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Imtech NTSC decoder modifications for PAL


As a TV, I use an old Sony PVM 2730 monitor. It's great quality, but mine doesn't have S-Video inputs, just multiple composite and a SCART socket with RGB (later versions of the monitor did support S-Video). Generally, these monitors, various Sony PVMs and also Barco and other professional monitors, are available for peanuts now on eBay and still give a great picture. Anyway, for a while now I've been considering building something to convert S-Video outputs from some equipment to feed the RGB input of the monitor.

In February 2005, ali110uk advertised some surplus Imtech NTSC decoders on eBay. I looked around for information about this, and could find none. As I live in a PAL country I was hoping I'd be able to adapt the unit to decode PAL as well as NTSC and as it was cheap I decided to take a risk that it might be useful to me.

When the box arrived, I had the lid off within minutes and almost straight away noticed that there is a 4.43MHz crystal on the board. This was very good news, as that's the PAL subcarrier frequency. It has nothing to do with NTSC. There is also a small additional board installed which contains a 7.16MHz crystal. That's double the NTSC subcarrier frequency, and it's obvious that the extra board is the one which supports NTSC. Good news so far.

I hooked it up, put an S-Video signal in the connector at the rear and looked at what was coming out of the connectors on the rear. It turned out that the 9 pin D socket contained a picture with signal levels compatible with SCART. A cable is needed as follows:

9 pin D pin number
SCART pin number
Usage
1
5, 7, 9, 14, 17, 18
Ground
3
15
Red
4
11
Green
5
7
Blue
6
8
Select
7
20
Sync

Note that the select line (SCART pin 8) is only raised to about 5v by the Imtech decoder. This is less than the SCART standard specifies, so may not automatically switch inputs on all TV sets / monitors.

I also wired up the audio lines to SCART pins 6 (left), 2 (right) and 4 (gnd) as you'll see below, I have switched audio going through my box as well as the picture.

A bit of investigation showed that the decoder was doing just fine with a 60Hz NTSC picture, but wasn't decoding PAL (the image is B&W) and it wasn't syncing with a 50Hz signal. Moving the JU13 jumper (shown below) was all that was required to make it sync properly with both 50Hz and 60Hz signals.



Next to make the unit decode PAL colour. I noticed a suspicious wire link was across jumpers JU11 and JU12. It connected the two furthest apart pins of the jumper. As this is right next to the TDA8390 PAL decoder IC, I decided to try different positions of the link. It turned out that connecting together the pin positions furthest from the chip resulted in perfect decoding of PAL colour.

If all you want is a PAL decoder instead of an NTSC decoder, you can stop here. Just remove the wire links and install jumpers across the two pins of JU11 and JU12 which are furthest from the TDA8390 chip, and move JU13 as suggested a few paragraphs ago and you have a PAL decoder. I wanted a decoder which would automatically detect the input format and switch. This only requires a little bit more work...

The photo below shows the jumpers after I'd connected wires to install a small extra board:


The following small circuit switches automatically between NTSC and PAL. It works because the centre pins of JU11 and JU12 have the colour difference signals with an 8v DC offset when PAL is being decoded. So, I use the offset to switch the relay. The transistor can be virtually any device. Mine is a BC172 from my my junk box. The LED does not light brightly (it's a very inefficient 1970s LED from my junk box), but makes sure that the nearly 2v from the TDA8390 in the "off" state doesn't switch on the transistor. The diode is required to prevent reverse EMF from the relay coil from damaging other circuitry.

You can find 5v and gnd very near the 7805 regulator on the main board.


Installing this small circuit results in a completely working PAL and NTSC compatible decoder which switches automatically depending on the input format. Really good performance, too.

I also made a couple of other small modifications. I decided to add a front panel switch to select between three different inputs at the rear, including the switching of audio for each source. The connectors at the right hand side, after the mains input were all added by me, as was the switch on the front at the left hand side of the box in the first photo of the unit at the top of the page.


The Y/C connection from my added source switch goes to the back of the 75 ohm terminating resistor switch on the rear panel. The red wire carries Chroma, white carries Luma. You should, of course, have this switch in the ON (down) position to terminate the signal correctly.


Remaining problem:

There is a bit of a jitter problem in the picture when playing video tapes. I think I need to re-route the sync somehow, and suspect that the left-most switch (MTA...) in the picture just above is someone else's mod to do something similar, as it picks up the signal from the back of the composite input.


I'd be very interested to hear from anyone who knows how these units came to be modified like this. It looks like they started off as PAL only decoders, and were modified to support NTSC at a later date. It is quite possible that there is some neater way of making them multi-standard than what I have done.


Anyway, electronics is a hobby of mine. I make wicker baskets for a living. You can see what I make by visiting my home page here:

Link back to my home page

Or contact me using the contact details here. I'd be very interested to hear of others results with modifying these units.

Also, I have a small number of S-Video cables for sale here.

You may be interested in the measurements I made some years ago of the performance of VHS tapes punched to enable them to be used for SVHS recording. They're here.



Lastly, a bit of a disclaimer. There are lethal voltages in this unit, around the PSU. I recommend you only take it apart if you know what you're doing, and of course at your own risk. Also, the device itself may well be damaged if you meddle with it. Lastly, if it doesn't work out for you, I'm not worth suing...

I recommend reading Samuel M. Goldwasser's excellent Repair FAQ, and in particular the safety notes.