Building and RGB input Circuit. Choosing between input options on Mux chip.

Thread Starter

vol.2

Joined Sep 19, 2020
4
Working on an old Toshiba TV with a composite input that I recently brought back from the dead (had to replace the vertical output IC).

I have a lot of older video equipment that has RGB outputs like microcomputers, etc, and I want to use this CRT to receive signal from those sources via RGB.

There are lots of tutorials on how to do it, but this set is just a little bit older, and the video MUX chip operates a bit differently, so I was hoping to get clarification on the route I should take.

The mux chip is called a AN5352N. https://datasheetspdf.com/pdf-file/185055/PanasonicSemiconductor/AN5352N/1
Here's the schematic of the TV: https://drive.google.com/file/d/1LnxW6jVgfAUmrCAgev7gYhk2rOPu3eFr/view?usp=sharing

Unlike newer MUX chips, which typically have Luma-Chroma inputs from the NTSC chip, there are 2 sets of RGB inputs on this MUX chip. One is for the TV signal, the other is for the OSD text. (TV and Tx) My main concern is which one makes more sense to use.

Here is what I have been able to determine for both options.

Using the TV inputs on the AN5353N MUX chip (pins, 15, 18, 21 +pin 11 for C-SYNC or LUMA):

1) TV input on this chip seems to bypass both the contrast and brightness circuit on the main AV chip. I'm unsure if I will get contrast and brightness if I used them.

However, looking at the TV schematic, it seems like the contrast and brightness controls are also hardwired to the screen control. I'm not sure if this would mean that it will still function or not if I snip the TV RGB inputs to the mux chip and inject my own signal.

Here's that signal path:


2)I'm not sure what voltage to input to the TV input pins, or at what point I would hack them in. The RGB lines between the main NTSC chip and the mux chip start out as 5.8V. Each have a 220pf cap (which is part of a network) in series, then a 2.2k resitor in series, and finally, a 10k resistor in parallel which is tied to the 12V rail. All RGB lines at the mux chip are labelled 6.9V. Typical RGB outputs are somewhat lower than that. Most are are 1vpp and some things are 3-5vpp.


Using the Tx RGB inputs on AN5352N (pins 22, 19, 16 +pin 6 for "clamp):


1) Tx has a 3x gain for Contrast, which adjusts in conjunction with the display contrast.

Perhaps this is a good thing, as it looks like it's expecting around 3.5V input, much closer to what I would be inputting.

This is the spec for the voltage amplification of the OSD input signal from the datasheet:

Here's the part of the AV chip's output that has the tie-in for the contrast (pin 8 output) and the brightness control (pin 15).

2) Tx has a brightness control input that isn't used on my set. (it's just tied to ground) I figure I might need this, but I'm still unsure if the brightness and contrast would already be taken care of by the connection shown between the main AV chip and the flyback, and if this functionality would be there for the Tx inputs as well. (I linked an image above)

The unused pin for brightness is PIN 9, you can see it tied to ground (together with 4, 5, 13) in the schematic below on the right side:



5) The Green Tx input on my chip isn't populated, it's just out to a capacitor to ground. I would have to reenable this input to use Tx inputs.


6) The RGB Tx inputs are referred to as clamp, and there is a single "pedestal clamp" at pin 6 that goes to Rtx,Btx, and Gtx.
I'm not sure if the RGB signals from the CHAR gen chip are substantively different from the TV inputs in that case. I'm not totally sure, but it looks like they are AC coupled. There is a hit to B6,B7 on the schematic before they go the mux chip. I tried to trace out the B rail on the board, but I couldn't actually find any connections to AC. I may be missing something.


Based on the research I've done so far, the pedestal clamp level should be the black level (just above sync). This is the circuit used by the TV to derive the pedestal clamp:




This is the character gen chip if that makes any difference. You can see the signals which end up at the mux chip:
Here where the signal comes out of the character generator for the on screen menu.



This is the Character Gen pin output, showing the RGB outs on pins 12, 13, 14:


Here is the pinout descriptions of the char gen chip:


I'm hoping that someone here can help push me in the right direction as to which route to take.

Thanks
 

MisterBill2

Joined Jan 23, 2018
7,021
I do not predict success for any efforts to hack into the circuitry of that set to gain an RGB input that works. What it appears to have, based on your description, is an NTSC (Composit video) input, which is narrowly defined in all aspects. The best video you will be able to see will not be better than CGA video. In addition, the TV sweep frequencies are fixed, and while some sources may be similar, most are different.
The work-around is to use an external converter circuit to produce the composite video signal from the other formats. You may even be able to purchase such a device as a commercial product.
BUT with used flat screen monitors selling for less than $50 at electronic swap gatherings it will be much cheaper to get a used monitor that can accept such input directly. And another option is that WalMart sells NEW tvs with VGA and RGB inputs for under $100
 

Thread Starter

vol.2

Joined Sep 19, 2020
4
Thanks for your reply.

I do not predict success for any efforts to hack into the circuitry of that set to gain an RGB input that works.
Hacking the On Screen Display chip (otherwise know as a jungle IC or video mux chip) is certainly possible, and I have done it myself to other, slightly newer sets.

If you would like more background on the process and theory, I would recommend the first post here: https://shmups.system11.org/viewtopic.php?f=6&t=56155

The TLDR is, you snip the RGB character IC inputs to the MUX chip, terminate your inputs to 75ohms with an input filter cap, and pull the blanking pin on the MUX chip high, so that the OSD is always on. The display button on your remote then toggles the RGB input on and off. (or you can use a switch)

The specific set I'm working on now is a slightly older set; it has an early mux chip with RGB TV line inputs in addition to the Text character inputs. My question, and all of the associated supplemental schematic material, was about the possibility of using the RGB TV line inputs instead of the usual Text inputs.

Also, if your not interested in reading about the process, but you'd still like to see it in practice, you might want to check out this quick video. It's a fairly rudimentary video, but he goes over the main points of the process, and I think his channel is pretty fun, especially if you're old enough to remember using computers in the 80's.

 

MisterBill2

Joined Jan 23, 2018
7,021
While connecting the colors can be done there is still the rather severe issue of sweep rates and synch. That may be the show stopper, depending on just what sources get connected. The older sets did not have wide sweep frequency ranges.
 

Thread Starter

vol.2

Joined Sep 19, 2020
4
While connecting the colors can be done there is still the rather severe issue of sweep rates and synch.
By "sweep rates," I'm assuming that you are talking about the horizontal and vertical sweeps? I wouldn't be connecting anything to it that didn't conform to typical NTSC 480i scan rates. I'm using things like a Commodore 64 and a PC with 16 color output, everything is a 15kHz horizontal output.

Thanks
 

Thread Starter

vol.2

Joined Sep 19, 2020
4
Thanks for the words of warning. Yes, I won't try to run VGA or anything like that. This is strictly being done for a way to hook up period-correct historical microcomputers.
 
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