AVI ELF II Detailed Assembly Notes
This page documents the detailed assembly notes for building the AVI ELF II Rev D Variant main board.
Note 1: The Clock Circuit for the AVI ELF II Rev D.
We have recently discovered that due to differences in production of integrated circuits since the original Netronics ELF II was produced, the values for the components in the clock circuit may produce erroneous results. Problems we have noted are: A clock that runs the CPU and the Pixie Graphics chip at 3.579MHz, A clock that runs it a harmonic of the clock crystal or A clock that is too misshapen to be useful.
1a: A15, the clock driver IC is a TTL 74LS00. We have found that in some cases a modern 74LS00 will not produce a useable clock signal. If you are able to source an older SN7400 or SN74LS00 or similar TTL 7400 chip, you will likely have better success than with a new 74LS00. We have tested our stock of TTL chips and have produced a good clock source with 7400 and 74LS00 IC's with date codes from the early 1970's to the late 1980's and 90's.
1b: The components for the frequency divider (A16, R30, R36 and C10) should be changed to work in accordance to the newer TTL chips on the, market; specifically the 74LS74; no changes to the PCB need to be made.
The changes are as follows:
- A16 should be 74LS74 not 74HC74
- R30 and R36 should be 2.2K resistors, not 10K
- C10 should be 47pF not 330pF
1c: If you can not manage to find a 74LS00 IC that will produce a decent clock signal, you can substitute a 3.579545 MHz TTL Oscillator 5V 14-DIP, 4 Leads (Full Size, Metal Can) in place of the 74LS00 IC. We have tested this and found that all functions work using a 3.579545 MHz crystal oscillator, including the 1861 Pixie Graphics IC. The unit we have used can be found at Digikey at: https://www.digikey.com/en/products/detail/ecs-inc/ECS-100AX-035/827253.
Photo of the 3.579545 MHz crystal oscillator in place of the SN7400 on a working AVI ELF II:
We will be updating the BOM, Schematic and PCB Gerbers. This note will be removed when we have updated them.
Download the zipped archive of Gerber Files to produce your AVI ELF II PC Board. You can send this zipped archive to most PC Board Fabricators to have a set of boards created at a reasonable cost.
Please download the AVI ELF II Bill of Materials Excel spreadsheet to help you to acquire all of the components for the board. We have included links to all of the components that we were able to source from known electronics parts houses and have included notes for the components that may be difficult to find.
We suggest that all integrated circuits for the ELF II main board are installed with high quality sockets. We have assembled many prototype boards for this project and have found that "cheap" turned pin machine sockets have failed us on more than one occasion and caused us to troubleshoot problems that ended up being directly the result of faulty sockets. If you can't find affordable high quality machine sockets, then we suggest you rely on dual leaf sockets which are rarely known to fail.
The Original Netronics ELF II kit was shipped with wooden triangular end pieces that attached to the main board with four screws. During assembly it will be handy to make a set of these triangular pieces and attach them to the bare PC board so that you have a way to hold the board off the table top when you place your components for soldering. The triangular pieces have the following dimensions: 1/2 inch wide veneered Wood 2-3/4 inches tall by 7 inches long with an 1/8 inch bevel on the very front tip so it's not sharp down to a point. The hypotenuse of the trangle is attached to the PC board and measures approximately 7-1/2 inches long.
Note: You will need to slightly notch the left side triangular piece to clear J11 and C4. During this example build, I only installed the right side triangle and then added the left side after J11 and C4 had been installed.
During the building and testing phase, Costas Skordis came up with a set of 3D printed end pieces. You can download the STL files to print them for your own AVI ELF II main board.
The AVI ELF II Main Board was designed to provide several options for displaying the data and in some cases the location on the address bus. This is achieved through two options on the main board and two options that use plug-in display cards.
The main board supports two options:
- HP5082-7740 7-segment LED displays. These are the LED displays that shipped with the original Netronics ELF II kit. These displays are no longer in production but they can be found on EBay and a variety of electronics vendors that specialize in new old stock or reconditioned vintage parts.
- HP5082-7340 dot matrix LED displays. These were used in the original Popular Electronics ELF Computer construction articles. Like the 7-segment LED displays above, these displays are no longer in production but they can be found on EBay and a variety of electronics vendors that specialize in new old stock or reconditioned vintage parts.
Three additional options are available through the addition of plug-in display cards:
- Two versions of a TIL311 dot matrix LED display card can be assembled, which then plug into a set of 4 headers just in front of the keyboard. The boards use Texas Instruments TIL311 dot matrix displays that are no longer commonly available but they can be readily acquired from numerous ventors on EBay and through a variety of electronics vendors that specialize in new old stock or reconditioned vintage parts, such as Jameco Electronics.
- 2-Digit TIL311 dot matrix LED display card: Download the Gerber files for the TIL311 Display Board here to have the board made up at one of the available PCB fabricators.
-
6-Digit TIL311 dot matrix LED display card: Download the Gerber files for the 6-Digit TIL311 Display Board here to have the board made up at one of the available PCB fabricators.
Note: The 6-digit board can run as a 2-digit data only board by only populating one 74HC244 and the two TIL311 displays for the data readouts.
- A modern dot matrix display card can be assembled and plugged in to the main board providing address and data values. The card uses parts from Pic Micro, MAXIM and easily acquired LITEKEY dot matrix displays.
- Download the Gerber files for the Modern Dot-matrix Display Board here to have the board made up at one of the available PCB fabricators.
The board in this example build will use the original HP5082-7740 7-segment LED displays which were purchased as new old stock from a US EBay vendor.
The hex keypad on the AVI ELF II Rev D Variant uses 16 normally open SPST push button switches for HEX characters 0-F, plus one more for the Input key. The switch footprint is known as an MX or MX1 type switch which are commonly known as Cherry key switches (Cherry Americas LLC). The set of switches we have recommended in the bill of materials are pre-lubricated, smooth action, non-clicky switches. We have chosen them because they closely approximate the action and feel of the original switches that were used in the original Netronics ELF II kit and which are no longer available. The switches we have recommended also have plastic positioning pins to ensure perfect alignment with the PC Board.
You will need to supply your own MX type keycaps for the 16 keys (A-F), plus the input key. There is no lack of suppliers to provide the keycaps. We recommend using XDA profile keycaps as they are consistently shaped without a slant in any direction. Reasonably priced MX type keycaps can be found on Amazon.
Raised Keyboard
Cherry MX type push button switches are lower than the original "stackpole" push button switches used on the Netronics ELF II, so we have provided an option to raise the key switches if that is your preference. If you intend to install your AVI ELF II Rev D Variant board in a Netronics ELF II case or a replica of the Netronics ELF II case, then you will want to raise the keyboard to approximate the original keypad's height. This is done by installing the key switches on a keypad daughter board that plugs in to five 4-pin headers that are on the perimeter of the main board's keypad footprint.
Download the Gerber files for the raised keyboard daughter card here to have the board made up at one of the available PCB fabricators.
VIP Keypad
Another raised keypad option is the VIP Keypad, designed by Josh Bensadon. The VIP Keypad mounts as a daughter board above the main AVI ELF II board with the RCA VIP specific keypad layout, a cutout to expose the ELF II data displays and a tiny speaker for VIP sound effects. If you intend to VIP your AVI ELF II, the VIP Keypad is a must have addition.
Download the Gerber files for the raised VIP Keypad daughter card here to have the board made up at one of the available PCB fabricators.
The easiest way to build the ELF II main board is to start small and move up. Begin your build with the smallest components and then start moving up to larger components. The entries below detail the order of component installation we have chosen for ease of construction.
The majority of the resistors and diodes has the exact same lead spacing which is at about 12mm on the inside edge. To assist in getting consistent bends, I suggest you make a jig out of heavy card stock or any sturdy material that you can cut to 12mm wide and with two parallel inside lines 6mm apart; that way, you can place the component between the lines on the jig and make consistent bends of all of the resistors and diodes. The following diagram is a line drawing of the "lead bending jig".
- R15 is an outlier and does not share the same lead length as the others.
Our build methodology begins with installing the 16 diodes; 12-1N914, 1-1N4001, 3-1N5819. Gather the parts and bend the leads with with small needle nose pliers, then insert the diodes and tape them in place on the top of the board with masking tape to hold them in place while soldering and then remove the tape after the diodes are soldered in place. Solder the diodes as quickly as possible to avoid overheating and damaging them.
Note: If 1N914 diodes are unavailable, for our purposes 1N4148 diodes are a suitable substitute.
After the diodes, resistors are the next smallest components so they are next to be installed. Use the same method you used to install the diodes as you use with the 13 resistors on the board.
The 9 ceramic capacitors that populate the board are the next smallest components after the resistors so they are the next components to install. The shape of the capacitors make them a little tricky to solder in vertically but with some care and attention they should be reasonably simple to install. Take care not to overheat the capacitors when you install them. It should only take a fraction of a second to solder each end with a good soldering iron and good quality solder.
In the bill of materials we are recommending a LM-7805 or V7805-2000R, which is a miniature DC-to-DC converter with the capacity to produce 5 volts at 2 Amps. The V7805-2000R is a really unique device, it runs with zero heat on this board, even with expansion boards but it isn't necessary; a LM-7805 will run this board with very little heat generated. If you are the least bit concerned about heat with an LM-7805, simply add a heatsink to it and it should be fine. For the example build in this document, I am using an LM-7805 without a heatsink.
We have taken care with the AVI ELF II Rev D Variant main board to orient all IC's consistently. All vertical IC's are oriented with the notch locating pin 1 up and all horizontally placed IC's are oriented with the notch locating pin 1 to the left. Please take care to install your IC sockets and IC's accordingly.
The number of sockets to install will be determined by the display option you have chosen for your build. I have chosen to use the original HP5082-7740 7-segment LED displays for my build which means that I will need two additional sockets for the display drivers (I will solder the resistor networks directly to the board). In my build I will be installing 18 IC sockets. If I were to use the HP5082-7340 dot matrix LED displays, I would only need to install 16 IC sockets. I've chosen dual leaf IC sockets except for the socket for the CDP1861 Pixie Graphics chip, which I had a quality 24-pin turned pin machine socket for.
NOTE: If you are planning to install the 7-segment HP displays that require RN5 & RN6 DIP resistor networks, we recommend that the DIP Resistor Networks are soldered directly to the PC board so that they do not interfere with any mezzanine type display cards like the VIP Keypad for ELF II.
Note: For mounting the HP5082-7740 7-segment LED displays, I have sacrificed a 24-pin turned pin machine socket, cutting the two rows of pins into 4 5-pin rails.
In my builds I have socketed the clock crystal so that I can swap it out with different crystals if I need to. I created a socket for my board by sacrificing an 8 pin turned pin machine socket, removing a bank of 4 conductors, splitting it into a pair of 2-pin sockets and soldering them into the 4 holes that are available for the clock crystal.
Install the 4 resistor network components next (RN1-RN4). These resistor networks have a common pin which is often marked on the resistor network with a circle or solid dot. If in doubt, use an ohm meter to assess which pin is the common pin and take care when placing them to make sure that you have them oriented in the correct direction. RN1, RN3 & RN4 have the common pin in the top location. RN2 has the common pin in the bottom location.
Note: As noted in step 5, RN5 & RN6 DIP resistor networks are only necessary if you are installing the HP5082-7740 7-segment LED data displays and should be installed directly to the board so that there is room above them for the mini speaker on the VIP Keypad for ELF II board should you decide to add on the VIP Keypad daughter board. The DIP Resistor Network footprints are longer than a standard DIP package and we found it necessary to substantially file both resistor networks in order to allow enough room for them to fit. Fortunately, the network is quite deep and there was no risk in exposing it when we filed the plastic packaging. You can also install separate resistors in place of the DIP resistor networks which may reduce the footprint further.
Install the two LED's for power and the Q-bit taking care to make sure that they are oriented correctly. Typically, the negative lead of the LED is dignified by the flat side of the otherwise rounded lens and the negative lead is generally slightly shorter than the positive lead.
The jumper pins are made up from Breakaway Male Pins Headers that come in rows of 40 conductors that are easily snapped into headers of however many conductors you need. In this case we are using some that were purchased from Digikey but they are commonly available from most electronic supply houses. In this example build I am installing four 3-pin, three 2-pin and one 1-pin header.
Note: The two 2-pin headers at the top, right side of the board, beside the power switch are not necessary unless you intend to install a Netronics Giant Board but I am installing them in this example build.
Install the horizontally oriented power switch next and after it has been soldered in place, install and solder in the 8-position dip switch that manages the mapping of the 32K of onboard RAM. Orientation of the 8-position dip switch is up to your preference. I tend to orient mine so that when the switch positions are rocked forwards towards the RAM, they are engaged (closed) and when they are rocked back away from the RAM, they are disengaged (open).
The four jacks that manage video, cassette in/out and power are up next. If you intend to run the 1861 Pixie Video chip, you will need to install the video out connector. The cassette in/out sockets are only necessary if you are going to add an expansion card (Netronics Giant Board, AVI Hyperboard) that provides the support circuitry to save/load programs on cassette tape or another audio device. I have included them in this build for completeness and the possibility of expansion later. The power jack is of course necessary.
Next, place and solder the 4 electrolytic capacitors onto the board. C1, C8 & C12 are likely going to be the same height and can be placed and soldered at the same time. C2, the 1000uF capacitor is likely larger than the other three and will be easier to place and solder after the shorter three capacitors.
If you are soldering the crystal directly into the board, now is as good a time as any to do so. If you are socketing the crystal, you can wait to do that when you install all of the IC's and begin check it out and testing.
Next up for installation are the 17 keys for 0-to-9 and A-to-F, plus the Input key. As noted above under KeyPad Considerations, the keypad key switches can be installed directly on the AVI ELF II Rev D Variant board or on a raised daughter board that connects via five 4-pin headers.
Below is an example of the key switches mounted directly on the PC Board.
In this example build, we are installing the keypad on a raised daughter board that will connect to the five 4-pin headers.
Providing you have installed the key switches directly to the AVI ELF II Rev D Variant PC Board or to a raised keypad daughter board, the last components to install are the three toggle switches for Run, Load & Memory Protect.
It is extremely helpful to have the triangular side piece installed at this point (as mentioned at the top of the page) so that you can insert the switches and use the right angle of the triangle to support the PC Board vertically while you solder the switches in place. For this example build, I found the most effective technique was to press the switches into place; the fit of the switches recommended in the bill of materials spreadsheet have just enough clearance to remain in place when they are pressed in. I soldered the middle conductor for each switch while ensuring they remained perfectly aligned and then put the PC Board on my soldering bench to solder the remaining conductors.
Note: If you intend to mount your AVI ELF II board to an original Netronics ELF II case or a reproduction case, you will want to slide the switches into the PC Board then mount the board to the case to find the perfect orientation for the switch. You may need to tack one of the pins for each switch in place so that you can orient them correctly in the case and then remove the board to solder the remaining pins. Josh Bensadon, who designed the new AVI ELF II boards, fine tuned the switch footprint locations to get the switches as close to the original to fit well with his authentic Netronics case.
If you have followed these build instructions, you should be able to install the IC's, displays, crystal (if it's socketed) and when you apply power your ELF II Rev D Variant will come to life. That said, it's probably a good idea to exercise caution and engage in a checkout routine before inserting all of the integrated circuits and displays.
Without any of the integrated circuits installed, plug in your DC adapter; an 8 volt DC adapter with centre positive is recommended. When you flip the power switch to the right, the Power LED should illuminate. The LM 7805 should remain cool to the touch and you should read 5v at the power/ground positions on your IC sockets.
If your tests confirm the above assertions, flip the power switch off, unplug the power supply and populate the board with the integrated circuits from the bill of materials, taking special care to insert the correct components and their correct orientation for pin 1 of each device.
The AVI ELF II Rev D Variant has 5 Jumpers at the bottom of the board. It is somewhat important to have them configured correctly to use the AVI ELF II as a classic ELF type computer with the Pixie Graphics chip for video output.
Note: J3 and J11 are a not necesary if a CDP1861 Pixie Graphics chip is not present. The 1861 is optional, but highly recommended for the full COSMAC ELF experience.
J3 located just below the Pixie Graphics chip (CDP1861) is Display off. For a basic non-expanded ELF II, this jumper should be set to the center and right pins.
Note: On the original ELF II, this pin was ground (no way to turn off the display, aside from reset). It cannot be set to I61 unless an expansion board that creates I61 is installed. Boards that create I61 and I69 are the Netronics Giant Board, AVI Hyperboard, STG ELF 2K Disk Board and the ELF II SD Card.
J4 which is located just above the 74HC74 is used for expansion boards such as the Netronics Giant Board. For standard use, the jumper needs to be in the lowest position and positioned vertically.
J7 (Synch) has worked best for me with a jumper across the two pins but that may be specific to my monitor. Try it with and without the jumper when you are using the on video graphics capabilities of the AVI ELF II and use the setting that works best for you.
J9 located on the bottom far left should go to EF1. Only the Netronics Giant Board and the AVI Hyperboard support the DISP ST functionality.
J11 located on the bottom far left should bridge the center and bottom pins unless the ELF II is expanded with a Netronics Giant Board, AVI Hyperboard, STG ELF 2K Disk Board and the ELF II SD Card.
Final checkout completed. Up and running!!
Now that you have your own fully functioning AVI ELF II Rev D Variant, here are a few short programs to test it out with.
In order to program your AVI ELF II, you will need to know how to Enter, Inspect (step through the instruction) and Run your Programs.
To Enter a Program
- Turn the power on for the ELF II and note that the power LED is illuminated and the data displays are lit. The values that show in the data displays aren't relevant as we will be entering a new program.
- Make sure that all three toggles for RUN, LOAD and MEMORY PROTECT are toggled in the off (down) position. This is the reset position.
- Toggle the LOAD switch up.
- To enter code from a program listing, use the keypad to enter a pair of hexadecimal characters from 0-F and press the "I", input key. Upon pressing the input key, the characters that you just entered will be shown in the LED data displays and they will be stored in the computers program memory (RAM).
- As a test, use the keypad to enter pairs of of the 16 hexadecimal characters starting with zero: 00, 11, 22, 33, ... pressing the Input key after each pair of characters until you reach and input the characters FF.
- Flip the LOAD toggle off to take the AVI ELF II out of programming mode. In the next section, you will inspect what you just entered.
Inspect Your Program
It is often helpful to step through your program to inspect it and make sure that you haven't made any mistakes. To inspect the series of codes that you just entered, perform the following steps:
- Make sure that RUN, LOAD and MEMORY PROTECT are toggled in the off position. This puts the ELF II in the reset state, with the address register at 00.
- Toggle the MEMORY PROTECT switch up and the LOAD switch up. MEMORY PROTECT will prevent you from overwriting your program, while you review your program.
- Press the Input button to review all of the keystrokes you entered in the previous section, in the order that you entered them.
Run Your Program
In order to Run your program perform the following steps.
- Make sure that RUN, LOAD and MEMORY PROTECT are toggled in the off position. This puts the ELF II in the reset state with the address register at 00.
- Toggle the RUN switch up. If you have entered a program, your program should now be running. To stop your program, toggle the RUN switch down (off).
Demonstration Programs
The following are some demonstration programs. Use the directions above to LOAD, REVIEW and RUN the programs below. The column on the left is a reference of the Address space that this program instructions reside in. The two digit codes to the right are the instructions. Enter each two digit instruction and then press the Input key to load it into the program RAM.
The following program will put the CPU into a loop where it turns the Q-LED off and then on, repeatedly. It will run at full speed so it will simply look like the LED is on. The first instruction 7A turns the Q-LED off. The second instruction 7B turns the Q-LED on. The third instruction 30 tells the CPU to jump to the next address which is the start of the program at 00. Enter the following listing, then flip LOAD off and RUN on. The Q-LED will turn off and on at full speed, appearing to be solidly on.
0000 7A
0001 7b
0002 30
0003 00
The next program will introduce a delay to allow you to see it blink off and then on.
This program is from Tom Pittman's Short Course in Programming and it employs some clever uses of skip instructions to count from 00-FF as a delay between off and on states of the LED.
The first instruction 91, checks Register 1 to see what the count is. The next instruction CE is a long skip if the count is zero. This means it will skip the two following instruction, 7A (turn Q-LED off) and 38 (skip one instruction) if the count is zero. The long skip occurs once every 256 increments. The instruction after the long skip is 7B which turns the Q-LED on. The following instruction 11 increments the counter in Register 1 and the last instruction, 30 jumps to the address 00 to start the process over again.
For 255 loops of the program the CPU will check the R1 register, execute the 7A instruction to turn the Q-LED off, skip the 7B instruction to turn the Q-LED on, increment the counter in R1 and loop back to repeat the process. Every 1/256 loops, it will skip the 7A instruction and the skip instruction to then execute 7B to turn the Q-LED on, increment the timer in R1 and then loop back to repeat the process. It will continue this until you toggle the run switch off or turn off the computer.
Enter this program using the programming steps discussed above and when you run the program the Q-LED should blink off and on slowly enough for you to see both states easily.
0000 91
0001 CE
0002 7A
0003 38
0004 7B
0005 11
0006 30
0007 00
This is another program from Tom Pittman's Short Course in Programming. In this program if you press the Input key, the Q-LED will illuminate. The program does this by testing the input key to see if it is not being pushed and if it is not being pushed it will jump to an address. The listing begins with the familiar 7A to turn the Q-LED off. Then it executes 3F which branches to the address 04 in the next instruction if the Input key is not being pressed. Address 04 contains the instruction 30 to branch back to the start. If the instruction 3F finds that the Input key is being pressed, it does not branch and the instruction 7B is executed to illuminate the Q-LED. Then the branch instruction 30 is executed to go back to the beginning and check again.
Enter this program using the programming steps discussed above and when you run the program the Q-LED will turn on when you press the input key and will turn off when you release it.
0000 7A
0001 3F
0002 04
0003 7B
0004 30
0005 00
The following program counts from 00-FF and displays the results in the data displays. It uses 3 of the 16 general purpose registers to handle counting, a delay to slow the count to human speed (about one digit per second) and a register to pass the count to the data displays.
Instructions
0000 F8 FF
0002 A7
0003 F8 00
0005 A9
0006 F8 40
0008 B9
0009 17
000A 87
000B 58
000C E8
000D 64
000E 28
000F 99
0010 CE
0011 C4
0012 C2
0013 30 03
0015 29
0016 30 0F
Check your work
After you have loaded the program completely, it is a good idea to check that you have entered it correctly. To do so, you will reset the computer and put it into memory protect mode to step through the program. Flip the Load toggle down so that all three toggle switches are in the down position. Then, flip the Memory Protect toggle up and the Load toggle up. Press the input key to step through the contents of the program memory.
Run the program
If you are happy with the contents of the program, flip the Memory Protect and the Load toggles down to reset the computer and flip the Run toggle up. If everything is entered correctly, you will see the computer count from 00-FF.
In the resources below, there are some programs that you can use to exercise the CDP1861 Video Chip but the simplest way to test the video chip to make sure that it is at least working is the following two instruction program that simply turns on the video and tells it to display the contents of the RAM.
Instructions
0000 61
0001 69
Run the program
Make sure that you have your VIDEO OUT jack connected to the RCA Composite Video input on your TV or compatible monitor and that you have set the following Jumper settings on the main board:
- Jumper J3 to "GND" by placing a jumper shorting connector on the middle pin and right side pin of J3.
- Jumper J9 to "N0" by placing a jumper shorting connector on the middle and bottom pins of J9.
- Jumper J11 to "EF1" by placing a jumper shorting connector on the middle and bottom pins of J11.
If you are happy with the two instruction contents of the program, flip the Memory Protect and the Load toggles down to reset the computer and flip the Run toggle up. If everything is entered correctly, your video monitor with have a black background with a rectangle of random video "noise" displaying the random contents of the computer's memory.
Learn how to program your AVI ELF II Rev D Variant through the lessons in the Short Course in Programming by Tom Pittman.
Here are A few simple programs to exercise the AVI ELF II that I found on the Internet.
The COSMAC ELF Website is a wonderful resource for information, articles and software.
We have done our best to make sure that the AVI ELF II microcomputer is as rugged and dependable as the original Netronics computer; nonetheless cutting edge 1970’s technology can be temperamental. Below are two troubleshooting areas that we have encountered and the steps that we followed to resolve them. An oscilloscope that can work in frequencies at 5MHz or better is a handy tool to have when troubleshooting microcomputers like the AVI ELF II.
Please see the note at the top of this page regarding an errata for the clock circuit components.
If you find that keystrokes are not being registered on the data displays each time you key in a pair of values and press the input button when the ELF II is in program entry mode, it may be that the crystal controlled clock is not working. If this is the case, more than likely the HEX data displays will register some random value and will not step through the program memory or accept entries into the program memory.
An oscilloscope that is capable of handling signals up to 5MHz would be a handy tool to see if the clock is oscillating and at what frequency. Probe pin 1 of the CDP1802 to see if it is getting a clock signal. It should see a clock signal of approximately 1.79MHz. You can also probe the expansion bus pads on the main board where it is marked 3.57MHz where you should find the corresponding clock signal.
We have recently discovered that due to differences in production of integrated circuits since the original Netronics ELF II was produced, the values for the components in the clock circuit may produce erroneous results. Problems we have noted are: A clock that runs the CPU and the Pixie Graphics chip at 3.579MHz, A clock that runs it a harmonic of the clock crystal or A clock that is too misshapen to be useful.
The clock driver IC A15: We have found that in some cases a modern 74LS00 will not produce a useable clock signal. If you are able to source an older SN7400 or SN74LS00 or similar TTL 7400 chip, you will likely have better success than with a new 74LS00. We have tested our stock of TTL chips and have produced a good clock source with 7400 and 74LS00 IC's with date codes from the early 1970's to the late 1980's and 90's. We will endeavour to test new 74LS00 IC's to determine how best to make the clock oscillate and update this Wiki page.
The Frequency Divider A16: The components for the frequency divider (A16, R30, R36 and C10) should be changed to work in accordance to the newer TTL chips on the market; no changes to the PCB need to be made.
The changes are as follows:
- A16 should be 74LS74 not 74HC74
- R30 and R36 should be 2.2K resistors, not 10K
- C10 should be 47pF not 330pF
NOTE: If you can not manage to find a 74LS00 IC that will produce a decent clock signal, you can substitute a 3.579545 MHz TTL Oscillator 5V 14-DIP, 4 Leads (Full Size, Metal Can) in place of the 74LS00 IC. We have tested this and found that all functions work using a 3.579545 MHz crystal oscillator, including the 1861 Pixie Graphics IC. The unit we have used can be found at Digikey at: https://www.digikey.com/en/products/detail/ecs-inc/ECS-100AX-035/827253.
Photo of the 3.579545 MHz crystal oscillator in place of the SN7400 on a working AVI ELF II:
If I you are not reading a clock signal at Pin 1 or on the expansion bus then check the 74LS00 located at the bottom of the main board beside the CDP1861 display chip. The 74LS00 is responsible for generating the clock signal in conjunction with the crystal. Although the board is marked for a 74LS00, a 74HC00 is a suitable substitute if you can’t find a 74LS00.
If you are reading a clock signal at pin 1 of the CDP1802 CPU but it is reading at about 3.57MHz, there may be a couple of things going on. It may be that the clock signal from the 74LS00 is overwhelming the 74LS74 and it is no longer dividing the frequency. Although most CDP1802’s will run up to 6MHz, some older 1802 chips are not able to operate above 2.5MHz. check the 74HC74 that is wired as a frequency divider; pin 3 should read 3.57MHz and pins 5 and 6 should read at 1.79MHz. If all three pins read 3.57MHz then the clock is overwhelming the 7474 and a a replacement should be tried.
If you are not receiving a video signal when you try to enable the Pixie Graphics circuit, it is most likely a problem with the clock circuit. If the computer is functioning correctly in all other aspects except for the video output, it is more than likely a problem with the 74HC74 frequency divider. check pin 1 of the CDP1861 chip with an oscilloscope to ensure that it is receiving a 1.79MHz signal.
If the CDP1861 is getting no clock signal, check pin 3 on the A16 74LS74 to ensure that it is getting a clock signal of approximately 3.57MHz from the 74LS00. Then check pin 5 of the A16 74LS74 it should be showing a clock signal of 1.79MHz which goes to the CPU. Check pin 6 of the A16 74LS74 which is the inverse signal of the CPU clock signal that goes to the CDP1861. It should read 1.79MHz. If you are not getting the expected readings, there is a problem with the clock or the frequency divider.
Happy Computing!!