NEC PC8201a laptop computer
EO tablet (1993)
Apple iPhone 5G clone
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
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
|5, 7, 9, 14, 17, 18
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)
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.
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:
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
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,
particular the safety