Help me with RF interference
 

Okay, this one is driving me crazy (as most things I can figure out do).

My Avante is having some RF issues. It covers most of the screen with dots and other static. When I move the antenna lead around it goes from better to worse and sometimes I can get it to go away. It seems if I get the lead in just the right angle, it goes away, but it won't stay like that for long as the slightest change in angle will bring it back.

This is definitely being introduced at the connections on the TV. If I touch either one of the screws with my finger, the interference gets really bad. If I touch one and then the other, it goes from bad to not so bad. I've looked inside the set for any issues with the routing of the twin lead and don't see any. It only at the connections. I did notice that one of the connections is not original. Looks like it must have been pulled out and they put in a similar one, but soldered the lead to it instead of it being a removable connection.

I really wish I knew what it was as I doesn't look like the normal interference I'm used to seeing with vacuums, compressors, etc. The static doesn't roll up or to the side. Each piece of it changes, but not location, just intensity.

Hope this makes sense.

Look for arcing in the HV (including the connection to the CRT), focus and yoke sections. Since the noise is synched to the screen I'd guess the source is internal, and probably due to arcing.

I have checked and found no arcing.

If it is internal, how does moving around the leads on the external connections clear it up or make it worse?

Would providing a picture of the best/worst of the interference help diagnose?

Thanks

Look for any coaxial cable having a broken/intermittent ground at one end.
E.g., a coax RF input cable, or the IF cable from tuner to chassis.
I've seen it happen with a pickup mic cable (in a remote set).

I have check every external cable. I have also checked the two twin lead connections inside the TV. I will check all the cables inside connected to the tuners ( I believe there are three and one jumper).

I am attaching a couple of pics of the static at its worse. I can adjust the external connection orientation to almost eliminate it, but it doesn't completely go away (but pretty close).

Could be a device inside your house, CFL light, Wall Wart, Computer, Router, etc.

I'd try unplugging everything you can, one at a time to see if it goes away.

This happend to us in the 80s and it turns out it was our nieghbour on his ham radio :D

If it were arcing the focus pin of the CRT socket was common
on these especially if it had open S-caps or divider.
Test by pulling all ground straps & check them for HV.

But this looks external. Try using an AM radio to track it.
It can be almost anything. I live out in the woods & the QRM
kills AM radio. During blackouts it comes alive with Philly
at high noon ( apx 300mi).

We used to have an FCC that worried about interference but now
they are just a rubber stamp for bad ideas.

73 Zeno:smoke:

Is it only on this tv.....? Put a similar tv in it's spot in the house, does it
have the noise too....?

.

Zeno, what is QRM?

Google it.

QRM = NOISE!
The owner of the business next to mine has a laptop that drives the analog TVs nuts with that kind of interference. I can tell when he turns it in and when he turns it off!
Also, check your focus divider for a small crack in it's housing. This will allow a slight discharge which can cause that kind of interference.
If the focus pin is green (corrosion) on the CRT it could be leakage on the socket. Usually this causes focus troubles too.

I did before asking, but I got a bunch of non-electronic related crap that was clearly not the same acronym being used here....

"Q codes"
http://www.qsl.net/w5www/qcode.html

jr

Well I found it. I checked everything I could think of. Turns out it was the only piece of equipment I needed. It turned out to be the RCA RF modulator introducing the noise. I got my hands on an older RS modulator and the problem is gone.

Thank you for all your input. It's so nice watching it without all those annoying dots!

You didn't look very hard, Wikipedia, the third listing down explains it.

I wrote too soon. The problem isn't solved.

It seems the interference is coming for a switching DC power supply I'm using to power all the components in the rack. I got it so I could get rid of several wallwarts. The unit provides power to an FM transmitter, Amped Wireless access point and Roku unit. It is supposed to be filtered, etc. When I unplug it, the static is completely gone.

Is there any way to clean it up?

This is the unit I got from Amazon:

http://www.amazon.com/Universal-Regu...g+power+supply

It is almost certainly a switch mode power supply. You can try plugging that supply in through a power line noise suppressor (keep the cord between the power supply and suppressor as short as you can so it don't act as an antenna), add extra filter capacitance to the output, and extra metal shielding around it, but the benefits of that are limited. Switch mode supplies are basically oscillators controlling power circuitry, and are MASSIVE noise generators. If you can't suppress the noise from it, then look into going back to the wall warts, or get a 120V-12V line step down transformer, a diode bridge rectifier, two good size 25V caps, a 12V regulator chip (if you are using it as a 12VDC supply), and build your own noise-less linear power supply (it is fairly easy).

How about moving the switching supply about 10 feet away? I could then run a lead from the DC output to the connections for the devices. Isn't the noise radiating from the supply? If so, moving it should help, right?

These wallwarts are still a bit of a mystery to me in how they provide their power. The FM Transmitter had a 12V .5A wall wart, but setting the switching supply to 12V created a problem with the transmitter locking on the frequency. I had to bump up the output to 13.8V to make it work properly.

I'd like to keep away from the wallwarts as they take up way too much space and create a wiring nightmare. I'm back to them, temporarily and now have 4 of them and three standard AC plugs. It looks just like the electrical octopus the fireman told me never to create back in elementary school. :smoke:

Switching supplies often radiate their noise thru the AC mains wiring and sometimes the DC output too. You may or may not get a noticeable noise drop if you move it 10' or half a block...

The electric octopus is not a danger as long as you don't overload anything (most are too dumb to check so they advise not to do it, assuming everybody is dumb). Electricity can handle as complex and messy a path as you can give it (the CPU chip in your computer is billions of times as complex as your little rats nest), and remain well behaved as long as you observe ratings and rules.

I found that combining all 4 in the little "nest" causes a buzzing as one of them is vibrating just enough to resonate when bundled.

I'm going to try and move the supply and lengthen the DC output line. Do you think a cap across the DC leads might help if there is noise there? Also, I don't think it's going back along the AC as it's being radiated to the antenna input of the TV.

Thanks

The cap on the DC may help. I'd try a lytic in the 1000uF range, and a parallel ceramic or film cap in the 0.01uF range, and maybe a series resistor in the 5-10ohm range between the caps and a terminal of the supply.

It don't have to travel the whole way through the AC lines. 6-10' of travel = a decent tuned VHF TV antenna (for it to transmit to the air), and 3-6" a decent antenna for UHF....

If you can grab a newer SS parts TV, video monitor, or computer powersupply most of them have a line noise suppressor built in. The new video gear used switch mode supplies too, but they build them properly so they don't make too much noise. Find the big 200V cap nearest the power cord on the parts board, it is fed by a diode bridge, everything between the bridge and line cord is the filter (usually consists of some ceramic caps and what looks like a small weirdly connected line transformer which is actually a dual line choke). Harvest that filter (or buy a more expensive one from the store if you don't have faith in your skills) and add it inline with your supply as close to the supply as you can.

QRM is man made noise like computers, arc welders etc.
QRN is natural noise like lightning crashes etc.

73 es cul de Tom KA1YHI sk:smoke:

It also may be one of the things you got hooked up
is radiating off all the extra wires involved. Try unhooking
one thing at a time ( power) if you havnt. As far as well
filtered goes take that with a grain of salt. Switching supplies
are wickid noisy. They are no different than the hoz
sweep in a TV. Both noise generators.

73 Zeno:smoke:

I disconnected all devices from it and the interference was still there. The only thing that got rid of it was to unplug the PS.

I have a small access hole to the closet with all the gear. I'm going to run a lead though it to the harness supplying the devices. The PS will be outside (other side of wall) in a different AC outlet. Hopefully this will take care of it. It's amazing how much space is lost with 4 wallwarts, extension cords and associated wires.

I've had the same problem. What fixed it for me was swapping the adaptor going to the antenna terminals, some of them seem more susceptible to receiving the interference than others. I tested several, and the vhf/uhf type seemed to pick up on the interference the most, and the shorter the length of twin lead on the tv end of the adapter the better.

I had similar interference which turned out to be a bad 75-300 ohm matching transformer. Switching to one that was about 20 years old cured it. Just reinforces how new stuff is made out of crap.

Gracious..... I thought it was the other way round!!

I would vote for building/sourcing a linear 12v power supply which would not have any power line induced or more importantly RF emitted noise and using that as your central 12v power distribution source. Probably lots of candidates out the on fleaBay to choose from as a building block. You could use a small car battery charger, whack in a full-wave bridge, add some 10Kuf filter caps, regulate it with an LM-338K (or a low dropout voltage LM1084IT-ADJ ) and there's a solution for a pretty cheap and plenty of +12vDC power supply for your needs (and output voltage adjustable with a pot, if you wanted). Total cost for the mod parts mentioned is about $10 or so.

http://www.ebay.com/itm/Schumacher-8..._Tools&vxp=mtr

Tom (PK)

I agree. But one thing to beware of is that some car chargers now are switch mode supply based. Make sure there is not switch mode supply in any car charger you buy lest you replace the devil with the same devil in disguise...

The "fleabay" battery charger example referred to would be a transformer type. I agree that everything nowadays (current new production stuff) is all switch-mode stuff either using buck or boost types of regulation to derive the D.C. output voltage (with the unavoidable R.F. noise they inherently generate).


Tom (PK)

I just went through this same problem, with the lines as shown on the photos, which were slowly getting worse weekly.
It turned out that the problem was the Magnavox DTV converter box. I replaced it with another identical unit and the problem vanished.