I am new to this forum and I also apologise for my English, it is my second language.
Anyway, I started building my Apple I replica about half a year ago. However, after I finished the terminal, I discovered that it wasn't working (sort of).
The main thing, that makes my replica hard to troubleshoot is that I built it on an universal PCB, wiring every connection by hand and using a mix of Czechoslovak (TESLA), Soviet and some western TTL chips (similar to this one https://www.applefritter.com/content/apple-i-replica-using-ussr-components).
So, when all ICs are installed (without 2519 and 2504) it dispalys "@" for split second and than the whole screen goes blank. After a while I discovered, that if I remove 7410 at B2, it will display 40 columns of "@" but the rows go to infinity. It might be some vertical sync problem. The monitor I am using is a Czechoslovak Tesla PMD 60, originaly designed to accept PAL composite, but the sync frequencies are fully adjustable by trimmers. I also tried modern LCD TV, which should sync to NTSC and got the same result, so there must be a fault in the terminal.
After some months, I bought an Oscilloscope, so I could make deeper measurments. I measured every signal and found out that a lot of signals got replaced by a weird 15,72 khz signal when the 7410 is installed. I know, that it is an NTSC frequency, but even the outputs of 74161 at D8 are replaced by this signal. So it does no counting and thats why the 2513 (in my case MHB 2501, equivalent from TESLA with different pinout but same functionality) is not displaying anything.
Also, when I remove the character generator, it displays 40 rectangles instead of 40 x 24 small squares.
Things that are not causing the problem:
- filter caps - At the begining I had 0,1 uf and I changed all four to 1 uf - no improvement
- LS, ALS TTL instead of clasic TTL - At the begining, half of the chips were LS or ALS, I changed all to clasic TTL - no improvement
- faulty ICs - It might be possible, but I bought 3 of each chip and changed them multiple times - no improvement
The PSU of my Apple is completly different from the original. I am using a PSU from scraped Czechoslovak terminal, because it has all the voltages needed for the Apple (+5 V, -5V, +12 V and -12 V). Thats why there are no voltage regulators or big electrolitic caps on the PCB.
While assembling, I also kept in mind, that signal VINH and /VINH are the same and that pins 3, 5 on D6, 3, 5, 11 on D7, 1 on D8 and 1 on D9 shouldn't be connected together.
I am attaching some photos of the terminal and the video output.
I would appriciate any suggestions!
PS: I wasn't able to upload the pictures of the video display normaly, for some reason it is blank. Here are some links, that should work.