Hugo Holden
Veteran Member
I have just finished restoring a Bytesaver II board and made some notes related to the attached image.
As is often the case with vintage S-100 boards there is a moderate amount of service work to do, that is if you expect the function to be reliable.
This particular board had one of the worst combinations of IC sockets and IC pin types. Cromemco must have had a deal going with TI at the time because all the 74LS series IC’s were TI types, not a mixed bag of brands often seen on vintage s-100 boards.
The IC sockets were also all made by TI. Generally there are two versions of their socket that look the same externally, though you can spot the difference looking into a hole on the socket. In TI’s early design, the socket claw grabs the IC pin from side to side across its wider dimension. In the other case the claw grabs the pin across the flat (as most dual wipe sockets do). The ones on this board are that latter type. These particular ones have a very low claw tension, even when new.
The fist thing I noticed was that the ROM sockets were totally “worn out” with the ROMs basically falling out of the sockets with very minimal force. I made a small tool with a wire loop connected to a defunct IC pin. Using this to test each socket claw I found many were stretched apart and had no spring force at all.
(It is very important that no other object other than a standard geometry IC pin is inserted into an IC socket, or it will stretch and damage the claw, leaving the socket unreliable or the connection lost altogether).
In some cases the claws can be stretched apart and still appear to work, especially if the IC’s pins are splayed apart and have not been formed properly to fit the socket hole entry points. In these cases one face of the IC pin only touches one side of the socket claw, making for an unreliable long term connection.
It required that all of the ROM sockets were replaced. With this type of socket, the plastic shell lifts off the top of the pin array, so each pin can be removed and the hole cleared with the solder sucker and temperature controlled soldering iron. This way there is no damage to the pcb.
#1 On the photo below shows the new dual wipe ROM sockets installed.
#2 The board also contained a type of DIP switch, which I have found notorious for poor connections as they age. They were removed and machine pin IC sockets fitted and high quality Omron DIP switched plugged into those. This give an overall height the same as the original switch and one day new switched can easily be plugged in as required.
#3 I have found over the years that the white thermal paste on power devices tends to go powdery and break up and it is better replaced.
#4 There is another problem better dealt with. Cromemco used a plastic screw to secure the 7905 IC. This is a very bad idea because with heat the screw stretches, the forces drop and the thermal contact between the metal flag of the device to the heat sink is degraded. Somebody also had trouble with this in the past, in that it was a 6-32 plastic screw with a 4-40 nut screwed on it with damaged plastic threads. So I replaced that with a metal screw where the screw’s length is insulated with heat shrink sleeving.
#5 The original screws and nuts securing the heat sink and the devices were very rusted. These were replaced.
#6 All of the IC’s were removed from their sockets and the IC pins cleaned with 2000 grade paper (I find this better than chemical cleaning), then a clean with CO Contact cleaner and then lubricant applied (Inox’s mx-3).
Vintage TI brand IC’s have silver plated steel pins which develop a black oxidation. Sometimes these pins are brittle and fracture even if they were bent and straightened once. Other times they rust through, often at the pin entry to the plastic IC body. Three of the 74LS05 IC’s had damaged or rusted pins and were replaced, I used TI types to help match the rest on the card.
Each IC socket hole was tested with the test pin, dipped in mx-3 to lubricate it. Fortunately, the claw tension in all the smaller sockets was normal and none required replacement.
#7 Two 74LS367’s required replacement due to pin rust.
After this restoration work, the card was then ready to try using Martin Eberhard’s software and I was able to program the 2708.
As is often the case with vintage S-100 boards there is a moderate amount of service work to do, that is if you expect the function to be reliable.
This particular board had one of the worst combinations of IC sockets and IC pin types. Cromemco must have had a deal going with TI at the time because all the 74LS series IC’s were TI types, not a mixed bag of brands often seen on vintage s-100 boards.
The IC sockets were also all made by TI. Generally there are two versions of their socket that look the same externally, though you can spot the difference looking into a hole on the socket. In TI’s early design, the socket claw grabs the IC pin from side to side across its wider dimension. In the other case the claw grabs the pin across the flat (as most dual wipe sockets do). The ones on this board are that latter type. These particular ones have a very low claw tension, even when new.
The fist thing I noticed was that the ROM sockets were totally “worn out” with the ROMs basically falling out of the sockets with very minimal force. I made a small tool with a wire loop connected to a defunct IC pin. Using this to test each socket claw I found many were stretched apart and had no spring force at all.
(It is very important that no other object other than a standard geometry IC pin is inserted into an IC socket, or it will stretch and damage the claw, leaving the socket unreliable or the connection lost altogether).
In some cases the claws can be stretched apart and still appear to work, especially if the IC’s pins are splayed apart and have not been formed properly to fit the socket hole entry points. In these cases one face of the IC pin only touches one side of the socket claw, making for an unreliable long term connection.
It required that all of the ROM sockets were replaced. With this type of socket, the plastic shell lifts off the top of the pin array, so each pin can be removed and the hole cleared with the solder sucker and temperature controlled soldering iron. This way there is no damage to the pcb.
#1 On the photo below shows the new dual wipe ROM sockets installed.
#2 The board also contained a type of DIP switch, which I have found notorious for poor connections as they age. They were removed and machine pin IC sockets fitted and high quality Omron DIP switched plugged into those. This give an overall height the same as the original switch and one day new switched can easily be plugged in as required.
#3 I have found over the years that the white thermal paste on power devices tends to go powdery and break up and it is better replaced.
#4 There is another problem better dealt with. Cromemco used a plastic screw to secure the 7905 IC. This is a very bad idea because with heat the screw stretches, the forces drop and the thermal contact between the metal flag of the device to the heat sink is degraded. Somebody also had trouble with this in the past, in that it was a 6-32 plastic screw with a 4-40 nut screwed on it with damaged plastic threads. So I replaced that with a metal screw where the screw’s length is insulated with heat shrink sleeving.
#5 The original screws and nuts securing the heat sink and the devices were very rusted. These were replaced.
#6 All of the IC’s were removed from their sockets and the IC pins cleaned with 2000 grade paper (I find this better than chemical cleaning), then a clean with CO Contact cleaner and then lubricant applied (Inox’s mx-3).
Vintage TI brand IC’s have silver plated steel pins which develop a black oxidation. Sometimes these pins are brittle and fracture even if they were bent and straightened once. Other times they rust through, often at the pin entry to the plastic IC body. Three of the 74LS05 IC’s had damaged or rusted pins and were replaced, I used TI types to help match the rest on the card.
Each IC socket hole was tested with the test pin, dipped in mx-3 to lubricate it. Fortunately, the claw tension in all the smaller sockets was normal and none required replacement.
#7 Two 74LS367’s required replacement due to pin rust.
After this restoration work, the card was then ready to try using Martin Eberhard’s software and I was able to program the 2708.
