May 142024
 
Location Capacitance uF Voltage V NOTES
C102 1 50
C107 22 50
C201 47 50
C202 47 50
C203 47 50
C211 2.2 50
C212 2.2 50
C213 2.2 50
C220 470 35
C221 100 50
C222 22 50
C280 10 50
Located on the Remote Adjustment Board
C332 10 250
Located on the Neck Board
C402 470 35
C403 100 50
Not present on all PCBs
C406 1000 35
C407 100 50
C408 100 50
C409 100 50
C411 22 50 BiPolar Cap
C451 22 50
C455 10 50
C456 10 50
C457 10 50
C458 22 50
C459 22 50
C503 100 50
C513 10 250
C515 100 250
C554 1 50
C556 1 50
C561 47 50
C562 100 50
C911 1000 200 Filter cap
C951 180 100
C952 220 100
C953 47 250
C954 680 35
C955 220 50
C956 680 35
C957 10 50
C958 47 50
C987 10 50
Not present on all PCBs
Some chassis PCBs are missing these ceramic caps – installing them can reduce EMI/RFI interference from AC power
C991 1000 pF 400 Ceramic
Kemet C911U102MVWDBA7317
C992 1000 pF 400 Ceramic
Kemet C911U102MVWDBA7317
C994 1000 pF 400 Ceramic
Kemet C911U102MVWDBA7317

Tutorial: CRT Color Calibration for Video Games

 General, Guides, Technical Info  Comments Off on Tutorial: CRT Color Calibration for Video Games
Apr 242024
 

Last Updated: 4/24/2024

This tutorial is a straightforward procedure for using a colorimeter to calibrate an arcade cab or consumer CRT. Using a colorimeter is perfect for people that want their displays calibrated to a high standard with no “eyeballing it” or guesswork involved.

Display color calibration in a nutshell is precisely matching the contrast, brightness and colors of a video display against a reference standard such that (A) the calibrated display looks the same as other calibrated displays and (B) subtle details in video games aren’t missed due to either color washout (white point too high) or black crush (black point too low).

Up front, here’s some vocabulary words that this tutorial uses:

  • White Point: The measurement of the brightest white your monitor can display.
  • Black Point: The measurement of the darkest shadow your monitor can display.
  • Contrast: Adjusts the White Point of the RGB color guns equally.
  • Brightness: Adjusts the Black Point of the RGB color guns equally.
  • Gain or Drive: Individually adjusts the White Point of each Red, Blue and Green color gun.
  • Cutoff or Bias: Individually adjusts the Black Point of each Red, Blue and Green color gun.
  • Color Temperature: Expressed in kelvins, color temperature is a parameter comparing the color of a light source (ie your display or a light bulb) against an real-world reference light source (ie the sun). Lower color temperatures (< 3000 K) are considered “warm” (red or yellowish) while higher color temperatures (> 5000 K) are considered “cold” (blueish).
    • The abbreviation for 6500 kelvin color temperature is D65 while the abbreviation for 9300 kelvin is D93.

Note that display color calibration is a VERY deep rabbit hole with a lot of information, detail and nuance. This guide is intended for enthusiasts that just want a procedure they can quickly step through to make their displays look nice. There’s much more you can do in addition to what I’ve written up here if you want to be very precise with your display’s calibration.

If you want to delve deeper into the world of color calibration, consider these resources and articles:

This tutorial steps you through a white point balance procedure for your display, but does not cover adjustment of the RGB primary and secondaries since adjustments to those are very specific to each CRT model. Geometry adjustment is also not covered here.

Necessary Calibration Components
  • Laptop or desktop Windows computer to run the Colorimeter
  • Colorimeter to accurately read color measurements off the display
  • Pattern Generator to push accurate calibration images to the display
  • Calibration Software that reads the values from the Colorimeter and displays them to you in a readable format
What I Use
  • X-Rite I1 Display Pro colorimeter – it’s around $30 – $100 used from Ebay.
  • MiSTer FPGA with MiSTercade add-on running HCFR core Pattern Generator and SNES 240P Test Suite – Download Link
    • Other good choices for a Pattern Generator are:
    • 240P Test Suite running on a game console
    • PGenerator running on a Raspberry Pi
      • PGenerator is a great pattern generation option since it supports 15 khz output for CRTs and the laptop you’re using to read measurements can control it automatically, but it is tricky to setup for the first time. There is an excellent YouTube tutorial by StickFreaks available here: https://youtu.be/D6z0wS5oRoE
    • DVD player running the FreeCalRec601 disc
    • HCFR – the same program that interfaces with the colorimeter – if the display you’re calibrating supports an output resolution that your HCFR PC supports.
  • HCFR calibration software

If applicable, use a color generator that matches what will be connected to your display the most often – ie, if you’ll be running a MISTer FPGA in an arcade cab, use a MiSTer as your color generator.

White Point Reference Selection

One decision you should make before you start is the white point standard you want your display calibrated against.

For retro video games, there are two target white point standards to consider:

  • D65 is considered the standard that US CRTs were calibrated for in the 80s-90s. It is a “warmer” white point and thus the display will have stronger red output.
  • D93 is considered the standard that Japanese televisions were calibrated for in the 80s-90s. Is is a “cooler” white point and thus the display will have a stronger blue output.
  • Choosing one or the other is a personal preference.
    Personally, I go with D65 for displays with US tubes (ie Zenith, RCA, Magnavox) and D93 for Japanese tubes (Sony, Toshiba, Panasonic, JVC, Hitachi)
  • There’s a thorough writeup on D65 vs D93 here: https://www.retrorgb.com/colour-malarkey.html

Procedure

These instructions assume you’re using HCFR as your colorimeter interface software on your Windows laptop.

One-time only software setup

1) Download and install the free HCFR software from here:
https://sourceforge.net/projects/hcfr/files/

2) Launch HCFR.

3) Click on the Advanced menu and choose Preferences.

4) Click on the References tab. Set the settings as follows:

D65 White PointD93 White PointAdvanced Tab

6) Click OK to close the Preferences window.

Technical Note: the values entered for the Red, Green and Blue references in HCFR correspond to the SMPTE-C phospher specification. https://en.wikipedia.org/wiki/NTSC#Colorimetry and HCFR Calibration – Custom Coordinates

Calibrating a display

IMPORTANT NOTE ABOUT TERMINOLOGY
CRT manufacturers often confuse the labeling for Contrast, Brightness, Drive and Cutoff.

“Contrast” is the term for adjusting the White Point level.

“Brightness” and “Black Level” are two interchangeable terms for the Black Point adjustment. I use “Brightness” in this guide, but yours may be labeled “Black Level”.

“Drive” and “Gain” are two interchangeable names for the upper-end Red, Green and Blue color gun adjustments. I use “Drive” in this guide, but yours might be labeled Gain.

“Cutoff” and “Bias” are two interchangeable names for the low-end Red, Green and Blue color gun adjustments. I use “Cutoff” in this guide, but yours may be labeled Bias.

CRT and colorimeter setup
1) Connect your test pattern generator to the display.

2) Power on the display. Display a 100% solid white test pattern.
In the 240P Test Suite, go to “Test Patterns” -> “White Screen”.

3) Wipe down the monitor glass with glass cleaner. Place the colorimeter directly on the glass in the center of the screen. Place a book or something heavy on top of the cable to stop it from sliding or falling off the screen.

4) Wait one hour with the screen on all-white to let the CRT and chassis warm up.

5) Connect the colorimeter to your laptop running HCRF with the USB cable. Launch HCRF.

6) Click on the “File” menu and choose “New”.

7) On the Generator Selection screen, choose:

  • DVD manual: if you’re using a pattern generator that can’t be controlled from the laptop, such as MiSTer HCRF core, 240P Test Suite or a DVD Player.
  • Automatic: if the laptop is the pattern generator and thus connected to the cab display as a secondary monitor.
    • PGenerator will also be detected and used by HCFR when Automatic is selected.

8) Select your model of Colorimeter from the sensor list, select “Do not use a meter correction file” and click Finish.

9) Your meter might now ask what kind of display you’re calibrating – choose “CRT” or “Refresh Display”. Also, Reading Type should be set to “Display”. Click the Calibrate Meter button. Some meters will then ask you to display a white image of 80% IRE or higher. You’re already displaying a 100% IRE all-white screen, so just click OK to finish the calibration.

10) In the main HCFR window that appears, click on the Green Triangle button in the menu bar to start taking readings.

Adjustments – Ideally, you’ll want to do this with the room as dark as possible.

Flyback Screen Dial – Initial Approximate Calibration
Source: https://emphatic.se/?p=710
Archived Copy: PDF

NOTE: If you don’t have straight-forward access to your CRT’s flyback, set each Drive, Cutoff, Contrast, and Brightness setting to the center position and move on to the next section.

1) Set the CRT controls as follows:

  • Each RGB Drive: All the way down.
  • Each RGB Cutoff: Center position.
  • Contrast: Center position.
  • Brightness: Center position.

2) Display a white grid pattern.

In the 240P Test Suite, go to “Test Patterns” -> “Grid”.

3) Turn the Screen dial on the flyback down (counter-clockwise) until the display is completely dark and pitch black. You want to go just past the last trace of the grid.

4) Turn each RGB Cutoff up until you see the black space around the white grid turn into the color you’re adjusting. Then turn it down until the black is perfectly black again.

5) Turn each RGB Drive up until the grid is really white without bleeding out / turning blurry.

Now the CRT is broadly dialed in, we can fine tune the adjustment using the colorimeter.

Contrast and Brightness – Precise Calibration Part 1
Source 1: https://consolemods.org/wiki/CRT:CRT_Color_Calibration_Guide
Source 2: http://www.curtpalme.com/forum/viewtopic.php?t=10457
Archive: PDF

1) Display a 100% IRE test pattern.
In the MiSTer HCFR core, choose “REC601 D65” or “REC601 D93” from the menu depending on which white point standard you’re using. Up and Down on the controller will cycle the IRE level up or down in 10% increments.
In the 240P Test Suite, back out and go to “Test Patterns” -> “100 IRE”. Depending on the console, some control buttons will adjust the IRE level up or down in 10% increments.

2) Click on the 100 Column in HCRF. If the colorimeter isn’t currently taking readings, click on the green Triangle button in the upper-middle of the toolbar.

The “Y” measurement tells you how bright your screen is. Typically, you want it at 100 nits. If you plan to always run your display in a dark setting, you may want to go down to either 95 or 90.

Adjust the SubContrast dial (if you have one; Contrast otherwise) until the Y readout hits your target.

  • If your chassis doesn’t have any Contrast adjustment dials or if you can’t get the Contrast to go high enough to hit your target, try adjusting the Red, Green and Blue Drive/Gain pots evenly upward.

3) Click on the 20 Column in HCRF. Change your pattern generator to display a 20% IRE test pattern.

4) Your target now for the Y value is 0.3% of the reading at IRE 100.
So if 100 nits is your IRE 100 target, 3.0 is what you want Y to read at IRE 20.
HCRF also calculates the target value for you – look at “Y Target”.

Adjust the SubBrightness dial (if you have one; Brightness otherwise) until the Y readout hits your target.

If you can’t go low or high enough try adjusting the Red, Green and Blue Cutoff pots evenly.

  • Note that aged chassis components and tubes may have trouble hitting these high and/or low targets. In that case, just get as close as you can.

5) When you change Brightness, it affects Contrast and vice-versa, so repeat Steps 2-4 until the Brightness and Contrast are balanced against each other with the appropriate Y target for each.

Drive and Cutoffs – Precise Calibration Part 2
1) Click on the 80 Column in HCRF. Change your pattern generator to display a 80% IRE test pattern.

2) Look at the gauges in the lower-left corner of HCFR. The Red, Green and Blue gauges show your levels relative to the target, while the yellow gauge shows your Delta E (deviation) away from the target. As the RGB gauges get close to 100%, the Delta E gauge will drop. Your goal is to get Delta E as close to zero as possible.

Adjust the Blue and Red Drive pots until the gauges are as close to 100% as possible and Delta E is as close to 0 as possible. Adjust Blue first before Red.

Adjusting Green will scew both the Red and Blue levels, so you shouldn’t touch the Green pot unless you can’t get Red or Blue to adjust far enough to reach your targets. You’ll notice as you adjust the Red and Blue levels closer to 100%, the Green will be pulled there as well.

3) Click on the 20 Column in HCRF. Change your pattern generator to display a 20% IRE test pattern.

4) Adjust the Blue and Red Cutoff pots until the gauges are as close to 100% as possible. Adjust the Blue first before Red.

Adjusting Green will scew the Red and Blue levels, so you shouldn’t touch the Green pot unless you can’t get Red or Blue to adjust far enough to reach your targets. You’ll notice as you adjust the Red and Blue levels closer to 100%, the Green will be pulled there as well.

5) When you change the Drive pots, the Cutoff target shifts and vice-versa, so repeat Steps 1 – 4, going back and forth between 20 and 80 IRE and adjusting Cutoff and Drive respectively until both of them are as close to 100.0% RGB and 0.0 Delta E readings as possible on the gauges.

Now, the display is pretty well dialed in at this point, but if you’re extra picky, repeat the Part 1 – Contrast and Brightness steps one more time, because adjusting the Drive and Cutoffs will have shifted those values a little bit. If you do end up readjusting Contrast and Brightness, you’ll also have to readjust the Drive and Cutoffs again afterward as well, but none of them will need very much readjustment.

Verifying the White Balance Calibration
If you’d like to check your cab CRT now and see how the White Balance calibration holds up across the gamma range, do the following:

1) In HCFR, click the Green Triangle to stop taking constant readings.

2) Click the Measures menu and choose “Gray scale”, and click Yes at the prompt.

3) The software will prompt you to “set 0% grey level”. Set an IRE 0% test pattern on your generator.
In 240P Test Suite, use the L and R buttons on the controller to set IRE to 0, and click OK.

4) After a moment, the software will now prompt to “set 10% grey level”. Set an IRE 10% test pattern on your generator.
In 240P Test Suite, use the L and R buttons to set IRE to 10, and click OK. Repeat all the way up through the 10 incrementing IRE levels.

5) When the measurement is complete, look at the DeltaE row. Recall that DeltaE measures your variance from the perfect white balance target. If all ten cells are green (Delta E < 2.0) then you’re in good shape – your white balance is approximately on-par with a professional CRT. If some or all of the cells are yellow (Delta E between 2.0 and 3.0) then you’re approximately on par with a consumer CRT. If some or all of the cells are red (Delta E > 3.0) then you’re off target – either the calibration is wrong, your chassis PCB needs servicing, or the color guns in your tube are worn out.

Also, with Gray Scale selected from the drop-down in the upper-left corner, make a note of your Average Gamma and your Contrast Ratio. Ideally, you want an Average Gamma of 2.2 and an Average Contrast Ratio of at least 1000 : 1, but your results will vary based on tube and chassis age, specs, etc.

This post-calibration result of a Panasonic Tau consumer CRT shows some drift at the 90 and 100 IRE levels.

Note that calibration drifts as the tube and caps age – you may want to “tune up” your calibration every couple of years.

Post-Calibration Examples

Seibu Kaihatsu SPI

 Encyclopedia  Comments Off on Seibu Kaihatsu SPI
Apr 182024
 
Last Updated: 4/18/2024
Article Credit: ShootTheCore, Ikotsu, rtw, Trap15, ekorz, twistedsymphony, channelmaniac

The Seibu Kaihatsu SPI platform was offered in two configurations:

  • Two Board – Main Board and cartridge ROM Board
    • Region locked: Main Board must match country of cartridge ROM Board. This protection can be bypassed – see “Protection Mechanisms” below.
    • Optional Stereo audio.
  • Single Board
    • Only runs the one game that was loaded onto it at the factory.
    • Offered with two different audio setups- Yamaha YM-271-F or dual OKI MSM6295
    • Mono audio only.

This article will address the two configurations separately.

Two-Board Configuration: Main Board

The SPI main board contains all the processing logic to run the game. It was released with two revisions, both of which can run the same ROM game cartridges. The marking on the board for each revision is:

  • (C)1995 SXX2C-MAIN V2.0
  • (C)1998 SXX2D-MAIN V2.1

Stereo sound output is available on the two-board SPI configuration through the CN1 connection header. Everten sells a nice add-on board that adapts the CN1 header to standard RCA stereo audio ports.
https://shewfly.com/products/seibu-kaihatsu-spi-stereo-board-by-everten
https://www.candycabclub.com/product/everten_spi

The video and audio quality is said to be better with the SXX2C-MAIN V2.0 revision.
Source: https://shmups.system11.org/viewtopic.php?t=43308

The SPI system has onboard flash memory used for both region lockout and to store part of the game. The V2.0 board restricts write access to the flash memory by a jumper labeled JP072. The V2.1 board does not have any flash write access restriction jumper.

When swapping game cartridges, you will need to change the jumper (V2.0 only), install the new cart, power on, wait for the game to write to the flash memory (the message “NOW UPDATING. PLEASE WAIT A MOMENT.” will be displayed), power down and flip the jumper back (V2.0 only). Even after flashing, the ROM cartridge must stay attached for the game to operate.

Notable ICs

ICFunctionPCB Label
AM386DX/DXL-25 – OR –
Intel NG80386DX25
25.0 Mhz Main CPUU0169
Zilog Z84C0008PEC7.15909 Mhz Audio CPUU1122
Yamaha YM271-FAudio Processor and Flash AccessU1020
Yamaha YAC513-MAudio DACU1068
Intel E28F008SA – OR –
Sharp LH28F008
8 MBit FlashROMU1053, U1054
SEI400Graphics RenderingU048
SEI600Graphics RenderingU0154

Two Board Configuration – ROM Cartridge Board

Cartridge boards include DIP socket EPROMs, SMD mask ROMs, SRAM and a custom decryption IC – see Protection Mechanisms below. It is not possible to reliably convert one SPI game cart to another at this time, although it is possible to upgrade the same game to a different version (ie Viper Phase One to Viper Phase One NV).

ROM Board model numbers

  • SXX2C ROM SUB
  • SXX2C ROM SUB2
  • SXX2C ROM SUB4
  • SXX2C ROM SUB8
  • SXX2C ROM SUB10

Common Issues and Quirks

CHECKSUM ERROR or HARDWARE ERROR (credit: rtw & Trap15 and channelmaniac)

The two-board SPI platform stores part of the game inside onboard flash storage. The flash storage is also initialized from the factory with a country code. If the country code stored in the first byte of u1053 flash on the mainboard does not match the country code stored in the last byte of the PRG0 EPROM on the ROM cart then you will see the message CHECKSUM ERROR XXXX or HARDWARE ERROR XXXX during the flash process where XX is a two or four digit error code.

It is possible to desolder the u1053 flash chip from the main board to reprogram it with a new code. A list of the country codes is in the opening comments of the MAME driver.

Alternatively, Trap15 developed a software solution named SPI Revive that can rewrite the country code on the mainboard by swapping the PRG0 EPROM on a Raiden Fighters or Raiden Fighters 2 cart. If SPI Revive is unable to change the country code then the u1053 flash chip should be replaced.

SPI Revive: https://bitbucket.org/trap15/spi_revive/src/master/

Source: https://shmups.system11.org/viewtopic.php?t=62636

There have also been cases where the flash chips are holding the correct country code data but the board isn’t reading that data correctly. The Yamaha YM271-F sound chip controls access to the flash storage, so check the connections and traces on it.

Reflowing the solder on all of the large SMD ICs is recommended for HARDWARE ERROR and CHECKSUM ERROR messages if the earlier methods of writing the country code do not resolve the problem.
Credit: ChannelManiac

Boot Failure or Vertical Lines (jailbars) through graphics (credit: twistedsymphony)

It is common for the large SMD ICs to have pins separate from the PCB due to board flexing.

If an SPI board fails to boot at all on a known-good ROM cartridge, try reflowing the solder on each SMD IC, starting with the 386 Main CPU – it seems particularly susceptible to this issue.

If an SPI board has vertical lines running through the graphics, reflow the solder on the two SEI400 and SEI600 graphics rendering ICs.

Single Board Variants

In some regions, Seibu Kaihatsu released some of their games on cost-reduced, all-in-one single boards. These boards are “locked” to the game they ship with, do not have stereo output available, and in some cases use lower-quality dual OKI MSM6295 ICs for audio instead of the Yamaha YM271-F IC that the two-board SPI platform utilized.

  • SYS386I -Raiden Fighters 2 Operation Hell Dive 2000 (AMD 386 CPU and dual OKI MSM6295 Audio)
  • SYS386F – E-Jan Sakurasou (Intel 386 CPU and Yamaha Audio)
  • SYS386H – Raiden Fighters Jet 2000 (AMD 386 CPU and dual OKI MSM6295 Audio)
  • SXX2F – E-Jan Sakurasou (Intel 386 CPU and Yamaha Audio)
  • SXX2G – Raiden Fighters Jet (AMD 386 CPU and Yamaha Audio)

The SPI hardware and games was never bootlegged or cloned by another company.

Daughterboards

The SPI platform only utilized one daughterboard – the SXX2C Mahjong adapter used with E-Jan Sakurasou.

Capacitor List – V2.0 and V2.1 (credit: ekorz)

LocationCapacitanceVoltageDiameter (mm)
C123470 uF26 V10
C051470 uF16 V10
C01210 uF16 V5
C091110 uF16 V5
C092110 uF16 V5
C092610 uF16 V5
C12710 uF16 V5
C121610 uF16 V5
C12110 uF16 V5
C096100 uF25 V6.3
C129100 uF16 V6.3
C1210100 uF16 V6.3
C1214100 uF16 V6.3
C122100 uF16 V6.3
C1215100 uF16 V6.3
C124100 uF16 V6.3
C1217100 uF16 V6.3
C125100 uF16 V6.3
C1219100 uF16 V6.3

Film / Ceramic Resistors – Optional

LocationCapacitanceLead spacing (mm)
C0920182K5
C0923182K5
C0927392K5
C008392K5
C0924222K5
C0913222K5

ROM Details (credit: MAME, twistedsymphony)

Due to the variety of ROM boards used across the SPI games, consult the MAME Driver for game-specific ROM assignments.

The DIP EPROMs can be replaced with a 27C040 EPROM. Replacement of a faulty SMD Mask ROM with an SMD flash might be possible depending on which mask ROM it is.

Protection Mechanisms

Sprite data is encrypted differently for each game and paired to a specific sprite data decryption IC on the cartridge board. This proved to be an effective deterrent against cartridge game conversions and bootlegging.

Schematics & Datasheets

Schematics for the SPI hardware are not available.

Additional Resources

MAME SPI Driverhttps://github.com/mamedev/mame/blob/master/src/mame/seibu/seibuspi.cpp
Trap15: SPI Revivehttps://bitbucket.org/trap15/spi_revive/src/master/
Ikotsu: Raiden Fighter Jet – Seibu SPIhttp://ikotsu.blogspot.com/2010/01/raiden-fighter-jet-seibu-spi-jet.html
Everten SPI Stereo Boardhttps://shewfly.com/products/seibu-kaihatsu-spi-stereo-board-by-everten

https://www.candycabclub.com/product/everten_spi
World of Arcades: Changing SPI region codeshttp://www.world-of-arcades.net/seibu_spi/

Silkworm repair log #2 and schematics

 PCB Repair Logs, Repair Logs  Comments Off on Silkworm repair log #2 and schematics
Apr 162024
 

Board is a bit dusty but very clean nonetheless. Some very specific chips are rusty, probably due to some rodent fluid. Tracks are invisible and located in the core of this 4-layer board making the inner tracks very fragile when pulling a chip out.

Game starts when powering the board with full sound and some graphics. The title screen has a white background instead of black as it should be. A similar white background appears during attract mode:

Cross-testing the board with a good one points the fault to the bottom board (graphics board). On this board I started probing the 8 EPROMs filled with the background graphics data. Some pins are quite corroded, so I decided to dump them to check their contents against MAME data:

As a result, EPROMs 10 and 14 are dead. I burnt two replacement EPROMs and that brought the background back:

Background is back but the sprites are blinking, they are misplaced and have bad colors. Sprites are generated by the custom chip Mitsubishi M60002-0118P and displayed through twenty 4164 DRAMs. These are known for having a high failure rate. I will focus my investigations in this area:

Probing the D-OUT pins of these DRAMs (pin 14), 8 DRAMs out of 20 shows an output signal stuck low or even floating when the game has to display sprites:

I pulled these 8 D-RAMs:

All of them were confirmed bad out of circuit.

I fit 8 brand-new replacement DRAMs. Some sprite glitches are still visible – two other DRAMs have just gone bad. I took those out and replaced them. Now the sprites and backgrounds appeared to be rendering properly. However, the sprites motion is still jerky.

Rygar shares the same hardware as Silkworm and has schematics available. In the schematics, we can see that two 6116 SRAMs handles foreground and background graphics and a third one is dedicated to sprite positioning (SP/POSITION.RAM as per the schematics). This is the area I will inspect.

Inspection of the SRAM chip at location 6L doesn’t show anything abnormal. While probing around the area, I came across a signal stuck high on a 74LS193 binary counter (BORROW output) located at 7K. I double-checked it with my logic comparator:

Red light indicated a bad output – I pulled the suspect:

Confirmed bad out of circuit:

Replacing this chip fixes the jerkiness. Game is perfectly playable. Sound and controls tested OK.

Summary :

  • 11 DRAMs (SAMSUNG),
  • 2 EPROMs,
  • 1 74LS193 (TEXAS INSTRUMENTS)

Some months after this repair, I found the proper original Silkworm schematics, scanned it and upload to my GitHub here:
https://github.com/DocteurPCB/schematics/blob/main/Silkworm%20%5Bschematics%5D.pdf