Hitex HT-767 - interesting famiclone

Discuss hardware-related topics, such as development cartridges, CopyNES, PowerPak, EPROMs, or whatever.

Moderator: Moderators

Post Reply
User avatar
krzysiobal
Posts: 1037
Joined: Sun Jun 12, 2011 12:06 pm
Location: Poland
Contact:

Hitex HT-767 - interesting famiclone

Post by krzysiobal »

I accidentally got into possession of this famiclone and it made me think this is some kind of very old design:
Image Image Image Image Image Image Image Image

1) The CPU and PPU are the old revisions (before mid '90). Never spotted this chip in any famiclone before, thought they appear on aliexpress.
The difference for CPU, compared to after mid'90 is the divider of 16 and around 20% bigger current consumption

2) The video amplifier is built using one NPN transistor, not PNP+NPN like in 99% of other units
3) The resistors used for mixing audio have also non-common values
4) The audio is amplified with NPN transistor and 74368 together (and the feedback resistor is also uncommon - 2k instead of 100k)
5) The PPU's VCC is not directly tied to +5V but by a jumper meaning that there could be soldered something else instead (diode for voltage drop and heat reduction?)
6) There is a unpopulated place for soldering second crystal and components to make second identical clock generator (for PPU). So as a result, CPU and PPU can be clocked using different crystals.
7) There is a closed jumper between PPU-/VBLANK and CPU-/NMI and a place to solder DIL14 chip and potentiometer which do something with those two signals (when the jumper would be open). No idea what chip would that be and what would be its role - delay (or maybe fasten) the edge of NMI?
Pinout of this chip resembles something from 74 or 40 family (7=GND, 13,14=NC, 12=+5V).

Image

8) I carefuly examined all parts and there is no resistor named R7, I suspect it could be here, but those pins are shorted:
Image

Though I spotted very similar design in another famiclone, named: EASTERN COMPUTER; similar to famicom, but totally black shell) and here pins 14,13,12=+5V
Image
lidnariq
Posts: 11432
Joined: Sun Apr 13, 2008 11:12 am

Re: Hitex HT-767 - interesting famiclone

Post by lidnariq »

krzysiobal wrote: Sat Apr 17, 2021 9:01 am Pinout of this chip resembles something from 74 or 40 family (7=GND, 13,14=NC, 12=+5V).[/b]
Neither KiCad nor Eagle know of a 14-pin part in those families that connect power to pin 12.

Eagle does know of a handful of 16 pin 40 or 74 parts that only have power on pin 12. And it knows of a bunch of 14 pins with power on pin 12 but they're mostly ADCs and DACs.
User avatar
krzysiobal
Posts: 1037
Joined: Sun Jun 12, 2011 12:06 pm
Location: Poland
Contact:

Re: Hitex HT-767 - interesting famiclone

Post by krzysiobal »

There is a strange +5V via to nowhere near pin 14, maybe this is a pcb design flaw and pin14 should be the VCC of this chip?
Image

Wonderful idea with parsing the eagle library set to determine which pins of each chip are inputs or output and then find something matching.
lidnariq
Posts: 11432
Joined: Sun Apr 13, 2008 11:12 am

Re: Hitex HT-767 - interesting famiclone

Post by lidnariq »

Maybe both pins 12 and 14 are intended to be tied to +5V? Pin 12 to safely disable the adjacent pins.

It's almost the pinout of something like a 74'02, except that pins 1 and 10 are shorted and exactly one must be an output, and I don't remember a layout that's compatible with that.
Joe
Posts: 650
Joined: Mon Apr 01, 2013 11:17 pm

Re: Hitex HT-767 - interesting famiclone

Post by Joe »

My guess is it was meant to be a 4011 to make a NMI delay circuit for compatibility with NTSC games on a PAL PPU, except they accidentally routed VCC to pin 12 instead of pin 14.
lidnariq
Posts: 11432
Joined: Sun Apr 13, 2008 11:12 am

Re: Hitex HT-767 - interesting famiclone

Post by lidnariq »

Ooh, good option. That'd want pins 12-14 to all be connected to +5V (12+13 to prevent it from oscillating)

Then the stuff on the left becomes an SR latch, and the result is a retriggerable one-shot after roughly R·C÷3...
Post Reply