I have this unrestored original Zenith 24-inch B/W I picked up off of eBay a couple of years ago. It gets turned on once a month to check it, otherwise it's just on display (it's not a daily watcher). It recieves OTA UHF channels clearly:
http://www.audiokarma.org/forums/attachment.php?attachmentid=82905&d=1205618790


There are no OTA VHF channels in range, so the VHF tuner hasn't really ever been used. I tried out the Insignia DTV converter today with really poor results. The converter works fine on other TVs. On either channel 3 or 4 I get almost no audio and these curious white spots:
http://www.audiokarma.org/forums/attachment.php?attachmentid=82916&d=1205619825

Any expert-TV-guru thoughts on this?

The VHF--particularly the low (2-6) band is highly susceptible to this sort of interference. It is usually caused by digital equipment and sometimes switching-type power supplies such as is used in computer and compact electronic equipment. The little plaything on top of that telly and to the right may be your culprit if it is even plugged into an outlet. This also is not uncommon when modern gear like that converter is used with older sets with 300ohm twin lead-in. Newer sets have much more secure shielding against spurious RF created by today's digital stuff.

The little plaything is the DTV converter. Would be rather sad if it generates interference to its own output signal, but I guess the way things are designed these days...

No, I doubt its interference. The DTV converter would have to output a lot of interference to screw the picture up that bad, and if it did, the FCC wouldn't allow it to be sold.

Methinks the VHF tuner is misaligned, or has a bad connection inside of it. Maybe a coil needs replacing.

It appears to be the interference as Old1625 described it. I can move the DTV box around, limited by its two-foot coax cable (supplied) and find a "null point" where the interference has little effect. Moving it farther from the TV in general diminishes it. It's obviously coming from the converter.

http://www.audiokarma.org/forums/attachment.php?attachmentid=82977&d=1205639237

The Digitalstream from Radio shack does the same thing.

Something to think about when designing your DTV-to-Vintage setup. Maybe the Zenith is particularly suceptible (or somewhat misaligned). I'm curious how others fare with this.

I have had issues similar to this with the ATSC tuners(not the coupon eligable converters) that I obtained from Newegg.com. When this occurs, I can either turn the ATSC tuner off and back on again or the TV on and back on again, then the interference is gone. I suspect that some sort of resonance is setting up.

I also have ATSC tuners from a now defunct CATV company. These have never had the issue. (Overall, it is my opionion that the CATV tuner build quality is better than the Newegg tuners).

Something's wrong with these DTV boxes if they can put out so much RF interference that they mess up the picture on a TV.

The FCC should be made aware of this...

Apart from that, maybe some foil or mesh around the DTV box will reduce the RF emaniating from it...

I've noticed a similar interference when I try to watch the tv at my desk (1960 17" b/w Zenith) with my computer on at the same time. I've also noticed it bothers the lower vhf channels (particularly ch2 and 3), but not the upper ones, it's at it's worst when I'm trying to watch ch3 through the uhf converter, but ch3 through the dvd player is better. I've also narrowed the source of the problem down to the monitor, if I shut the monitor off and leave the computer on, most of the interference goes away.

I got the same problem with a cheapo VCR that does the same thing . I have the 54 Crosley in the upstairs on a seperate antenna , and the VCR downstairs , 20+ feet away , and I was still getting this interferance .
I am not happy with some of the newer electronics

Something's wrong with these DTV boxes if they can put out so much RF interference that they mess up the picture on a TV.

The FCC should be made aware of this...

Apart from that, maybe some foil or mesh around the DTV box will reduce the RF emaniating from it...

The shielding of the DTV box may cause it to overheat, and there is no need, as any vent slot on that device is small enough not to be of sufficient aperture to admit signals that would cause interference in the low VHF band. But there is probably enough of that digital hash spilling out via its power cord--despite efforts to filter it at the point of power entry. Attempts to shield this will probably prove futile, or at least unreliable, as the radiation would simply come from the house wiring after that.

Also the set being using as a monitor for this DTV is of a bygone era.

Let me explain: Late model sets with the coaxial antenna input have better shielding; the input connector is actually part of the completely shielded VHF/UHF/Cable tuner assembly, so there is no exposure to in-the-air interference. A shieled coaxial cable pipes the signal directly from the output of the DTV converter to that tuner without outside interference being able to penetrate the shield of the cable. And all the electronics in the typical set are at the bottom of the set, and away from any nasties you might place on the top. OK, now that we've covered this let's look at the older set....

In the old console there is usually a set of twinlead terminals at the back of the set. While some sets have a "balun" to convert the balanced 300ohm to a shielded cable at 75ohm impedance to run to the tuner at the top corner of the cabinet (right under that converter in the posted pic!), the balun is usually an unshielded coil assembly just behind the screw terminals. But many sets of that day had the balun assembly on the tuner and a short length of twinlead running from there to the screws at the back of the set. That twinlead is fair game for any extraneous interference.

If the latter best describes the situation the best bet may be to disconnect the balun assembly, and try to tap into the direct unbalanced input of the tuner, and run RG-6 to a coax nipple such as used in wall plate applications fit into a drilling in the set's back. It is best to obtain a schematic for the set to ascertain what this conversion will actually involve.

The trash emanating from the DTV is still likely within FCC guidelines as regards how much spurious radiation it may produce. While your own set may complain it is not likely to bugger up your neighbor's TV--or even a set in the next room.

I hooked up one of the D-tv converters (RCA) to my roundie color Zenith using a 75 ohm to 300 ohm balun on the back of the set to the factory antenna terminals. No noise what so ever.

I hooked up one of the D-tv converters (RCA) to my roundie color Zenith using a 75 ohm to 300 ohm balun on the back of the set to the factory antenna terminals. No noise what so ever.

Luck is on your side. Maybe RCA was having a good day when they made yours. :thmbsp: I still think the chief radiation of digital hash from those is likely out the power cord. There are isolation coils that are used, but perhaps some of those filter setups are better than others. I suppose the cord could be turned through a small toroid a few times right at the back of the unit to solve any such problem.... :scratch2:

BTW on a brief side note I got my first VCR when my main TV was indeed a Zenith roundie. Some movies had a bend at the top of the screen, indicating a horizontal sync issue. I pulled the back off the set and disabled the sync circuitry long enough to fiddle with the horizontal and vertical controls to study how the horizontal and vertical blanking bars crossed each other. There was a skew or offset of the horizontal blanking bars at each vertical blanking bar. (Hmmmnnn... The horizontal phase detector must be having trouble recovering from the obvious change in phase...) I solved the problem by altering the value of some capacitors in the circuit so that it would act more quickly. The result was no more bend of vertical lines in the picture at the top. The penalty was more susceptibility to horizontal jittering with noisier pictures of fringe-strength stations. I was tempted to put in a switch so I could choose between one way and the other, but never got a round tuit.

Mixing old tech with new is bound to prove amusing from time to time.... :D

I noticed similiar intereference with my DVD player (running through a cheap adapter to hook up to old sets.) Most sets showed no problem; with my DuMont color set I needed to tighten the connection at the back of the set to eliminate it. On my Zenith roundie (25MC30) I could not eliminate it. I'll have to try a longer cable so I can move the unit around some..."The Lucy Show" DVD really looks great on a color roundie, but the intereference ruins the effect.