This can be the first of (eventually ... hopefully) a few hardware musings from Bluehaze ...
This is an account of some modifications I did to an ancient (circa 1980) Sanyo TV of ours to fix an annoying side effect of the so-called "Macrovision" protection of rental VHS video tapes.
NOTE: If you came to this page as a result of looking for
Macrovision removal methods (eg: to enable your DVD player to run
to your TV set via your VCR without having the picture go dark every
30 seconds or so), what you really need is a video timebase
This page doesn't provide that. You can buy kits for such items (for the Oz and UK PAL TV standard) from your nearest Jaycar Electronics shop and make your very own in one evening (if you aren't frightened of soldering). Just type "stabiliser" into Jaycar's search box and then look for "Dr video Mk II". They're about $100.
Or maybe even search around for a TB (video) stabiliser on Ebay or Google.
By the way - a quick and dirty trick that I use is to connect our DVD player's outputs into the RCA inputs of our old Sony video camera's modulator box instead of running it into the VCR. The RF output of that modulator box (on Channel 1) is then plugged into the TV set's aerial socket. This means re-plugging everything just to watch DVDs, but by bypassing the VCR, it eliminates the problem.
Or if you have a more recent TV that comes with RCA AV sockets, just plug your DVD into those. Or if you're really lazy and you want to avoid re-plugging things to watch old tapes - get a TV with two sets of RCA inputs (AV1 and AV2), and plug your DVD into one and your VCR into the other!
Okay - back to the story of our old Sanyo!
This old warrior has been kept in service over the past 22 years for the simple reason that I fitted a nice big new 63cm (25") tube back in 1985 after the green cathode of the original had gone low in emmision. (Well, re-gunned actually ... you would give Thomas Tubes here in Melbourne your old CRT, and they'd flog you a re-gunned one of the same type for around $150.)
Anyway, in June of 2003, this old Sanyo had gone "bang" and stopped working completely. This happened to coincide with a water fight the kids had been having in the living room, and I then learned that "Rosie had thrown a glass of water at Andy but Andy had ducked out of the way and it had hit the TV instead."
"Hmmmm ... okay, guys - thanks heaps!", I thought. Then I learned that when the set was later turned on once again, there was now a strong, acrid burning smell in the room and no picture or sound at all.
Great. Just what I'd needed to nicely round out the week.
Anyway, to cut a potentially long story short, Anna (my beloved) and I dragged the old monster out onto my back room workbench (see Fig.1 below) - and over the next few days, I probed around and finally dug out the culprits.
Surprisingly, there was no water damage at all - just an old original electrolytic capacitor (4.7uF 350 volt) which had shorted out, plus an incinerated 1 ohm series resistor which obviously hadn't enjoyed having 120 volts across it as a result (see Fig.2). These were part of the overall 120 volt feed to the horizontal output stage.
I dragged an old 8uF 450 volt electro out of my junk box, and a new 1 ohm 5 watt resistor (out of the not-so-junk box), soldered them in, and sound and picture returned as good as new.
Okay - so far, pretty straightfoward.
At this point, I wondered to myself "Should I just take the set back to the living room now, or try to do something about it's Macrovision susceptibility?" (see Fig.4) Because, whenever we'd play rented tapes over the years, every 30 seconds or so, we'd get all these bright white dashed lines coming up over the top half of the picture for 9 or 10 seconds. A most annoying effect.
Finally, I decided to have a shot at getting rid of the problem. Walking over to the adjacent bench (the one with this web server and my office PC), I did a search via Google and sure enough, turned up a site that explained the mechanism. Basically, it seems that Macrovision sneaks in a series of high amplitude white-going pulses during the vertical retrace interval. "Aha", I thought, "okay then, let's have another look at that Sanyo circuit (see Fig.8). What could be wrong with its vertical retrace blanking?"
Sure enough - there was no evidence of any CRT-retrace blanking circuitry at all! One could only assume that more recent (or better designed) receivers must include such blanking, because our old chassis was one of the few on which I'd ever noticed this effect. So - how to get rid of the problem?
It seemed obvious that what was needed here was some additional circuitry that would clamp the video signal down to black (or below) during the vertical (field) retrace interval. I quickly sketched up a circuit for a single NPN transisor, some resistors and a pair of coupling capacitors, grabbed the components out of the drawer and soldered myself up a little rats-nest for the job. I then connected the transistor input circuit up to the vertical output sawtooth (via a 100k resistor and a 470pF cap) and experimentally tried hooking its output up to various spots in the video-signal chain.
I eventually managed to get this single transistor switch system working to a greater or lesser extent. I could clean out a certain amount of the effect - but it was still disappointingly short of the mark. For one thing, it wasn't staying on for sufficiently long. For another, because I was taking my driving signal off the 120 volt analogue sawtooth vertical output and differentiating via an RC network into the transistor base, the transistor was only acting as a switch as it turned ON. The turn-off process was really slow and sloppy.
At this point, the idea of some digital logic went through my mind. In fact, what would obviously be ideal would be a PLC (programmable logic controller) - one could probably write some code to do exactly what was required. The next day, I even asked our Electronics department (unofficially) if by any chance they had any old PLCs lying around that I could garner for the task. But they didn't, so that was that.
Because I certainly wasn't about to spend several hundred dollars for a PLC ... to say nothing of the effort of learning how to code the damn thing.
Over the following weekend, I put the CRO on the set once more and checked out the waveform and gave it some more thought. I still had "PLC" logic in the back of my mind, and went off to make cups of coffee, and returned, and paced around, and wasted some time surfing the web, then made some more coffee - and then suddenly thought "Why not a bistable flip-flop? That should allow some control over the pulse length, if nothing else, and it'll be absolutely "on" or "off", ie: a nice simple, predictable switch. Why not give that a try?"
So once again it was out with the pen and paper, now drawing up a bistable. I then wired it up and tried it, and this time things looked much more promising. Initially it would turn off too early, but this was soon traced to my poor choice of sampling point (on the vertical output circuit) and a coupling capacitor which at 470pf was too low. So I fixed that, moved the input to the actual vertical oscillator circuit (which is an 18 volt square wave instead the 120 volt sawtooth wave I'd been sampling) - and now the timing was much closer to what I wanted. Spot on, in fact.
My only other problem now was that there seemed to be no convenient spot in the circuit to which I connect the output of this little rat's nest circuit to completely blank the video. I could reduce it, sure - but not completely kill it. It was frustrating in the extreme. All I wanted was a spot where I could ground the video signal or pull it up to 18 volts for black, and yet there seemed to be no such point! I could hardly believe it - every obvious output connection point produced problems of one sort or another. So once again I seemed to be going around in circles.
Thinking about it in bed that night, it finally occurred to me that I should just try running the flip-flop output into a high voltage NPN transistor and use that to ground the CRT control grid. I'd already checked that circuit and the control grids (all wired together) were normally sitting around +100 volts, with the cathodes on about +250 volts (see Fig 6 and/or Fig 8). In other words, the control grids were sitting around -150 volts with respect to the cathodes during normal running, but grounding them would increase this negative bias by a further 100 volts.
The following evening, I checked the effect of grounding the grid circuit, and going that extra 100 volts negative certainly blacked things out. So I grabbed an MJE340, wired it up to the CRT board, and connected the output of the flip-flop to it. And hey - bingo - success at long last!
Having now spent about a week (on and off) struggling with this modification
and getting almost nowhere, I finally had reason to feel pleased with myself.
And it was at that point that I also thought "Why not put a description up
on the web? That's where I found the original description of the Macrovision
system and the timing specs. Who knows, someone else with an old Sanyo or
similar may be putting up with the same problem and wondering what the hell
is going on."
Here's a few pictures if you're interested. You can click on them if you want to see a bigger version (but the resolution is limited anyway - they're just some stills taken via my video camera).
|The old 65cm Sanyo on the workbench with the back removed. There's almost enough spare room in there to hide from burglers if we ever need to. As it is, plenty of space to hold cups of coffee, toasted sandwiches, books, my Scope soldering iron and it's 3.3v transformer, and the cat from time to time ...|
|The original fault was caused by this little electrolytic capacitor on the left. The poor resistor then died as a result.|
|Having fixed the above fault, I decided to have a look at our Macrovision problem. Looking at the video signal coming out of the video detector with my old Telequipment D66 CRO, I could clearly see the Macrovision interference signal|
|And here's the delightful effect on the screen. Believe it or not, not only had the family been tolerating this for 10 years with the more recent rented tapes, they told me not to worry about it! (I do know that it severly annoyed our friends, though)|
|This is what the signal should look like, as seen between Macrovision interference periods|
|This is the final version of the add-on circuit to provide vertical retrace blanking - a bistable flip-flop (using discrete transistors, no less). When the vert oscillator waveform swings positive (ie: at far left of lower trace on CRO screen above), the flip-flop switches the MJE340 on and pulls the CRT grid down to 0 volts. When the vert osc swings negative (about half way across the screen), the bistable flips back and the MJE340 turns off again.|
|I couldn't be bothered making up a nice circuit-board for something as small as this, so I simply wired it onto the back of the Sanyo's main video processing board. Quite a rat's nest, as you can see.|
|If you'd like more detail on this Sanyo chassis, here's the full circuit of the receiver (a poor copy, unfortunately, although still good enough to help one find their way around). It's a 1+ Mb GIF file.|
BTW - you may notice that
the schematic is missing the 2 base-emitter protection diodes in the movie.
I added those whilst creating this page, after realising to my horror that
I'd left them out of the schematic. (The protection diode on the RHS is
uneccessary, of course - I only included it because I was feeding signal to
that side in one of my earlier experiments)
Personal note: Currently I work full-time as a programmer and computer systems person, but I've always been fascinated by electronics and enjoy reading magazine columns such as "The Serviceman" in Oz magazines such as "Electronics Australia" and "Silicon Chip", "Electronics World" (UK), and so on. And that led me (as a teenager) to taking up audio, Ham Radio, digital logic design, and so on.
Playing with hardware is such good fun (once you learn to solder). Makes a
nice diversion from computers and boring old software ... :-)