[App_rpt-users] Bizarre Issue

[Steve Agee]

Having worked with app_rpt for the past five years, I've run across most 
of the common issues that everyone else has. But every once in a while, 
I run across one that is so bizarre, I feel compelled to post it, so 
others might know about it. Such was the case yesterday.

I have a site with a cabinet with three repeaters in it. The two meter 
repeater was installed last as an app_rpt repeater from the beginning, 
and has never had any issues. The other two are 70 centimeter GE EXEC II 
mobiles, full duplexed, connected to an RLC-4 controller, with the other 
two ports being wire line interfaces. Since the wire line has been 
having intermittent issues lately, I wanted to move the two GE radios to 
URIs. I built two conversion cables, with a female DB-9 on one end (to 
accept DB-9 male from radio to RLC controller), and a male DB-25 on the 
other (to connect to the URI). This leaves the flexibility of going back 
to the RLC-4, without rewiring everything. So, I get to the site and 
plug everything in and I'm ready to set levels. But wait, whats this 
gawd awful loud crackling noise on the transmit audio of both GE radios? 
Is it present on the discriminator line of the receiver? No. Is it there 
when the DB-9 from the radio is unplugged from the DB-9 coming from the 
URI? No. Now things are getting narrowed down. How about swapping URIs 
since there are three in the cabinet? No difference. What about swapping 
USB cables? No change. But wait, plug the two meter repeater in to one 
of the two new URIs, and it now has crackle. What!? It turns out the USB 
hub on the computer motherboard is oscillating! This box had 3 hubs. 
Moving both of the GE radios to a new hub solved the issue. I have never 
run across this issue before, on the twenty-something number of boxes 
that I have either built myself, or helped others build. I may never run 
across it again either, but if I do, now I'll know where to look. By the 
way, that noisy hub ... it works fine. I plugged the keyboard in to it 
and used it without any issue whatsoever.

Many thanks to KE5PTZ for tirelessly tracking this issue down, and 
putting up with me pulling my hair out for hours on end while doing so!

N5ZUA