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  #1  
Old 08-03-2017, 08:25 PM
CPQ5360 CPQ5360 is offline
Trent
 
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Question about Blonder Tongue modulators

I'm thinking of getting a Blonder Tongue modulator for use as a TV transmitter to use with my portable TVs. Looking on eBay, it seems that in addition to the Agile Modulator that people on here talk about, there are other Blonder Tongue devices that look similar, and are selling for cheaper prices.

I see some called "Audio Video Modulator", with model designations such as MAVM-s, SAVM-b, and MAVM-864. I also see ones called "Channel Processor". Would these serve a similar purpose of broadcasting a low-power TV signal? If so, how are they different from the Agile Modulator models?
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  #2  
Old 08-03-2017, 11:36 PM
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If you decide to get one, please make sure to get the AM-60-550. I cheaped out and got the AM-40-550, and it is completely useless. It used to work sort of OK, but it totally crapped out a few months ago. When it did work, it was extremely weak.
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Old 08-03-2017, 11:43 PM
CPQ5360 CPQ5360 is offline
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Hi Dave. What's the functional difference between the AM-40-550 and the AM-60-550?
It sounds like your unit was just on its way out in general. I've read that there are a few electrolytic capacitors that sometimes go bad - have you tackled those?
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Old 08-04-2017, 12:18 AM
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I'm not totally sure, but I've heard the 60 does have more transmitting power. Shango066 did a very good video on making a tuned antenna for one though. I tried it, and my unit still wasn't powerful enough. And yeah, the electrolytics are probably bad. I left it on 24/7 for about a year. I just have coax cables all over the place now.
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Old 08-04-2017, 10:42 AM
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dtvmcdonald dtvmcdonald is offline
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The Blonder Tongue BAVM series of single channel modulators work great. The ones
with the SAW suffix have a slightly better frequency response but the difference is
impossible to tell on old TVs. They are very cheap now, but this can't last forever
as the cable companies will eventually sell them all off.

The agile ones are of course nicer but you likely need only one or at
most two of those. They are much easier to find with stereo sound.
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  #6  
Old 08-07-2017, 10:27 PM
CPQ5360 CPQ5360 is offline
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Thanks for the info. Any idea how the MAVM and SAVM units differ?
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Old 08-07-2017, 10:47 PM
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Electronic M Electronic M is offline
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Both the AM-40 and AM-60 series as well as the BAVM-Z series can transmit a decent distance. They all seem to work best around channel 7 and worst around channel 2. You need a dipole cut to the midband wavelength of the channel your using (shango66 has a good youtube video on that) for best TX distance.

With such an antenna and using channel 7 I get a full power 1 block radius with the 60 and ~200' radius with the 40 the BAVM-Z falling in the middle....Which makes sense if you look at the output power (dBm) specs of the three.

If you live in europe or a non-NTSC region don't import NTSC units from the states. Our channel assignments, modulation schemes and audio carrier offsets will not necessarily match those of other broadcast standards in a usable manner. I don't know if BT made modulators for foreign standards, but if you live outside the USA make sure the broadcast standard of your box matches your TVs.

Dave, it sounds like your box(s) were defective. These units are hard on lytics...I recapped a BAVM-Z with good testing used caps, and ~1-2 years later those caps are going bad too. Some of these units occasionally blow power regulator chips and or diodes. It is wise to google the regulator part numbers during repair. ID the regulator output voltages and pins and confirm voltages are on spec....Even if the lytics are good if B+ is low the units will not perform properly.
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Old 08-07-2017, 11:01 PM
CPQ5360 CPQ5360 is offline
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This reminds me of another question I forgot to ask - what do the channel letter labels on a BT modulator mean? For example, if I'm looking at one that has a "Channel J" sticker on it, what does that mean? What is channel J? Is that an ordinary TV channel?
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Old 08-08-2017, 08:57 AM
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IIRC the letters are cable channels...Most are probably not useful for vacuum tube era TVs.
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Old 08-10-2017, 12:45 PM
kf4rca kf4rca is offline
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Channels A thru I are 14-22. Some of the old CATV converters still have letters on them.
I have 2 B-T MAVM modulators (channels 9 & 13). They are old fixed tuned crystal controlled units. They work very well. They use the Motorola MC1374 chip. You should be able to buy one on Ebay for less than $10. They occupy 1 RU.
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Old 08-10-2017, 01:47 PM
CPQ5360 CPQ5360 is offline
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Thanks for the info. I see MAVM modulators for quite cheap, so I may grab one.
Thanks for the info on the channels. I'm still a little confused - I found charts online listing the different cable channels and their frequencies, and some list J as 525.25 MHz, the same frequency as UHF 23, and others list it as 217.25, which is just above VHF 13. O_o I just don't want to accidentally get a modulator that can't be picked up on a standard VHF/UHF TV set.
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Old 08-11-2017, 10:38 AM
kf4rca kf4rca is offline
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A CATV processor is quite different. They do not take AV in. Their input is a VHF (or UHF) channel. They convert it down to an IF (typically 41 MHz.), then heterodyne upconvert it to the output channel. No demodulation to audio/video occurs.
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