Test results
Test results
OK, I've had a chance to run some checks and am posting the results. I wanted this to be a learning experience for me and possibly give other members an idea of what to look for when troubleshooting these systems.
This is a MAI Basic Four S/10 computer with the following symptoms:
Appears to be dead. Disk drives aren't being accessed and the screen is blank.
The computer has 5 boards in it:
Power Supply
Terminal Board
CPU/Floppy Controller board
Video Board
Main board similar to a backplane.
The power supply should be one of the first modules to be checked. I replaced the electrolytic capacitors in the power supply only because of their age and potential to leak. Then checked the power supply voltages without it connected to the system and they are OK. This supply outputs +5V, +12V and -12V. I then connected it to the system and checked for voltages on the Terminal and CPU/Floppy boards. They are reading OK.
Next I checked for voltages and signals going to the monitor section. There are 5 lines going to it:
+12V supply
Ground
Video signal
Horizontal Sync
Vertical Sync
The +12v is reading fine and the horizontal and vertical sync signals look good coming from the Terminal board. The video signal has a problem. When the computer is powered up I get the video signal for about 3 seconds and then it stops. I'm not sure what is going on here. I would think that the video signal should be a steady stream.
I noticed there was no glow from the CRT neck and decided to investigate to see what was causing that. I have a manual for this computer and it has schematics for all of the boards except for the Video board. It does have a description of the circuits and references to some of the components that are labeled on the board. However without a schematic and not much experience with video circuits, I had to do some studying of the video circuits from other computers and trace through this board with a multimeter to see where the signals were going. I came up with the following:
There are 6 pins on the neck of the CRT. If you are holding the socket that mounts on the CRT neck with the pin connectors facing you, the pinouts are:
Counterclockwise from the "key"
1 - Video signal
2 - H+ Heater
3 - Ground
4 - G1 Control Grid
5 - G2 Screen grid
6 - G4 Focus Grid
The only pin getting voltage is the Heater. (Dwight Elvey was right. 12v from power supply). I measured the resistance between the heater and ground pins and got 30 ohms resistance. However when powered up the neck does not glow. Do the grid voltages also have to be present to get that glow?
Most CRTs use 12V for the filaments. The open there is a bad indication.
Have you measured the filament at the CRT pins?
Dwight
Found that all of the grid voltages are derived from the secondary of the flyback. Tested the Horizontal Output Transistor (HOT) with the diode setting on my multimeter and found the Base -> Emitter is shorted. That explains the high voltage section being down and no video. I would like to replace the flyback at he same time I replace the HOT just in case it is shorted but I can't seem to find a replacement on the web. The part # is KFS-20492. Any leads would be appreciated.
One last item. I tested the pins on the flyback to see if there was a short somewhere. There are four connections to the primary and there is continuity between all four pins (0.2 ohms). The secondary has 3 pins and a ground. Resistance is 1.7 ohms between it and the other two, the others were 0.2 ohms. There is no continuity between the primary and secondary windings.
