New User - What about N-PAL modulators?

[zxMarce]

Hi there, I just signed up for some TI goodness.

A brief history, off the top of my head - Actual facts may slightly differ.

 

The first home computer I saw was a TI-99/4A with its PEB at a computer dealer in the early '80s.

It was being used by a young person, and before my eyes a Parsec-style game slowly started to appear - the person was coding the game right there, live!

At the time, Texas Instruments did have some HQs here in Argentina (guess it still does); they not only had representation but they actually did manufacture transistors here (sadly, this is no more so). It appears that "our" TIs had a national modulator (PHA-2031) designed and manufactured locally for our own PAL-N systems (50Hz vertical, 15.625kHz horizontal, 3.582056MHz chroma burst, 4.5MHz audio subcarrier). Not the same as the European (PAL-B, as we know them here) ones.

Googled hi and lo, but could not get a schematic of it, so I got my hands dirty. I have a very rough and somewhat incomplete schematic of the beast (pencil!). I did not get to the RF part, basically because I just wanted to get to the A/V signal, and the RF part is just an L-C tank for the LM1889N it uses plus some support passives.

 

I did get some A/V out of the modulator, albeit video is slightly noisy (maybe a cap or two need replacing, be it in the modulator and/or the TI power supply itself).

 

I have the modulator board scan, a Pinta project with board's track and pad layers (flipped, so I could just pencil-draw over it the components by copying their positions from the board), and the handmade -partial- schematic of the modulator.

I did not make a second schematic with my modifications yet; I will do it and upload the whole bunch later to this thread.

My hopes are that someone sees what I did and yells me something along the lines of "Hey, if you get video this way [.....] it will be clean" or even "change caps Cxx, Cyy [...] and video will work".

 

Regards,

zxMarce.

Edited December 14, 2020 by zxMarce

[+Ksarul]

Schematics are always useful--and I may actually have one of those PAL-N modulators with one of my Argentine consoles, so this one will definitely be helpful once you post it.

[zxMarce]

Well... Got most of the original circuit for the PHA-2031 N-PAL modulator on a KiCAD 5 schematic. Attached.

 

The circuit does not include the LM1889's RF parts (for the TV channels), as I only wanted to get composite out of the device.
Basically, these RF parts are a L-C tank for the desired TV channel and that's pretty much it. Maybe an extra decoupling cap.

 

I'll make a second schematic with my modifications.

 

Regards,

zxMarce

Modulator N-PAL Schematic (KiCAD 5).7z

[zxMarce]

Here are some pics I took today of my modded N-PAL modulator, PHA-2031.

The last pic is the image as I see it on a 32" Philips TV via A/V Input. Sometimes better, sometimes worse.

 

The modification I made -I realized later- is one from the TI99IUC. Only changes I made was to replace the 68 Ohm resistor with a 75 Ohm one by connecting two 150 Ohm resistors in parallel (did not have any 68 Ohm at hand), and I used a SS9018 transistor (from a disassembled Timex/Sinclair 1000 RF modulator) instead of a BC547. I also connected my transistor between LM1889N's pins 13 and 14, given that pins 14 and 16 are both positive supplies.

 

I suspect something in my chroma subcircuit is dodgy because whenever I touch some component leads (mostly anything on LM1889N's pin 15, audio subcarrier input) I can make the circuit output color (noisy of course, but color), while lately I mostly get noise-free B/W. Maybe my 3.582056MHz crystal' s gone; I played a bit with its tuning trimmer without result.

 

For the eagle-eyed viewing my pictures, I also changed the non-shielded cable to shielded in the video mod; this I did after taking the photos.

Hope this helps someone; as I said, I'll do a second schematic with the modifications.
 

The modulator front:

 

The back:

 

The side with the A/V RCAs:

 

The guts:

 

Detail of the TV/Computer switching relay:

 

The horrible mess - I added the multi way connector to be able to separate the board from the input cable (and forgot to include my own outputs - oh well):

 

Detail of the multi way connector:

 

Detail of the video modification:

 

Same, but from the other side:

 

The... er... "picture" I get from my TV:

 

Regards,

zxMarce.

Edited January 3, 2021 by zxMarce

[zxMarce]

For the ones after it, this is the same KiCAD 5 schematic (corrected an omission of two caps' references and values), but including the italian-like video out modification.

 

Hope it serves someone.

 

Regards,

zxMarce.

Modulator N-PAL - Video Out.sch