Thursday, October 29, 2009

Solder Bridge Found...

A few days ago, I noticed that the LED for address line A4 on the expansion card was not illuminated. I removed the expansion card and connected it up to a 5v power supply and probed the header connectors. All of the LEDs were working fine. I figured there must be either a bad chip somewhere or a solder bridge. After poking around for about 30 mins with a magnifying glass, I finally found the solder bridge under one of the 74S257 DIP sockets. After dragging a hot solder iron through the bridge, and powering up the clone again, I had an illuminated LED for Address line A4. Very hard to see! (I need to get some flux remover I guess .... that would probably make things a bit easier to see).

Saturday, October 24, 2009

Clock Signal Fixed...

Note the square wave on the scope... I finally have a clock input signal on pin 37 of the 6502 processor. I found that replacing the Philips 74HCT04N with a National Semi 7404N (D12 in the schematic) gave me a square wave. I found this problem by first checking pin 37 on the 6502, then checking the schematic, and seeing that the output from the crystal clock circuit is sent to pin 9 on the 74175 (C13 in the schematic). I then checked pin 9 with my scope and what I saw was certainly not a square wave, so I immediately thought this chip must be bad and replaced it (the 7404). After powering down, then back up again, I was pumped to see my expansion card LEDs flashing away as they should be! Finally. In the image above, you can see the square wave generated while I have my left hand holding the oscilloscope probe on pin 37, and the camera in my right.
The video is still not working, but it is late (5:30 AM) so I will take a closer look at the video issue "in the morning".

-- 9-dec-2009 UPDATE: The video section needs a 7404. 74HCT04 and 74LS04 will not work. I replaced the 7404 with a new one and now have video.

Tuesday, October 20, 2009

2513 Character Generator

This description of the 2513 Character Generator is from Don Lancaster's TV Typewriter Cookbook. The description explains clearly how this thing works. Compared to the datasheet, I thought this was easier to understand, so figured I'd share it with the community.

I've been spending a bit more time on the video section of my Apple 1 clone, trying to locate the source of the problem (I have a signal, but no video). I re-soldered most of the points on the back of the board thinking maybe I had either a solder bridge or just a bad connection. I pulled and reinstalled the chips. Still no video. I tried removing and reinstalling all RAM chips, still no video. I tried swapping out a few of the chips, still no video. I checked all of the 1404 voltages, and everything looked fine. I'm going to have to spend a little more time with the scope, the schematics and the datasheets to figure out what's wrong here....

Monday, October 12, 2009

On Sunday, Oct 11, 2009 I decided to put together an expansion card to plug into the Apple 1 motherboard. This card provides the ability to send a "reset" signal to the CPU, read the address bus, and provide +5v and ground pins for a logic probe or oscilloscope probe. The above image shows the card with power applied, and the logic probe connected. The card is connected using the exact same 44 pin connector that is on the motherboard.

Lights off, first test. The green LED is the power indicator letting me know the board is getting power from the motherboard : +5 volts

Here you can see the 16 address LEDs, with sibling 300 ohm resistors. Note just to the left of the "+ 5v" label (bottom left) and the ground label (top left) there is a single molex pin. These 2 pins are for debugging the motherboard (i.e. the pins are used to hook up a logic probe).
The LED for the reset will be removed -- mainly because I wired it up wrong, and the signal for reset was blocked. Cool to have, but not necessary. Just above the switch is a .1 uF ceramic capacitor - for debouncing. The cap is connected to each pin on the switch.

Rat's nest on the back of the board. What you see here are the address lines, reset, +5 volts running across the top of the board from connector pin 1, ground across the bottom of the board from pin 20. The reset line is held high with a 10k ohm resistor. When the momentary switch is pressed the line is forced low, sending a reset signal to the motherboard / CPU. Testing with a logic probe shows we are good to go.

I attached the 44 pin header to the card using super-glue and a couple of C-clamps. What you see taped onto the board are (6) 16 pin DIP sockets which were soldered into place later on. These will eventually be used for hex displays.

Friday, October 9, 2009

I added the IN914 diode previously mentioned to protect the Apple 1 from unwanted external video surges, and I also added a "power on" LED indicator just for kicks.

Thursday, October 8, 2009

On October 6, 2009 I built this external circuit to bring the video, ground and +5v out to a separate PCB with an RCA jack. Eventually I'll add an LED and I should have included a diode to protect the Apple 1 from any surges coming from the video terminal. It's on my list.
In any case, I did not see anything on the video monitor other than a gray screen. I don't have a keyboard set up yet, with reset and clear, but I was thinking I might at least see random characters.

Monday, October 5, 2009

Miscellaneous images while populating the PCB

More power supply images...

Above: prepping to solder the ON/OFF switch.

Above - prepping to solder the female connector to the power supply lines. I don't know why I waited so long to buy "Helping Hands"... let's see how long it takes before I finally break down and purchase a solder-sucker...

Above: soldering the molex housing pins, one at a time.

Above is the completed power supply. At some point the transformers will be bolted to the floor of a home-made case, or maybe just a piece of plywood...
BTW, that is a 0.5 Amp 250v fuse.

Building the Power Supply

Note that the 4 pin header looks a little rough, because it's actually a cut down version of the 6 pin header. If I took crapier pictures you wouldn't notice :D.

Today I pulled out the 2 Stancor transformers and put together the power supply. I didn't have all of the parts. After spending too much money on hard-to-find chips and carbon composite resistors, I decided it was time to do what we all should do, a little spring cleaning!
So I dug through the basement and garage and found a decent 2 prong power cord, and a fuse housing wire (from an old Panasonic car stereo equalizer) . There were also 4 nice nuts and bolts probably for mounting the stereo equalizer that I snatched out of the equalizer box to use for mounting the (2) Stancor transformers. While digging around I also found a brand new 125v ON -OFF toggle switch that was perfect for this project, in one of my parts bins down in the basement. I grabbed a handful of various old male/female wire connectors I thought I'd find useful as well.

I first started with assembling the 6 pin Molex connector housing and pins. I soldered each connector pin to each of the corresponding Stancor transformer wires after crimping them. I added wire connectors between the fuse wires and also for the ON/OFF switch. Every minute or so I referred back to the Apple 1 manual I had displayed on my computer screen just to make double sure I wasn't screwing up. I had never built a power supply before, so I was a little worried I was either going to hurt myself or fry the board. I took my time, and everything came together just fine. I powered up the board (after soldering the 6 Pin and 4 Pin headers to the board) with the supply and checked a few points to make sure I had the expected voltages (-5, +5, -12, +12). I did this using 3 tools:

1) Oscilloscope
2) Logic Probe and
3) Multimeter set to measure DC voltage

Everything looked OK to me except for one area, which I am still confused over....
The Apple 1 schematic (drawing 101 - Power Supply) shows J3 pins X, Y, and Z where X = GND, Y = +12v and Z = -12v. I measured 22.5v and -22.5v for both of these points. However, the manual said that these 2 pins should not be used, and if they are used need to be regulated (the voltage is not regulated as is).

After cleaning up a bit, I went ahead and soldered the jumper points above the 74154. I used a combination of the Apple 1 manual - schematic notes, and the scant instructions (from the web) from Obtronix. They are slightly different, but I don't know why:

Apple 1 Manual (schematic notes) jumper connections:
Y => CSF
Z => CSD
W => CS1
X => CS0 (zero)

Y => F
Z => D
W => E (Note Apple jumps W=>CS1)
X => 0 (zero)
R => C

I took a look at a few Apple 1 images, and found a couple with jumper wire from W=>E so that is what I went with as well.

I powered up the board again and checked a few more points. I shut everything down, cleaned up a bit more, then proceeded to install the chips. After installing all of the chips, I powered up the board again looking for smoke, arcing, fire, flames, explosions, and nothing happened. But this time the board started producing heat. The 74154, the ROMs and the LM323k were all generating heat. The 6502 was not, however. I checked the voltages for 5v on the 6502 and everything looked fine.

So, enjoy the photos, and tomorrow after work maybe I'll see if I can knock out the keyboard and video.