the primary power supply (the metal box in the bottom of the case where the transformer and most of the fuses sit). Look for ripple on the 10.6v output. If you see any, replace the big capacitor mounted in the primary supply. If you don't see any with no load, then check for ripple with the main/aux boards disconnected, but with the DC output cable connected. If there is no ripple in either the +5 or +10.6 supplies at that point, then it is possible that there is an overdraw condition, but this is a bit unlikely (a dead short should be blowing fuses).

If you see ripple in the +5 supply but not in the 10.6 supply, then you'll need to debug the regulator circuit on the audio/regulator board.

CPU or ROM trouble

At this point it looks like the CPU is just not running correctly. Using a scope or a logic probe, look for pulses on CPU pin 37. If they're not there, then check the clock generator circuitry. Next, check for them on pin 39. If they're not there, then the CPU may be dead. Check also for square waves on all of the address and data lines (A15 may be mostly high). Check E2 pins 1 and 2 to make sure the ROM-to-data bus gateway is working, then check the ROMCS lines and the contents of the ROMs themselves.

It is possible that the CPU is ok and that both the VSM and the audio are bad. But if the game can't make video or sound, it's going to be a bitch to debug.

RAM error

Hit the RESET button and count the number of low, short beeps (or short blinks on the start LEDs) that take place before the first long beep. Each short beep counts a working RAM chip. The first long beep flags the bad one. If the bad RAM chip is one of the first four, then it is the CPU's private RAM that is in question. If it is one of the last 8, then it is the VSM's RAM. If it is the first, second, fifth or sixth RAM then also look for trouble in the path between the CPU and either the private RAM or the VSM RAM (as the case may be).

Beep to RAM table:

  1. R2
  2. P2
  3. R4
  4. P4
  5. M3
  6. M4
  7. L3
  8. L4
  9. K3
  10. K4
  11. J3
  12. J4

Self Test Passes

At this point it appears your game has passed its self-test. If you're still having problems, then it's something far less severe.

Self Test Gets Errors

The self-test is complaining about something specific being broken. If failures are indicated on the aux board, I strongly suggest you check and resolder the molex connectors for the interboard cable, and the cable itself.

Mathbox error

The mathbox has failed diagnostics. You might try making sure the interboard cable is working. If it is and if the mathbox is truly broken, then I have no idea how to help.

EEPROM error

The EEPROM chip has failed diagnostics. Turn the test switch off, then back on again to get into the bookkeeping screen. Use the encoder to erase the scores and times and repeat the self test. If it still fails, then you probably have to replace the EEPROM chip.

ROM error

One or more of the ROMs has failed diagnostics. Ignore the numbers in the 2nd row. The failing ROM(s) can be identified by the number(s) in the top row of the display:


Bad bus buffers

If a ROM or RAM chip is flagged as bad, it may be that the buffer chips between the CPU and that chunk of ROM or RAM are bad. This is particularly a possibility if the chip(s) containing the first byte of a particular section (private RAM R2 or P2, program ROM D1, vector RAM M3 or M4 or vector ROM N/P3) show up as bad.

At this point you presumably replaced the device. You did use a socket, right? Remove the device and run the game with the device missing. Check the address and data pins of the device to make sure none of them are 'stuck.' With the self test going, most if not all of these lines should show activity. Trace any stuck pins back to the device side of any buffers and see if there's activity on the opposite side of the buffer. If so, then the buffer is likely to be the problem.

Tune the Video

You should be in the self-test mode on the first screen. Momentarily close the SLAM switch and you should see a rectangle with diagonal lines in it and a character set near the bottom.

  1. Adjust the X and Y center pots to center the pattern
  2. Adjust the X and Y size pots to put the rectangle roughly within half an inch (1 cm or so) of the edge of the tube
  3. Adjust the X and Y BIP pots so that the corners of the diagonal boxes rest exactly on the large rectangle
  4. Adjust the X and Y linearity pots so that the diagonal lines are straight. When this is done the size pots may have to be readjusted.

Spot Killer

The spot killer has activated indicating either X or Y deflection is not occuring properly. Take a scope or a voltmeter and look for +/- 6 volt swings on the X and Y out loops on the main board.

Monitor faults

Your monitor appears to be broken. Check the archives on Wiretap for help with your monitor

Problems with gameplay

Turn the test switch off and try to reproduce the fault.

Unknown problem

If you get to this point, then I don't know what is wrong. Drop in on the Tempest Forum and I or someone else will be happy to help you further.

DIP switches

There must have been something wrong with the DIP switch settings you were using. Check the archives on Wiretap for the correct DIP switch settings.

Problems with the controls

You should be in the first screen of the self test.

Push each button on the control panel. Each one should change one of the 0s into a 1. Do the same with the coin inputs.

Rotate the encoder and watch the hex digit. Rotated clockwise the digit should decrease. Rotated counterclockwise the digit should increase.

Problems with the video

At this point you should be in the 2nd self-test screen (diagonal crosshatch) and it should be tuned correctly.

What color are the lines?

Check R, G and B outputs

You should be in the 2nd self-test screen (diagonal crosshatch) still. Operate the SLAM switch twice. The 4th self-test screen is a color test screen. You will see 7 groups of vertical lines. The three groups on the left will be (top to bottom) magenta, cyan and yellow, the three on the right will be (again, top to bottom) green, blue and red. The group in the center will be white. Within each group of lines there are 6 lines. The leftmost should actually not be visible. The 2nd from the left should be barely visible (adjust the brightness control on the monitor if this is not so).

Bad R, G or B outputs

One or more of the color outputs are not working. Using a scope or a voltmeter, trace back from the color outputs to the three color output transistors (Q2-Q4) and to the intensity D-to-A network near H9.

Monitor bias/drive adjustments

Find the neck board on the monitor. On the top (as the monitor is mounted in the game. It's really on the left side) of the neck board going from left to right you should see the red drive, red bias, green drive and green bias pots. On the bottom again from left to right is the blue drive and blue bias pot.

Adjust the three bias pots until the 5th line from the right (it is longer than the rest) is pure white. Adjust the three drive pots until the right most line is pure white.

Monitor purity adjustments

You should be on the 4th self-test screen. Close the SLAM switch to advance to the 5th screen, the convergence/purity crosshatch. Using a voltmeter or scope rotate the encoder until only the green color output is active (red and blue should be 0).

Loosen the yoke-retaining clamp so that the yoke can move. Slide the yoke toward the back. Remove any glue that may be holding the purity ring magnets in place, and rotate one or both purity rings in a line so that they face opposite directions from each other.

Slowly slide the yoke toward the front of the CRT until the pattern displayed is an overall pure green. Tighten the yoke retaining clamp lightly, making any slight physical adjustments to the yoke to maintain a pure green pattern. Rotate the yoke to level the pattern on the face of the CRT.

Turn the encoder wheel to the red and blue patterns, and slightly readjust the yoke for a uniform and pure pattern for each color. Tighten the yoke retaining clamp to prevent yoke shift or rotation.

To make fine adjustments to the purity, turn the purity magnets for the best overall purity of each color. Reglue the purity magnets with a small amount of glue.

Proceed to convergance...

Converge the tube

If you are on the 4th self-test screen, close the SLAM switch to advance to the 5th screen, the convergence/purity crosshatch.

Rotate the encoder until magenta (red and blue) is displayed. Adjust the tabs on the center pair of adjusting magnets so that the red and blue lines are superimposed, making magenta (do this at the center of the screen).

Turn the encoder wheel until white (red, green and blue) is displayed. Adjust the tabs on the back pair of adjusting magnets so that the green lines up with the magenta, amking white (again, at the center of the screen).

Encoder doesn't move

Check the two outputs of the encoder itself. You should see two sinewaves 90 degrees out of phase (the phase shift depends on which way you turn the encoder). Check to make sure the sine waves arrive at E6 on the aux board. Then make sure square waves make it from E6 to D6, which selects which set of controls to use for cocktail games. Next, check C6, which is the flip-flop that changes the two sets of square waves into a direction and clock pair. If that doesn't help, check to make sure that the counting is taking place on the input to POKEY #1. If it is, then the POKEY is suspect. If not, then the counter is.

Encoder moves only one direction

One of the outputs of the encoder is CLK, the other is DIR. There is a D flip-flop on the aux board that turns the two out-of-phase square waves into a direction and clock signal. If the encoder is stuck moving in only one direction, it is because the DIR output is not making it to or past the flip-flop. Check both outputs of the encoder to make sure you get sine waves as you turn it. Similarly make sure that the sine waves arrive at the aux board. Then check E6, which is in charge of squaring the sine waves, then D6 which selects which set of controls to use for cocktail games. Lastly check the flip-flop at C6.

Problems with the buttons

Use a voltmeter or a scope to check and see if pressing the button grounds the input on the board. If not, check the wiring. If it does, then follow the signal through the input chips until it either stops changing or you hit a data bus.

Problems with the sound

The 3rd self test screen is a large + sign and an audio test. In the self-test mode, use the SLAM switch until you see the +.

You should hear four descending tones. The tones should be continuous, one tone moving straight to the next, each about a second long. Each tone is actually made of two identical tones, one coming from each of the two POKEY chips. You may therefore hear a slight imperfection in each tone half way through. This is normal. If there aren't four tones or they aren't descending or they aren't ajoining, then the test has failed.

SLAM switch stuck

Check the slam switch. It is on the coin door near the top on the side where the lock is. It is a small metalic strip with a weight on the end. There is a second small strip of metal. When someone kicks the machine, the weight will move, making the two strips of metal touch. During gameplay, this will cause the machine to emit a loud sound for a couple seconds (an alarm of sorts). If the slam switch is stuck closed, this sound will be constant.

Trouble with the POKEYs

It would appear that there is something wrong with one or both of the POKEY chips on the aux board (B/C2 or C/D2). POKEYs are still commercially available, though they're not getting any easier to find as time goes on. To be sure, try power cycling the machine into self test again and make sure you don't get indications of a mathbox error. If you get mathbox and POKEY trouble, check the interboard connection cable.

You can also try swapping B/C2 and C/D2. If the letter changes from either P or Q to the other letter, then the associated POKEY chip is bad (P is B/C2, Q is C/D2).

No audio at all

Listen very closely to the speaker and crank up the volume (don't do this when the game is either making noise or could decide to start making noise). Do you hear either digital noise or white noise that increases or decreases in volume with the control?

Audio amplifier dead

Check the audio/regulator board inside the cabinet. The main audio output amplifier is located there and is suspect.

Audio output dead

Using either a small amplified speaker or a scope, check for audio on the two audio out loops of the aux board.

Dead audio preamp circuitry

Trace the audio from the POKEY outputs to the output of the aux board. The fault is in that area.

Possible VSM analog power problems

The most common cause of an inability to tune the video properly is the power supplies to the analog vector section of the main board. Check the +15, -15 and +6.8 volt power loops and make sure they are absolutely correct. Any variance beyond 2% or so is a Bad Thing.

VSM Digital faults

This, unfortunately, is the hard part. It would appear that you have a problem with the vector state machine. Debugging it is going to be rather painful.

If when entering the test mode an error in ROM N/P3 is detected, the game will make a continuous tone. It does this because it may not be able to generate characters from the character set and therefore it may not be possible to see the self-test screen. If you get indications of a RAM error and this continuous tone, you can almost be sure that something is either wrong with the VSM data or address busses in the area of the ROM and RAM or the CPU-to-VSM interface. You might try removing one or more of the components called into question. If the game starts acting better with components removed, then look at the chip select lines for those parts for possible shorts (this technique won't work for the first pair of VSM RAM chips, but it will for the other ones, since the test patterns on the whole are less than 1K of vector operations).

If the VSM ROM and RAM check out as ok (likely you can only deduce this because of dead silence upon entering the self test, but audio changes when pressing buttons or the SLAM switch) then the state machine itself or perhaps the program counter/stack section may be bad.

Use the SLAM switch and advance to the self test screen that has a four tone audio test pattern. Do you see a large "+" sign (it may be distorted into a vertical line and a pair of horizontal lines that go to the right and left of the vertical line)?

VSM ROM suspect

Try replacing N/P3 and R3.

VSM Analog faults

One or both of the deflection outputs is stuck at this point. I will presume it is the Y deflection that's not working. You will have to adapt a bit if X is the problem.

Turn the brightness up on the monitor SLIGHTLY until you over-ride the spot killer. Don't leave it like this very long! It is very bad for the tube to not be deflecting the beam a lot. You should now see the screen "crunched" into a flat line. Wiggle the Y centering pot. Does the line move up and down?

Trace deflection signals

You'll have to trace the deflection signal from the voltage multiplier (C13) back to the D-to-A converters to find the fault.

Be sure to carefully check the resistances on C13's pins. C13 is an analog multiplier. If the resistance between pin 5 and 6 (for example) is infinite, then the chip will multiply the input by zero!

You can pretty safely ignore A/B13 and A/B12. They provide a small correction signal to C12 and C13 to compensate for the tube's roundness, but even if these chips are out of circuit the video on the whole should still be visible.

Check deflection output amp

It appears that the final output amp of the deflection section (D/E13) is bad. Also check C13, the analog multiplier.

Transient faults

Does the video seem to "implode" during the intermission between players in a two player game?


Does the game do weird things, like give lots of free games or go bonkers after the end of a game whose score matches a particular pattern?

Bad ROM revision

The faults described in the previous question are caused by bugs in the game code.

The "implosion" is actually a feature of later revisions of the monitor. Extra capacitors were added so that if an input were "pegged" to a particular voltage, the circuitry would "drag" the input back down to zero. In combination with the spot killer, this circuitry would better protect the tube from bogus inputs by not leaving the deflection amplifiers "on full blast". At the same time that this revision was made to the monitors, Atari made changes to the intermission screen to add extra "black" vectors to insure that the "cursor" would visit the bottom half of the screen and avoid tripping the new amplifier protection circuitry. 15 years later, however, you don't always get the game board that came with the monitor, and if the game doesn't draw the extra vectors, you'll see this effect.

Both of these bugs are fixed by installing new x17 and x22 ROMs.

Of all of the versions of Tempest I have seen, there have only been 3 differences:

Between version 2 and 3, ROM 2 had two bytes change (actually, this change occurred at the same time that Atari went from 2K to 4K ROMs, so this change is actually in the 2nd half of the new 4K ROM 1).

Between version 1 and 2, ROM 3 had two bytes change.

Between version 1 and 2 a rather massive code change happened in ROM 8.

I am not sure which changes affected which behavior. There is certainly no harm in installing v3 versions of ROMs 2, 3 and 8 in any machine you happen across.

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