April 23, 1987
	
		    To All TVS501A Users
	
	Dear Sir:
		A problem has recently been discovered with the 
	later versions of the TVS 501A repeater controller. Units
	in which device U1 is a 74HC04 may experience problems 
	with the crystal oscillator or the countdown timer. These 
	problems result in:
		-Failure to decode DTMF tones reliably
		-Intermittent unexplained processor resets
		-Total failure of the unit

		Because of these problems, we are enclosing a 4069 
	IC which should be installed to replace device U1. Even if 
	you are not currently experiencing these problems, you 
	should make this correction, because sometimes the problem 
	only shows up with aging or ambient temperature changes. If 
	you have an old TVS501A in which a 4069 is already 
	installed, you just got a free spare!
	




				John Moore NJ7E
				Tiny Vital Systemse
				7525 Clearwater Parkway
				Paradise Valley, AZ 85253


				john@tinyvital.com