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Old 06-21-2018, 09:28 PM
cluelessgame cluelessgame is offline
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Possible Fix for Contrast Buzz?

I've been reading about contrast buzz lately. It plagues two of my sets, my Motorola 21BT130BE-1 and my Majestic 70, and I predict my Bendix C172U will suffer from it too once restored. Has anybody developed a fix for this? According to an older Antique Radio Forums post: (http://www.antiqueradios.com/forums/...ic.php?t=64442)
Modern televisions (relatively speaking) use SAW filters in their IF stages to eliminate this issue. I haven't really found any information of what exactly they do/how they work (or any schematics other than block diagrams). Would it be possible to build and/or add one to a vintage set?


the post also mentions in that this issue mainly stems from modern RF modulators not exactly conforming to the 12.5% minimum rule. would there be any way to possibly add some sort of regulator to the modulator output to fix it there?
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Old 06-21-2018, 09:57 PM
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Quote:
Originally Posted by cluelessgame View Post
I've been reading about contrast buzz lately. It plagues two of my sets, my Motorola 21BT130BE-1 and my Majestic 70, and I predict my Bendix C172U will suffer from it too once restored. Has anybody developed a fix for this? According to an older Antique Radio Forums post: (http://www.antiqueradios.com/forums/...ic.php?t=64442)
Modern televisions (relatively speaking) use SAW filters in their IF stages to eliminate this issue. I haven't really found any information of what exactly they do/how they work (or any schematics other than block diagrams). Would it be possible to build and/or add one to a vintage set?


the post also mentions in that this issue mainly stems from modern RF modulators not exactly conforming to the 12.5% minimum rule. would there be any way to possibly add some sort of regulator to the modulator output to fix it there?
Check/adjust AGC settings n your sets first (too high can cause or worsen contrast buzz) if that fails...

Just pop for a better modulator. Get a Blonder Tongue AM40, AM60(ideal for transmitting wirelessly to your sets), or a BAVMz family modulator, and adjust it properly....With a video signal containing high contrast text(most prone to causing buzz) turn up the video level pot on the modulator till it buzzes then back it down till just slightly past the point where it stops. Last I checked BT modulators could be had on ebay in the $20-40 range working...Much cheaper, easier and safer than trying to re-engineer 2+ TVs IF systems.
Granted some BT mods need the Power Supply lytics changed, but it is still much harder to fubar a lytic swap job than to fubar the IF alignment on a TV...Don't fubar your alignment...Fixing alignment will cost enough in specialized test equipment to make a bunch of BT modulators seem cheap.

I run all three BT modulator families I've mentioned and can adjust all of them to work excellent with no contrast buzz.
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Old 06-22-2018, 12:47 PM
cluelessgame cluelessgame is offline
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I have tried adjusting the AGC (labeled "noise gate" on my set) with no success.
Unfortunately the setup I have doesn't allow me to use a different modulator. I have comcast's basic package with the little (POS) "digital adapters" that only have an RF out and outside of plugging it into a demodulator to composite and then back into a modulator there's not much I can do on that end. I do, however, have tons of alignment equipment and even a spectrum analyzer if it comes to it so I'm happy to attempt to modify the TV's IF if a quasi-universal fix could be developed.... I'll certainly post my findings here if I do, but I still need more info on SAW filters before I can proceed
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Old 06-22-2018, 01:40 PM
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"Noise gate" adjusts where the sync circuit starts to ignore high amplitude RF interference, and you don't have any. You need to find the real AGC adjustment if there is one.

The IF would have to be modified to a whole different configuration -it's not just that it's a SWIF, but also may be that it has separate video and audio second detectors, each fed by a different IF response, or other complex fooling around.
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Old 06-22-2018, 02:26 PM
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You can also try adjusting the audio detector coil A.K.A. quad coil.
Flag your diddle stick to be SURE you can go back !
Adjust for less buzz. Center at the least buzz point. If it takes
more than a turn I would go through the audio with new det tube & a fine toothed comb.
The SIF transformer will only effect hiss.
BTW Motos often had an "optimizer" adjustment, try that first.

SAW filter is over the top IMHO. A fun project if you are into
such things would be trying an IF strip. Get a Zenith 150-190-01
IF module & adapt it ( must be the -01 version ! ). You will need 24 V
to run it & AGC voltage. Output will be composite video & SIF. A
crazy project but who knows ?

73 Zeno
LFOD !
BTW BUZZ SUCKS
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Old 06-22-2018, 03:55 PM
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Quote:
Originally Posted by cluelessgame View Post
Unfortunately the setup I have doesn't allow me to use a different modulator. I have comcast's basic package with the little (POS) "digital adapters" that only have an RF out and outside of plugging it into a demodulator to composite and then back into a modulator there's not much I can do on that end. I do, however, have tons of alignment equipment and even a spectrum analyzer if it comes to it so I'm happy to attempt to modify the TV's IF if a quasi-universal fix could be developed.... I'll certainly post my findings here if I do, but I still need more info on SAW filters before I can proceed
Same boat the spectrum boxes only have HDMI and RF no composite...I prefer wireless TV RF transmission since 10 TV sets are a pain to hardwire RF to and the ~70 I have would be a complete nightmare to hardwire. I simply tune channel 3 with VCRs and DVD-recorders(that have tuners) and feed the audio/video from those to the blonder tongues. Modern tuners found in VCRs, etc will remove the buzz in the composite A/V they output, but their internal RF modulators are not always good enough to not re-add it to their own RF out (which is why I favor the BT modulators).
You can pick up used working VCRs for $5 at thrift stores in my area and probably similar $ where ever you live. You don't need anything fancy tuner wise to make this work.
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Old 06-22-2018, 11:39 PM
cluelessgame cluelessgame is offline
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Quote:
Originally Posted by Electronic M View Post
Same boat the spectrum boxes only have HDMI and RF no composite...I prefer wireless TV RF transmission since 10 TV sets are a pain to hardwire RF to and the ~70 I have would be a complete nightmare to hardwire. I simply tune channel 3 with VCRs and DVD-recorders(that have tuners) and feed the audio/video from those to the blonder tongues. Modern tuners found in VCRs, etc will remove the buzz in the composite A/V they output, but their internal RF modulators are not always good enough to not re-add it to their own RF out (which is why I favor the BT modulators).
You can pick up used working VCRs for $5 at thrift stores in my area and probably similar $ where ever you live. You don't need anything fancy tuner wise to make this work.
Broadcasting the RF is actually what I'm doing! I fed the RF output through a channel master distribution amplifier and then to a set of rabbit ears and it broadcasts to all the sets in my apartment! I also found you can do the same thing by daisy-chaining 2 or 3 of those cheap antenna amplifiers they sell at Walmart together, but it splatters all over the spectrum and you can pick it up on multiple channels
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