This is also a good place for the 'finger test'. If there are suspect components you can get to that can't easily be removed (by pulling boards, etc.) then a calibrated finger will often lead you to the defective component. If it's really a cap, it should be generating a lot of heat. Sometimes your nose is a good indicator of what's getting hot too, but you really don't want to be sniffing the cap when the purple smoke and sparks decide to leak out!
Every Tech Order I ever used in the Air Force started with 'Perform a through visual inspection'. Often times, that was good advice. 73, Mike WM4B From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Eric Lemmon Sent: Saturday, May 22, 2010 10:48 AM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Lost 10 volts in a Master II UHF Repeater Whenever you have a shorted component, such as a tantalum capacitor, the current drawn by that short might burn out a PC board trace before burning up the capacitor. If that happens, finding the original problem becomes much more difficult. Perhaps a better and safer tack would be to use a small current-limited power supply to energize the 10 volt buss, but with the current limit set to 1/4 ampere or so. Following Jeff's suggestion, start unplugging and/or disconnecting loads until the buss voltage jumps up to 10 volts. I don't know what current is normally drawn from the 10 volt regulator in a properly-operating station, but that value should be measured and recorded for future troubleshooting efforts. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Jeff DePolo Sent: Saturday, May 22, 2010 6:36 AM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Lost 10 volts in a Master II UHF Repeater > I would look for a shorted tantalum capacitor hanging > somewhere on the > 10V rail. I agree. > If you hook 10V from an outside source to the 10V > buss, you'll > probably find it's drawing all kinds of current. The 10V regulator > circuit will go into fold back before burning up. This is by > design. I > usually hook a source of 10V at about 1.5A and look for smoke. It's > usually one of the tantalum capacitors that starts to smoke. > Once it's > done smoking, problem solved!! Put a DMM on the 10V line, then start disconnecting things until you narrow it down, divide and conquer. Pull all of the cards out of the cage (except the 10V reg card obviously), disconnect the exciter, remove the receiver, etc. With a good ohmmeter that measures fractions of an ohm, you should be able to narrow it down further once you've found the suspect module/board. > I have lost track of how many shorted tantalums I have had over the > years. When they occur in the B+ of the high current PA supply, they > simply burn up and th problem fixes itself. They make a cool purple smoke with lots of sparks when they flame out! --- Jeff WN3A
