Thanks Nate, I have read most all of the documents on RB and learned quite a bit about repeaters and the GEs.
73 John --- In Repeater-Builder@yahoogroups.com, Nate Duehr <n...@...> wrote: > > > On Oct 6, 2009, at 10:12 AM, Douglas wrote: > > > > > Let me tell you a little story about this kind of problem. I had a > > similar situation on a VHF mastr II repeater where a signal sounded > > great one time and noisy the next. The problem turned out to be a > > hairline crack in a solder pad that was used to bridge two circuit > > boards and the output connectors to a circuit board, within the PA > > assembly. It would run great power and then low power from one > > transmission to the next. The output power would drop noticably as > > seen on a relative signal meter on my rig. That was how I localized > > it. > > > > When we would put a meter on the output, it had just enough sstress > > on the cable to make a good connection and all was well, hookk it > > back to the duplexer, and the intermittant problem returned. > > > > I suspect that same kind of thing could happen on a receiver > > connection. > > > > The only real way to fix is to touch all the solder terminals that > > are used for primary signal in the input or output route. > > > > Good luck. I liked to have lost my brain over that one (what brain > > is left from the 60s). > > > > Doug > > > > KC0SDQ > > MASTR II PA's can also make funny noises (really ugly stuff on a > spectrum analyzer) if they're experiencing this type of crack at the > PA output going into the harmonic filter board... you literally can > HEAR it happening on the repeater's output. Often the arcing > happening over that joint "modulates" the FM signal in the form of a > low-level squeal in the background of the repeater's signal, and/or a > "frying" sound. It's not pleasant, but also not always so bad you can > hear it. > > http://www.repeater-builder.com/ge/pix/mvc-474f.jpg > > From: > > http://www.repeater-builder.com/ge/mastrIIparepair.html > > Look carefully. Sometimes it's hard to spot. There's a little strip > of metal under that solder blob, that bridges that gap, which is > almost under that capacitor. Re-heating this area, you'll often find > the problem, but be careful not to cook the cap. We've done the > "bump" fix, and we've also done the "just clean up the pads and re-use > the factory metal strip" fix. Can't say that either way is "better" > when the screws surrounding the joint are torqued to factory > specifications. If either side is "loose", or "too tight" the boards > will flex during heating and cooling and break the connection again. > Had one that was so burnt the board itself and the "charcoal" became > conductive and it would visibly arc over every time it was keyed. > That one had to have both the final board and the harmonic filter > board replaced, and was stinky. Usually they're sneakier than that, > though. > > I was fighting with VHF PA's a couple of years ago, and when I finally > enlisted the help of someone FAR more patient than I to completely > rebuild one, including pulling ALL of the boards off the PA heatsink, > scraping all of the old/dead heatsink compound off of everything and > re-applying new, re-torquing all the screws to FACTORY specs (it's in > the LBI, and many late model repeater PA's, not the mobiles usually, > have stickers with the EXACT torque specs of all screw types on the PA > board), resoldering every joint with good quality solder... etc. A > real "re-work" job. > > This was done slowly and carefully by Scott W0KU... then following the > "rule" we've given ourselves that ALL MASTR II PAs get an isolator... > > His rebuilt PA has gone through antenna hell (long story, but let's > just say it wasn't radiating, feeding into the isolator at 85W > continuous for hours and hours) plus has also worked great on-the-air > for three years now. > > I now really believe that these 30 year old PA's just need a little > bench TLC before they go up to a site, and I'm MORE than willing to go > work them over thoroughly, and buy beers for Scott to enjoy in his > backyard shed after the soldering and tricky stuff is done with. > > I rebuilt two, they died. Scott rebuilt one, after I explained what I > thought I'd rushed through, or done badly, and how frustrating it was > to see two of them die... we made a plan to meet up and work slowly > and carefully and do a complete tear-down and rebuild on one utilizing > whatever boards/parts we had on-hand. That rebuilt PA has kept > working now for a long time. > > I definitely credit the removal and replacement of the completely > dried out, dead heatsink compound as a component of much of this > "success." After seeing the flaky, useless "stuff" that came off the > heatsink (the date stamp read 1981 on this particular PA), I highly > recommend the full tear-down and rebuild of MASTR II PA's before > giving them a week long "stress test" at 110W into a dummy load in the > basement for a week... before they ever go anywhere near a repeater > site. > > Also I mention this, but don't think from your symptoms it's what's > happening, but it's good info for anyone taking over a MASTR II... > since what you're fighting is on and off receiver sensitivity loss, > you MIGHT be suffering from the "dendrites" or "tin whiskers" > problem... but I doubt it... > > http://www.repeater-builder.com/ge/mastr2/m2casting.html > > Lots of great info on RB about the MASTR II and known problems. > They're simple bullet-proof radios, if cared for and set up properly. > Literally. One of the repeaters I had donated to us by a local group/ > person, had a bullet hole in the cabinet and two dents where the > bullet hadn't punctured the steel MASTR "short" cabinet. The bullet > nicked a duplexer cable on the way through the cabinet. > > Definitely dig through all the great MASTR II info on RB, because > everything here that folks have taken time to write articles on -- has > "bitten" more than one "someone" on the list MULTIPLE times. > > -- > Nate Duehr, WY0X > n...@... > > http://facebook.com/denverpilot > http://twitter.com/denverpilot >
