View Full Version : MomPC: add video and audio outputs to Combo VCR/DCD/TV

04-13-2009, 02:09 PM
Hello, my name is Kevin Waite, I am working on a complex project for a stroke patient. One part/phase of this project is to add a 2nd TV flat screen to their environment. Very low cost is a driving factor. The patient has a MAGNAVOX 20MC4304/17 (alt. model no. 27MDTR20/17) combo TV/DVD/VCR -- and it does not have video/audio outputs -- this sucks -- because solving this -- conventionally would mean adding a cable turner DVD recorder -- and that leaves out the VCR -- this would greatly increase complexity of the IR computer controlled efforts for MomPC.

Creating (hacking) an audio outputs is trival -- because I can tap the speaker left or right or both or tap the headphone jack..... Audio output is not my concern. . .

The video ouput is. How does one test for a video signal?

The main Tuner / VCR board found inside is a:
BTD850F01012 x4 Triple Main PCB -- the VCR box is found
above this board. I have found this board for sale as:

This board is connected to the DVD player board (or at least this 2nd "DVD" board is found under the DVD player box, see (b) below. (I have photos I can post.) These two boards are bridged (wire connected) via two 19 wired connectors labeled CN2804 CN2803

I can describe in great detail the various connections between the 4 primary circuit boards found within this 27MDTR20.

a) BTD850F01012 x4 Triple Main PCB w/ VCR drive box above
b) BTD930F01031-A DVD / power board with DVD drive box above
c) high voltage TV tube board mounted vert. on the left upper part of the
TV (looking at TV from the backside)
d) small circuit board attached to the business end of the TV tubes cathode.

This TV allows the user to select between AUX, TV, DVD, VCR -- if a DVD is started or if a tape in the VCR is started -- then the scope of TV output jumps automatically to the corresponding play beginning device either DVD or VCR.

Bottom line -- if I could find the TV signal in composite / or / s-video / or / componet form -- at any pathway within the inside of this TV -- that would allow me to hack and get my desired Video output.

There are 8 wire ribbon cables connecting these boards, and there is
a 5 wire ribbon cable going from board (a) to the business end of the cathode of the TV tube. There is a logical flow of 8 wire ribbon connectors -- I mean -- it appears that there are both DVD and VCR 8 wire ribbon connectors going up to the vert. mounted circuit board that includes the high voltage circuits for the picture tube.

Please send me any help you know about to carry out or understand if this is impossible or not. These 8 wire ribbon cables are most likely carrying the video and possibly audio signals -- does anyone have spec sheet for these or any of these boards? If these are carying a video signal what type of video signal would it be?

Thank you in advance.

You can learn more abour MomPC at:
http://forums.trossenrobotics.com/blog.php?b=165 (http://forums.trossenrobotics.com/blog.php?b=165)
http://forums.trossenrobotics.com/blog.php?b=168 (http://forums.trossenrobotics.com/blog.php?b=168)
http://forums.trossenrobotics.com/blog.php?b=169 (http://forums.trossenrobotics.com/blog.php?b=169)
http://forums.trossenrobotics.com/blog.php?b=172 (http://forums.trossenrobotics.com/blog.php?b=172)

Kevin Waite

04-13-2009, 02:30 PM
It's not impossible. Rather difficult, but not impossible.

Get yourself a decent oscilloscope and go looking for the NTSC combined composite signal. It'll look something like this:


Should be around -1 to +1v peak-to-peak.

04-14-2009, 10:48 AM
Now that is very very cool. Thanks.

I do not have access to a scope . . . . (on my wish list) . . .
if I were to get one (on Ebay or outlet or borrow) what
freq range is required/good?)

Logically, this NTSC signal should be moving from board to board,
I meam, since this TV has 4 signal sources:
1) TV tuner
2) VCR
3) DVD
4) AUX in

Paying close attention to the 5 wire and 8 wire ribbon cables --
they must have the TV signal, no? I was just not sure what the
normal convention was within a COMBO TV such as this in
terms of selecting the input signal from the 4 input choices,
based on your answer -- they all are composite NTSC, right?

(Is there a term describing the transition from say TV Turner
logic/circuit -- to the circuits that drive the cathode?)

I am not sure what a TV's conventional signal is between
TV tuner signal output and input to cathode driver circuits.
If you know of some online info to explain this -- please
post any such links/info.

Focussing on the board connenting ribbion wires
and finding pairs that are +1 to -1 v -- and then just
testing with a video in on an old VCR / TV ? ? ? When I
see a picture I got it. . . Crude, I know. But still possible?

Thanks again.

Kevin Waite

04-14-2009, 11:37 AM
Not sure if I'm helping or hurting...

Go video makes a combo DVD/VCR...and there are a number of pc based remotes out there that can talk to multiple devices. My dad has one...it was pricy, but it works for controlling everything from the TV to the lights in his apartment. its actually sort of cool.

IR to sound converter (http://forums.trossenrobotics.com/downloads.php?do=file&id=135) Seams like a good path to check out!

In anycase, seams like your going way too deep...I would look closer to the surface before poking around in the back of a Tube TV...(good way to die)

good luck.


04-14-2009, 12:34 PM
I'm not sure what your overall goal is but if you want computer control of the system then an irBlaster may help:

04-14-2009, 12:37 PM
The waveform is about 63.5 (63.556 if memory serves) microseconds in periodicity. The Chroma carrier is about 4Mhz (just a little under, iirc, 3579.545khz) So as we can see - it doesn't take much of a scope to scope it out. I used to work on television stuff with a 5Mhz scope but always dreamed of a 10Mhz-15Mhz scope to make my life easier. It's hard to find anything under a 50Mhz scope these days.

I figure there's two ways to look at it... You could buy a VCR and DVD player, or a dual player as DB mentions. Or for the same money, you can get yourself a scope and have it around for future questions like this, and have the joy of hacking. :)

Ideally, look for a 100Mhz scope. That will cover about anything you'll want to do later.

That -1 to +1 peak-to-peak is changing thousands of times a second, way faster than any multimeter can lock-up the voltage. Readings will appear random and not really useful. Putting the wrong signal into the video in on a television or similar pretty much assures that you can also plan on buying a new television. They're not super-hyper-sensitive, but they can get annoyed without too much trouble.

As DB alludes to, you need to stay south of the tripler and flyback. 30,000+ volts of RF is not uncommon, and depending on the set, 60,000 - 120,000v+ isn't unheard of. [Which I immediately steal for Tesla coil building. ;)] It's a b* getting bit by that. I still have limited range of motion in a couple fingers and thumb when it gets cold and damp outside. That'll burn all the way along the bone destroying any nerves it comes into contact with. If that path happens to be through the body - they'll be fitting you up for a toe-tag. I knew a guy in passing (friend of my dad's) who was literally cut in half when two back to back televisions he was working on were arcing between them near invisibly...

I repaired TVs most of the way through college for extra money. I have my own share of hairy stories, most related to those stupid very early 36" picture tubes. [shiver] ~300lbs of major suck.

04-14-2009, 12:58 PM
Check my MomPC blog -- the first two posts describe
the need to control TV & DVD etc and focusing
in on using an IR blaster to control this.

The key problem (downside) with all IR Blasters is that
they do not have any sort of feedback --- so there
are failures of context -- as you add multiple boxes
to control (instead of just this single TV/DVD/VCR) the
more likely of dropped IR commands leaving
programmatic control no longer knowing
what state is current. Also adding more video boxes
introduces training problems for the nursing staff
to know how to manage getting TV or DVD
working for Mom manually -- they are very use
to her current TV/DVD/VCR --- changing this
now if avoidable would be a positive.

In fact -- even with the this lone TV/DVD/VCR --
the only way to best know you have selected
DVD - is to turn the TV off then ON which set
TV mode by default -- and then issue [SELECT]

If any of you have learned tricks to better
use and or improve using IR blasters please
let me know.

The audio hack is to solder 3 leads Ground, Left, Right.

The video hack -- is to solder 2 leads on this yet
to be found composite NTSC circuit.



04-19-2009, 03:59 AM
Significant progress!

Found a scope on Ebay from someone very nearby. Today I took readings from many logical locations on the Magnavox combo TV’s motherboard and came up with the following details:

Please check this web page:
http://www.kevinwaite.com/MomPC/ (http://www.kevinwaite.com/MomPC/)

Before I start connecting other video equipment to jumper J1298 – if anyone out there knows about the following measured voltage increases.

J1298 (a) appears to be a composite video signal – but it also appears to be 2 times greater voltage than the input composite found on Ext. V-IN J1297. J1298 (a) carries the selected signal of DVD/VCR and TV.

The RED J1831 (and it shows more trace activity was more red is shown) (same for Green and for Blue) – these signals are about 6 times greater than the input composite found on Ext. V-IN J1297. Test points RED GREEN BLUE test points carry the selected signal of DVD/VCR and TV.

Any input or guidance – is greatly appreciated.


Kevin Waite

The main Tuner / VCR board found inside is a:
BTD850F01012 x4 Triple Main PCB -- the VCR box is found
above this board. I have found this board for sale as:
VHS PLAYER ASSY BTD850F01012 X4 from MAGNAVOX 20MC4304/17 or 27MDTR20/17