What's the frequency, Kenneth?

The Cifer is proving to be an interesting project. I've never worked on a computer that had soooo many discrete logic chips. As identified here I managed to find some that were dead across all of the boards in the machine. Today, I shall be focussing on the graphics board.

This board had the following broken chips:

• 74LS166 at ML9
• 74LS163 at ML37
• 74LS163 at ML38
• 7425 at ML45

Graphics board - I don't think NVidia will be too concerned..

All of these broken chips were replaced, but before we get to the board, a slight detour.

Across all the boards I noticed that every 74LS166 and 74LS01 came out as faulty on my chip tester. This did make me wonder but I ordered some new ones of each variety anyway. The first to arrive were the 74LS01s and these were brand new old stock. Every one came out as 'bad' on the tester. Hmmm. 

Bad chip!

When the 74LS166s arrived I checked them too. Every one came out as 'bad' on the tester. Double hmmm. Then, second batches of both chips arrived, and every one came out as 'bad' on the tester. 

Bad chip?

So, a word of caution. If you have a chip tester labelled TES-200 then take what it says about 74LS01s and 74LS166s with a pinch of salt. 

Incidentally, you may have noticed that I 3D printed something to bolt the tester to. This was because the display was way too wobbly and I couldn't plug the battery in without the display pushing up out of the socket slightly. 

Desoldered headers

Desolder pins

Sophisticated and delicate fixings.
OK, so I used a glue gun. Don't judge me.

So I de-soldered the pins and socket and wired the screen to the board and bolted it all into a 3D printed box thing. It's rough and ready but it works and it's far easier to use now.

And now, back to our regular scheduled broadcast.

The graphics card in the Cifer offers Tektronix 4010 emulation. This graphics standard actually used a 'storage tube' for its display. This is a type of CRT that retains the trace of the electron beam on the display, which meant that the computers that used it did not require additional display memory to store the image on screen. Think electronic etch-a-sketch. Here's a youTube video showing a display in action.

Obviously, the display in the Cifer is a standard CRT so this mode is, apparently, more about emulating the commands of the display than the look and feel of the display. No matter, it needs fixing so fix it we shall.

To get to the 4010 mode there are a couple of key presses required. First, I have to press F6. Then to enter graphics mode, I press the (can you guess?) 'Graphics' key. What should happen is that cursor should change to a triangle, and text should appear when keys are pressed in the same way as in standard text mode, but the text is generated from graphics card instead.

What actually happens, is that the cursor sometime changes to a stretched out triangle, sometimes it disappears. When keys are pressed, they occasionally get displayed but are very tall and spaced out. Also, random garbage frequently appears, almost like there is a RAM fault, even though the RAM has been checked several times.

One other detail is that, when the board is plugged in there is a high speed 'chirping' noise from the machine. This noise is only present when the little plug in connector PL1 on the board is connected. The other two connectors (PL2 and PL3) are for the serial comms to the main board and the IEEE488 bus respectively. So, it was over to the technical manual and the schematics to see if I could work out what was happening.

One page of several of schematics

First thing I discovered was that it looks like the IEEE488 chips are pointless as they don't do anything for the graphics board. Possibly a 'future improvement' that never got implemented.

So, why is the board chirping and displaying garbage? I started at 'VIDOUT' and worked my way backwards into the board. I checked TR1, TR2 and TR3 and they all matched the expected values. Eventually I ended up at TR5. This also checked out OK but I noticed on the schematic a variable capacitor. I wonder if the technical manual mentioned this? It did.... It is the control for the overall frequency of the video signal. 

Reaching for a small flathead screwdriver I switched the Cifer on and listened to the chirping. Then I started to gently twist the variable capacitor. The frequency of the chirp changed and the garbage on screen also changed, pulsing more or less rapidly depending on which way I twisted the screwdriver. After a few seconds I hit a sweet spot and the chirping stopped and the garbage on screen became a stable picture - still garbage, but stable. 

Variable Cap

Casting caution to the wind I pressed the reset button. Still no chirping. OK. Press F6 and then press 'Graphics'. The cursor changed to a nice solid flashing triangle. I pressed a few keys. They ALL worked. I pressed Ctrl+Disc. The usual '...Is the IEEE cable connected?' message appeared but in the graphics font. OH. YES.

This graphics board is fixed. 

Tektronix emulation in action. Nice.