I recently purchased and set up a Blonder Tongue AM40 agile modulator connected to a DTV coverter so I can use my Sony Watchman and other small televisions. I have the modulator broadcasting on channel five. I leave this set up on continuously so I can use my handheld tv anytime without having to do anything but turn on my Watchman. The modulator runs very hot which I already knew they do. My concern is that this is a very hot unit, especially on the right side of it where I expect several high power components are located. I put some feet on the bottom of it to allow air flow underneath. The surrounding area where I have it located is not getting hot but I wonder how safe is it to leave this running all the time? Do any of you leave these running for a permanent analog home tv channel?

Well when used at a TV station or something I'm pretty sure they were run constantly.

I have mine switched though since when my TV is turned off it turns off also.

I would think it's okay since they were originally used by cable companies and the like 24/7, they had them rack mounted and there may have been a cooling fan or at least A/C to keep them cool.Worst that could happen is they burn up a component and pop the fuse.

you might want to recap the power supply inside the unit to increase reliability, use 105c Nichicon or Panasonic Caps for long life.

What you are feeding it with is on 24/7 also? Cable box maybe, it's at least as safe as that.

I don't leave mine on but that because electricity is expensive in California, also I use it with a DVD player so it wouldn't make sense to leave it running 24/7

Mine has run 24/7 for years. It sits alone on a wire shelf, so heat is not a factor (I just checked, and it's barely warm). After a couple of years, I replaced electrolytics in the power supply when it developed horizontal hum bars, but I figured that was just part of the game.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

I have it connected to a dtv converter box tuned to Antenna TV for the old programming. I also put my tube type RCA 8 inch portable on rabbit ears tuned to channel five. It really is neat to see Fifties tv shows on a Fifties set receiving over the air on a VHF channel. I wish I had done this right after the digital switchover.

Just provide reasonable cooling. Host CATV headends are kept in the low-mid 60F range. Otherwise the oscillators start drifting...

Unless you have a matched antenna for the channel you are transmitting on much of the power is being reflected back into the modulator. (bad SWR) This will cause the output package in the BT mod to fail in short order.

This is transmitter 101

My experience is the DTV converters dont last if you leave them on 24/7. Take note on how hot the microprocessor runs in the converter. Running it with the top off will help but a small fan would be better. Still, dont expect it to last very long, They were designed to pacify the public not to last.

Heres a video I made some time ago on how to make a basic antenna. Having the antenna analyzer or a spectrum analyzer to sweep the antenna is a must.
http://youtu.be/z4dhRt7tmd4

I made an antenna following the info in that video and it's definitely an improvement over the old rabbit ears.

Phil Nelson

Could you get the same results by adjusting the length of a rabbit ears antenna?

The modulator wants to see a 75Ω load, not some random match through a crappy matching transformer. This is why you need several feet of RG6 or 59.

All the radio guys know antenna is everything. Connect some random antenna to just about any transmitter over a few watts and the final transistors will instantly short.

These modulators are fairly robust, but the hybrid package they use for a final is not feasibly replaced. If it fails best to buy a whole new mod

It looks like I'll be constructing an antenna today. I don't have an antenna analyzer so I'll go with the 42" recommendation and make my leads exactly the same length. I can use whatever length of RG6 so what would be recommended? Is there anything else that I can do to aid in success without the need for equipment that I don't have?