(Solved) - Looking for knowledge on Vizio TV antenna input stage

[CatalinaWOW]

I have a Vizio MD801d-a3 TV and have apparently blown the antenna input.  I was using a battery power cable tracer to find existing coax runs in my house, and (probably) ended up connecting the coax to the TV antenna input while the RF output of the tracer was connected.  After this the previously working input reports no signal when connected.  Signal has been verified using other TVs.

This TV is huge and it will be a physically large project to dive in and troubleshoot, so I am looking for any guidance before I start.  Does anybody know what the first stage looks like?  If there is a fuse or a transistor pre-amp I will probably tackle the job, but if it is a custom IC with limited availability I will just continue to use the HDMI inputs.  Hooking the antenna up was just a way to get around a cable company dispute with one of the local TV stations.  I can live without that station.

[simmconn]

Most likely the RF part is a silicon tuner in a shielding can. There would be an ESD protection device at the antenna input jack before the input filters. You are in luck if it is the ESD protection device that went bad. Things would be more complicated if the silicon tuner chip got damaged. The main board can often be had for less than $50. Do a google search if you don't want to spend the time messing with component level repairs.

[BMack]

I'd simply look into the cost of a main board first, check eBay and shopjimmy. If it's cheap and available, buy it. If not, check availability for the tuner. It's probably not worth your time to try to fix it on the component level.

[DaJMasta]

Agreed with the above, Vizio isn't a premium brand so they probably aren't going to be designing their own input conditioning or tuner when there's prebuilt modules that work just fine on their own.  I'd expect a tuner can as well made by some other company just built into the board.  I can't imagine they're particularly pricey parts to replace, though, as they're going to be in every TV.  Maybe you can salvage one from a dumpster dive or find a replacement if it's a standard part - but if it's too integrated into the board or doesn't use a shared footprint/interface with other tuner modules, it may not be worth trying to fix - then just replacing the board would be the better choice.

Crack it open and see, though!  You may get lucky and the jack solder joint is bad or something 

[jh15]

Checking it with what? Be sure that is not the problem. You disconnected cables and such, used I guess a dvm. I cannot see that damaging a 75 ohm tuner input.

Make sure.  Go through menu, scan for cable/broadcast channels.

I've worked in TV RF on both sides of the signal path (xmitter/home).

Now on a some ham antenna analyzers, they are easily blown out by esd or rf, never say a consumer tv tuner blown by a multimeter.

Vizio is a good performing product, but there is no longer a repair channel. Same for Ipad, Iphone, Ibook etc, just toss it or play here.

[CatalinaWOW]

Thanks to all for jolting me out of my antiquated repair philosophy.  I found that a replacement main board is under $70.  Well worth it.

I am surprised that the input was damaged.  I haven't yet checked the actually voltage/power output from the cable tracer, but it runs for days off of a 9 volt battery so it can't be much.  It is FCC approved so it should be under 1 milliwatt. 

One thing I will check is the simple possibility that the connector solder joint cracked during connect/disconnect of the cable inputs.  I suppose there is some chance of a static discharge, but the problem occured on a day when I wasn't noticing static problems, and an F-connector doesn't expose the signal lead too badly during connect/disconnect.

I am quite certain the input is dead.  It was properly programmed and working when the cable was disconnected (standard cheapo F-connector).  Went through antenna programming and channel search a few times.  Even disconnected power to the set to get a complete reset.  Connected another TV to the same cable and had strong signal.

I am spending the time gathering information because this TV is so large and heavy.  Two man lift and will require either working on the floor or a very large table.  Not the kind of thing I can leave in the shop and work on from time to time.  When I start I want to get it done.

Again, thanks for the comments and ideas!

[TheBay]

If you do swap the board, swap any eeproms over 

[Richard Crowley]

I would have guessed that the RF antenna input is either capacitively coupled or transformer-coupled.  It seems unlikely that you could apply any kind of voltage to the antenna input (besides an electrostatic discharge) though a small capacitor (appropriate for 100s of MHthat would actually damage the circuit. But if the RF input were transformer coupled (typically with a rather tiny transformer made with a few turns of wire on a 2-hole ferrite bead).  If one put power into that kind of component, it would blow the input winding (like a fuse) which would make the input non-functional. That kind of ferrite-bead transformer is pretty easy to "re-wind"  because it has only a very few turns which are easy to replace.

[jh15]

I'm thinking more of a physical problem like a bent over or broken f connector center wire or ground crimp contact in the tuner input. I just redid a connector on a 4 bay UHF antenna were it started to spin under strong finger pressure.

I would normally use a wrench for final tightening, but this let go under fingers, twisted the stake loose, and then the center conductor twisted off, never noticed until it seemed to never tighten.

BTW this antenna works well once repaired, and re-soldered well with good flux. Will mention in another thread.

Please post the meter you used, this is new to me.

Maybe it was a megger? hehe

[CatalinaWOW]

I will check the center pin on the connector when I get it the thing out.

The cable tracker is a cheapo

http://www.harborfreight.com/cable-tracker-94181.html

The one I have was picked up from a returns table at about 1/3 normal price.  Like all Harbor Freight stuff quality is an issue and this one was a return that didn't work.  The problem was a bad solder joint on the power switch, easily fixed.  It isn't magic but can help on a number of cable tracing problems.  It has two modes.  In one, the TX generates an AM modulated RF signal.  The receiver is fixed tuned to the RF.  In the other mode it acts like a continuity checker, lighting an LED on the TX when there is a short between the two signal wires.

[jh15]

This tool should no way blow a front end.  I have similar pro tools. Maybe if you rubbed a cat across it on a dry day if it is esd sensitive.

Be glad HF is around, RS kept a lot of primary broadcast studios on the air on Sundays. Miss that.

If the set is designed for OTA, it needs to expect esd from snow or local t-storms through the outdoor antenna, so will have a drain path like a megohm resistor. Nothing to do with electrical code grounding of the mast.

I see physical damage or programming error, not tuner damage. Just trying to save you bucks.

One of my tools: OK, pretend I got the replacement part and it still doesn't work. Try that.

Always have an open mind though, I'd like to be proven wrong.

Post your results a as solved when time comes.

Or make a bench light out of it

[CatalinaWOW]

The problem is doubly solved.  First the cable company and TV station have kissed and made up so the Mrs. can see her shows over the cable.  Secondly, I have the over the air TV working.  The resolution of that is a long story that may aid others.  Bottom line is my signal path fell below the input sensitivity of the Vizio.

I live in a valley about 30 miles (50 km) from the transmitting antennas.  The transmitters are about 90 degrees apart, and I have no line of sight to the transmitter broadcasting the channels in dispute.  My home has a steel roof and I knew from cell phone and FM radio results that an indoor antenna was not an option.    I wasn't at all sure that I would be able to get over the air TV so I bodged up a quick experiment.  I purchased a moderate gain antenna (single direction pattern), grabbed a couple of pieces of coax out of the junk box along with some barrels and ran the antenna some 50 feet (16 meters) to an outdoor location.  Holding the antenna by hand I was able to receive the desired station with no artifacts of low SNR.  I also got good signals from the transmitter at 90 degrees to my aiming direction, although the station likely to have used the lowest transmit power had severe artifacts, updating the image only once a second or so.

I concluded from this experiment that there was plenty of signal.  The hand held location wasn't optimum, nor was it perfectly aimed.  The cable pieces were a mix of RG-6 and RG-59.  I figured there were significant losses in the cable, losses from aiming, and losses from the ground level.

The next step was to mount the antenna permanently and come up with a permanent route for the cable.  This was intended eventually to cover three TVs in the house, although the problem cropped up before any attempt was made to split the signal.  The house has coax routed to the Vizio TV location, left over from a previous owner, although the source was unclear.  I traced the cable, found that it had several junctions between various kinds of coax, and the route from the antenna was closer to 100 ft (30+ meters).  Hooked everything up and no signal reported by the TV.

At this point, I was disappointed, but not surprised.   Even though I thought I had lots of signal margin, lots of losses in the actual connection was not surprising.  So I bought a roll of RG6 and tested a direct connection from the antenna to the Vizio TV, routed across the floor.  I didn't want to go to the work of fishing the coax through a lot of walls and finished ceilings without knowing that cable losses were low enough. 

I was really disappointed to find no signal reported.  As a check I ran the test cable over to another TV, which collected all signals from both transmitters over the longer cable length.  I concluded something was wrong with the Vizio.

I was still stalling over diving into that big piece of gear when the cable came back and took the urgency down.  Other projects beckoned.  But as time became available I decided to at least wire up the other two TVs (and the Vizio while I was at it, so that it would be ready whenever I got around to fixing it).  Since I would be doing a three way split, and since there was at least some evidence that my signal margin was not as large as I would like I put a gain block in.  In the course of doing this I found that distribution amplifiers, which used to be cheap and ubiquitous at all hardware stores and big home improvement stores and stores like Best Buy, are no longer stocked at retail outlets and have gone way up in price in on line sources.  Happened to find one on a remainders shelf at one of the local stores.

When hooked up with the added gain block, the Vizio worked.

So after all of that I conclude that the Vizio has a very pedestrian RF sensitivity, significantly lower than the other two TVs and that I got very lucky in my initial test to exceed its input threshold.  I never damaged it, it just wasn't ever very good.