Blonder Tongue on Roundy

[iong]

I have Zenith ATSC converter with video output feeding BT. AM-550 goes to 3 TV's with splitter but only the Zenith 25cm-- has slow 3" hum bar runs entire screen from bottom to top. but OK with my other BT. AM-550. So I would think PS filter caps in modulator. But why only on the Zenith? The rest are Solid State sets. Has anyone seen this?
Thank you,
Perry

[old_tv_nut]

Interesting. Maybe a difference in AGC or DC restoration in that set (not bad, just different?).

[zeno]

The 2 SS sets usually will have 75 ohm coax direct to the tuners.
The Zenith has 300 ohm twin lead then a balun to bring it to 75 ohms
at the tuner. So you can have a few feet of twin lead to pick up
trash. A few other ideas... One of the other sets has AC leakage,
the matching transformer is bad. The RF amp, 6HA5 has H-K leakage,
if you try one get the short version.
Try the set with a generator & everything else removed.

73 Zeno
LFOD !

[kvflyer]

I have several Blonder Tongue modulators and I did have to replace the filter capacitors in a couple of them. They are easy to change and not an odd value.

[dtvmcdonald]

It ca modularion be signal from ground loop 60hz modeulation. It can be RFoverload. Cure is to make up T attentuatrors from plain carbon resistors, one for each set. 15-25 dB. Roughl 300-n outputy 75 Ohms on signal side, 300o output assuming twin lead.

Don't ground coax to TV That's critical

[dtvmcdonald]

It ca modularion be signal from ground loop 60hz modeulation. It can be RFoverload. Cure is to make up T attentuatrors from plain carbon resistors, one for each set. 15-25 dB. Roughl 300-n outputy 75 Ohms on signal side, 300o output assuming twin lead.

Don't ground coax to TV That's critical

[Electronic M]

Some of the solid state sets are more tolerant of signal hum. That said you should be able to make the hum almost unnoticeable either through recapping the Blonder Tongue, the TV or by breaking ground loops. My main BT rack I run on an 120V isolation transformer with the ground prong disconnected... It's fed by a rack containing about every consumer video tape and disc format to ever be popular as well as some digital sources....One of those digital sources (a PC) had a grounded power cord that I had to also break the ground connection from to stop hum...That PC eventually got upgraded but I kept the cord that way for its replacement.

One thing you have that I don't is a wired RF connection. All my modulators feed antennas and all my TVs have antennas. The spaghetti mess I'd get it I tried to connect 100 sets across 3 floors and more rooms would be insufferable....One thing in wireless mode is sometimes if there's a large metal object (metal cabinet roundy, refrigerator, etc) for the signal to reflect off of the distance of either antenna relative to it can cause hum, signal cancellation and other oddities.... It's possible to achieve the same effect with a wired system as well...If the coax is not the exact impedance the system is designed for such reflections become possible, and their severity varies based on cable length as a function of RF signal wavelength...More speciffically fractional wavelength....The effect of 3/4 wavelength is different than half wavelength, but 0.5, 1.5 and 2.5 wavelength coax lengths should behave roughly the same. The advantage wireless has over wired is it's easier to find and adjust out reflections than coax....The disadvantage is that when things and people are moving around the house your optimal adjustment changes (sometimes just from someone walking or where in the room you sit).
RF is a strange and interesting world...