[BC] TFT 734/724 FM mod monitors

[RichardBJohnson at comcast.net]

You need to hook up a scope to see what is happening. The most likely cause is that the antenna feeding the RF amplifier is too near the monitor, allowing leakage from the monitor to feed back into the amplifier, causing oscillation on audio peaks. Another cause could be that your receiving antenna is in a carrier null of your transmitter so that the sidebands received are too high for the carrier received. Carrier nulls can be caused by nearby radiating objects. At WARE, one of the first AM stations allowed to have remote control in the nighttime, Chris Payne designed a Rhombic Antenna for the studio roof-top, that had a natural null in the direction of the studio below, and a directional lobe towards the transmitter site. This resolved a problem of feedback. In these "olden" days, the General Radio modulation monitors required about 5 watts! This meant LOTS of gain in the (homemade) RF amplifier, and lots of output capability to remain linear. I designed that RF Amplifier. It used a 2E26 running class-A, fed by a whole bunch of 12BY7 class-A pentodes, each producing about 10 dB gain. It was all required to get the 5 mV/m signal up to 5 watts in 50 ohms for the monitor! 

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Ronald J. Dot'o Sr." <ron.doto at comcast.net>

I have a TFT AM monitor that is fed with a Balar RF amp and displays false 
highs (>125%) when fed by the RF amp.  When the mod monitor is hooked 
directly to the RF sample port on the TX it reads accuratly.

Ron D