Small HEF4050 A/V circuit for Atari VCS in EagleCAD - Page 1
Forum section: electronics / projects

author: arkanix38
author_rank: Contributor
timestamp: May 01, 2015, 04:04:17 pm
content: Hi everyone,I had some HEF4050BT's lying around and decided to upgrade my Atari's Video circuits with them. The circuit is a tried and true design, and the only thing i'm unsure about is my board layout. I've linked an image of the layout below, so if I've committed any substantial breaches of convention please let me know!ThanksArk

author: Andy Watson
author_rank: Super Contributor
timestamp: May 01, 2015, 04:11:02 pm
content: What aspect is this buffer supposed to improve?I don't see any supply decoupling for the buffer IC.

author: arkanix38
author_rank: Contributor
timestamp: May 01, 2015, 04:16:35 pm
content: The buffer replaces the existing 4050 buffer circuit feeding the UHF Modulator, with tweaked resistor values for composite video. It pulls the LUM0,1,2, CSYNC, COL and AUD0 directly from the TIA chip in the VCS (Bypassing the crusty ~35 year old video circuitry). The +5 and GND are pulled straight from the regulated +5 V and GND lines inside the Atari and are decoupled in the regulator circuitry near the supply input.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 01, 2015, 04:31:15 pm
content: Quote from: arkanix38 on May 01, 2015, 04:16:35 pmbut notnearthe buffer chip!

author: rdl
author_rank: Super Contributor
timestamp: May 01, 2015, 04:38:31 pm
content: You basically want one end of the decoupling capacitor as close to touching the V+ terminal on the 4050 as you can get it and the other end to ground in a reasonably short distance.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 01, 2015, 05:08:25 pm
content: Also, what are you going to connect to the video output? The output impedance matches neither 50 ohms, nor 75 ohms.

author: arkanix38
author_rank: Contributor
timestamp: May 01, 2015, 05:21:21 pm
content: Thanks for the replies! You are completely right about the decoupling cap, it was a gross oversight on my part. As for the rest of the functional circuit, I am trying not to toy with the schematic too much. As stated beforethe circuit is known to work well for PAL VCS systems. From what i've read the circuit is simply acting as a level shifter and mixer for the existing video signal. The output of the video is PAL Composite.I have some earlier prototypes of the board on their way, so i'll try your suggestions and see how they impact the quality of the output video (i'll test on both a standard LCD and on my old CRT Monitor for comparison).Pin Header Signals for future reference:1- Vcc2- LUM03- LUM14- LUM25- CSYNC6- COLOR7- AUDIO8- Vss

author: SeanB
author_rank: Super Contributor
timestamp: May 01, 2015, 05:36:02 pm
content: Pin 1 is on top of the board, so simply scrape off some of the soldermask around there and solder in a SMD capacitor from pin 1 to the ground plane. Decoupled and done, any value from 100n to 22uf ceramic will work, it just has to have a 10V rating if over 1uF. 100n will probably be 50v rated in any case. or just use another capacitor like C1 for it.

author: arkanix38
author_rank: Contributor
timestamp: May 02, 2015, 03:33:06 am
content: Fixed some of my silkscreening, added a 100nF decoupling cap and a 1k6 pullup resistor on the audio (was suggested elsewhere to improve the audio level to both L and R outputs)