ZX Spectrum Max 128 PAL/RBG/Composite video out advice

T-Man

Tinkerer
Oct 30, 2021
70
39
18
Tennessee USA
Hello all. A bit of a shot in the dark but after buiding and then after some delay programming the ROM and FPGA I was excited to get the following screen. At first I thought this was a PAL/NTSC issue, but on the schematics the RBG output comes before the video processor (AD724JR0 IC). The AD724 converts the RBG to composite and S-video and yes the chip is set to PAL. But I am getting the image on RGB and composite. Also, it's not the same sync appearance I've seen with other PAL NTSC incompatibilities. I have tried multiple SCART to VGA, Scart to component, SCART to composite, direct composite, S-video on multiple monitors (Sony CRT with component and composite; Samsung sync master VGA, Apple 2 green composite, Panasonic plasma with component composite s-video; overhead projector via and composite). The picture here is Apple IIgs with VGA converter set to PAL is the most stable although all either simply do not detect or have a similar over sized image without sync.

Anyway, in a big picture short version, is there anyone who has seen a similar output from RBG (overscanned and poor sync) who can give me a starting point for trouble shooting this?


Monitor.jpg
boaard.jpg
 

T-Man

Tinkerer
Oct 30, 2021
70
39
18
Tennessee USA
Thank you.

Strange but fixed. Lots of focus on the video circuit and resolution. Checked the main oscillator and it was slow. I thought there would be no way I was getting any kind of picture if this wave form was accurate. One of the caps connected to the oscillator had a trace, like manufactured, shorting the capacitor. At first I thought the I just didn't understand the circuitry, but after looking at the schematics just cut the trace to see......it worked.

Output is PAL (50mhz) RGB and composite and to get both color and no roll, only the II gs monitor with an adapter works. The Commodore monitor shows a picture on composite but no color. Strange, but I have a 1990's Sony TV that I thought would take PAL but it willnot.

IIGSMon.jpg
ComMon.jpg
 
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T-Man

Tinkerer
Oct 30, 2021
70
39
18
Tennessee USA
Anyone out there with PAL/NTSC experience? I'll start by saying I'm amateur so my terminology below may not be correct.

This computer is powered by a 9V DC jack. So it is not getting any type of 60 hz from the AC. The design of this ZX 128 MAX is for PAL output and I'm pretty sure that is what I get, on RBG and Composite and Chroma/Luma. That is to say ALL of these signal s come out of the computer at 50 hz. I have a PAL to NTSC cheap composite converter that works. I have a RBG to HDMI converter that does Identify the input RBG as PAL (via SCART) and does convert to a 60 hz HDMI signal that works.

I really I'd like at least composite that is NTSC and ideally output of S-video and RBG at 60HZ for NTSC. The chip in this design is the AD724JR.
and it is implemented in this design as below:
Image 5-24-26 at 2.14 PM.png


When I read the SPEC sheet from the AD724JR, it looks like maybe I can get S-video and Composite for NTSC just by putting PIN one to 5V and replacing Y2 with a 3.58 MHz crystal? Am I missing something here?

It does look like the RBG is at 50 mhz is coming from the main chip so I doubt I'd be able to change its freqmesny without an Alera code update.