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OT: inner workings of a cable box?

 
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The Robman
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Joined: 01 Aug 2003
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PostPosted: Wed Oct 29, 2003 11:36 pm    Post subject: OT: inner workings of a cable box? Reply with quote

I'm got a cable box here that I'm looking at that is brand new but it has no picture (the sound works fine). My goal is to try and find out what might be wrong with it.

I fixed one of my cable boxes before, but that was one where all the capacitors had blown due to over heating, so that was easy. As this one's new, there's nothing like that that's easy to spot.

There appears to be 3 IC chips and an EEPROM on board, but I can't see anything that screamed out "video processor".

Does anybody here know anything about this sort of thing or at least have any good ideas about where to start?
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Agix



Joined: 24 Sep 2003
Posts: 16

                    
PostPosted: Thu Oct 30, 2003 8:46 am    Post subject: Reply with quote

I have not worked on any cable boxes but I will give ya my opinion.

Have you tried to just probe the video output with a scope and work your way backwords? I am not sure how many layers those boards are, but it might not be that difficult. If I remember correctly, the video signal should be an obvious looking signal on a scope. Maybe some op-amp output gave out. If the box appears to be working in all other cases, I would think the power supplies are good. If you think something is shorted, do the finger test, that could give you an idea where to look. If a finger scares ya, get some freeze spray to find the hot spots.

Sorry I couldn't be of to much real help, I am sure you already tried some of these.

Good Luck
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The Robman
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PostPosted: Thu Oct 30, 2003 11:33 am    Post subject: Reply with quote

I have checked the power supply and it is outputting the correct voltage (12v). I have also checked a couple of points on the PCB where the voltage is printed and the output is correct.

The box does appear to be working perfectly in all other regards.

So, what sort of scope are you referring to and how would I use it? While I've become very good at soldering and finding obvious problems with burned out components, etc - I'm still totally a beginner at electronics, so don't assume I've already tried anything or even know what I'm doing! Smile
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jamesgammel
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Joined: 03 Aug 2003
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PostPosted: Thu Oct 30, 2003 12:00 pm    Post subject: Reply with quote

scope=oscilliscope. Not exactly something everyone just has lying around. What's your ultimate goal in this project? Is this something you're just going to personally use, or is it something you want to "fix" and re-sell? The fact that the audio portion was working indicates that the basic power supply was working. How about a little bit more info: ie. does it change channels? (does the audio at least change to indicate that it is changing channels?). Cable boxes typically use RG-59 cable in and out, although some may also have the yellow/red/white outputs as well as the RG-59 cable out. Which are you using? Is the video signal going directly to the TV from there, or is (are) there other devices in-between? If there are other devices between, have you ruled out them as a source of the problem? (IE receiver, switchboxes, etc.)
Jim
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The Robman
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PostPosted: Thu Oct 30, 2003 12:05 pm    Post subject: Reply with quote

I'm trying to help someone who sells these and has alot of them that don't work. I'm hoping to find a fix that I can pass back.

EVERYTHING else works on this box, it changes channels, the audio changes when the channels change, etc, etc.

I have connected both the COAX out and the red/yellow/white A/V outs directly to a TV and get the same black screen in both cases.
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jamesgammel
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PostPosted: Thu Oct 30, 2003 12:23 pm    Post subject: Reply with quote

OK, that helps a little. Did your friend get these "surplus" from some cable outfit ie. via some form of auction, sale, etc.? Are they all the same, ie, they've been "upgraded" by a different box?

What I'm trying to find out is if they inherently had a design flaw, or were deliberately "sabotaged" by the cable company that was using these to make them non-functional so buyers can't "steal" cable signals. Are these for "basic" cable service, or (as many cable outfits do) are these the ones issued for "premium" channel viewing? Since you mentioned an eeprom, it's quite possible that they erased the eeprom, and that could very well disable the video portion. Is the eeprom soldered to the board, or is it in a chip socket? I'd also suspect that there's some sort of connector on one of the boards for the cable company to program the eeprom, do you see any pcb edge connector where following some traces goes to that eeprom? I've already ruled out the built-in tuner as the culprit, since the audio portion is dependent on it as well, and it's apparantly working ok since it will change channels.

Does he have at least one that works? If not, can he get one of the same model, etc that does?
Jim
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The Robman
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PostPosted: Thu Oct 30, 2003 1:55 pm    Post subject: Reply with quote

These are direct from the manufacturer, these boxes probably represent the normal number of defective units that you get when you order in bulk. He's just looking to see if there's a way to fix them rather than trash them.
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Agix



Joined: 24 Sep 2003
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PostPosted: Thu Oct 30, 2003 3:59 pm    Post subject: Reply with quote

All guesses here, but I am surprised that a manufacturer would ship bad units. So all of these are manufactures defects or product returns that they are removing from stock? They must figure that the cost to fix them outweighs the cost to just build a new one.

So does he your friend have some good ones to compare against? You can make sure all IC's are properly orientated. Maybe the manufacturer received bad parts, you could check the date code if any are visible? I would think that there should be some kind of Video IC, maybe a bottom side component?

It does depend on whether or not this was a product defect or external damage when looking for this problem.
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The Robman
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PostPosted: Thu Oct 30, 2003 9:49 pm    Post subject: Reply with quote

There are four chips on the PCB.

1) the first is a simple DIP EEPROM, 24C04

2) Next is a flat 14 pin (2 x 7) chip labelled MC14066BCP, I found the following references on the web:
http://www.onsemi.com/site/products/summary/0,4450,MC14066B,00.html
http://www.onsemi.com/pub/Collateral/MC14066B-D.PDF (data sheet)

3) Next is a 9 pin (9 x 1) chip that is mounted vertically. It's labelled C1406HA. I found lots of references to it on the web in "part's lists" etc but nothing really useful. The following site makes reference to it as being used for a TV's audio:
http://www.electronicsrepair.net/spravka/spravtelesams.shtml

4) Finally, the big one. This looks like the main processor. It's on the underside of the PCB and it's pins seem to connect to most of the board. It's a Japanese 28 pin (14 x 2) chip and has the following writing on it...

CX6181FD6
87C405AM
3E72
0243HAI
465445
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usblipitor



Joined: 10 Oct 2003
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Location: Greenbelt, MD

                    
PostPosted: Thu Oct 30, 2003 10:41 pm    Post subject: Reply with quote

google searching for 87C405AM turns up some pages but they are written in japanese. I don't speak japanese, and as far as I know, google doesn't translate japanese pages.
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Lenroc



Joined: 05 Oct 2003
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PostPosted: Fri Oct 31, 2003 3:50 am    Post subject: Reply with quote

No, google doesn't.

But babblefish tries:
http://babelfish.altavista.com/babelfish/urltrurl?url=http%3A%2F%2Fwww.ic37.com%2Fgyxx%2Fsearch.asp%3Fxh%3D87C405AM&lp=ja_en&tt=url

Not helpful though...
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GeorgeIoak



Joined: 11 Dec 2003
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PostPosted: Fri Dec 12, 2003 4:34 pm    Post subject: Reply with quote

the 87C405 chip is a Toshiba microcontroller and would be the brains of the cable box. since the box powers up and changes channels then the MCU (microcontroller) is working fine as this is it's job.

the video output is going to come from the tuner which is a metal rectangular box usually standing on end.

the eeprom is usually the decription code to unscramble the video signal that the cable company uses.

there are usually several channels "in the clear" with the cable feed but since your putting the cable signal thorough the box it is probably processed along with the scrmbled channels.

my first guess would be that perhaps the eeproms are blank or have the wrong image. you might try swapping a eeprom from a good box to one of these bad boxes and see what happens.
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TimH



Joined: 26 Dec 2003
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Location: Panama City, FL, USA

                    
PostPosted: Sat Dec 27, 2003 4:48 pm    Post subject: Reply with quote

Not knowing what type (brand) cable boxes your working with, and not knowing what type cable plant your system is, I'll throw a couple things out. I work in a cable lab working mainly with GI and Motorola cable boxes, and we have a DAC system (primarily Motorola equipment). On this type system a cable box won't show any video, base band or rf, unless it is "on plant" meaning it has been added into the cable system. It won't even show the unencoded channels. One thing that should always work is diagnostics. On most boxes I work with you can access diagnostics by turning the power off and immediately press and hold the "ok" or "select" key (typically the key in the middle of the arrows). Another issue can be the "out of band" frequency that the box is setup for. Typically the freq is set in hardware, though some boxes the freq can be selected in diags.

FWIW there are a couple different refurb shops that can fix, and have the ability to test, cable boxes. One is http://www.dbtronics.com/
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