Jim, Good question! My new General Dynamics (Motorola) R2600D service monitor addressed that problem by equipping both the antenna input and duplex jacks with 1/16 ampere fast-blow fuses that are easily replaced using a 7/16" nut driver to unscrew the jack. Despite my lengthy experience with test equipment, and due diligence (whatever that is!), I did manage to key up a mobile radio while it was connected to the antenna jack. I was monitoring the RX LO signal and simply forgot where the power would go when I keyed the mike. Thankfully, the fuse blew instantly and protected the antenna circuit from damage. It only took a minute to replace that fuse (the manufacturer provided five spares- I wonder why?), and I was back in business. No doubt, that same fuse can be wired into older monitors without too much effort.
73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of James Sent: Tuesday, January 05, 2010 2:30 PM To: [email protected] Subject: [Repeater-Builder] Service Monitor Input Protection Hello to the group. Knowing most of you have service monitors for the Repeater Builder Hobby, I wanted to pose the following question. What method do you use to protect the input of your service monitor from excessive RF Power input? I am referring to the input/output jack that is limited to 2.5 watts on MANY service monitors. I know I can use an attenuator, but that gets removed for sensitivity checks and may not get put back on. One suggestion is a RF Relay that would trip on say one watt and put the power to a dummy load. The problem is I have used one of the cheap circuits in the past and toasted transistors in a Pre-amp, before the relay reacted as the book said it should. However I know there are circuits in say the Mirage Amp that also has a Pre-Amp that is removed from the antenna during transmit, that works well. What are you doing to protect your monitor? Appreciate your thoughts. 73 JIM KA2AJH
