From w0rw1 at msn.com Mon Jul 4 07:53:47 2016 From: w0rw1 at msn.com (Paul Signorelli) Date: Mon, 4 Jul 2016 05:53:47 -0600 Subject: [Motorola] Flaming FHTRU Message-ID: I had my old FHTRU out for a battery check and power check the other day, July 2016.I was adjusting the squelch control, (some FHTRU's have a squelch control in the battery compartment),When all of a sudden Smoke & Fire came billowing out.When was the last time you had Smoke and Fire coming out of your HT?I found burned marks on the 1 Meg Pot and took it out.It was an open version (not sealed) and I found carbon crumbshad migrated out of the Pot and shorted to the case ground.The carbon stator had deposited some conductive material at the edge and the 50V battery gladly ate it up.The Pot was still OK but I replaced it with a sealed AB Type J.It was bigger so I had to trim the bracket for it to fit.This control never gets used so the carbon crumbs were probably not scrapped up by the wiper.Just one of those old reliability failures for the book.Paul w0rw From w0rw1 at msn.com Tue Jul 5 16:50:47 2016 From: w0rw1 at msn.com (Paul Signorelli) Date: Tue, 5 Jul 2016 14:50:47 -0600 Subject: [Motorola] Flaming FHTRU In-Reply-To: References: Message-ID: Update on the failure analysis of Flaming FHTRU1AN8 s/n 20205 Handie Talkie. The carbon track inside the pot had deep groves cut into it on the CCW end. That is where the arc occurred. This battery case might not be the original case. No way for me to tell how or when these groves in the carbon resistive element occurred. They could be a result of me turning the pot when I was trouble shooting. A lot of carbon got dug out of the track and that probably feed the avalanching arc. The spacing between the B+ terminal and the case ground of the pot was only about 0.005 inches. Paul w0rw From w0rw1 at msn.com Thu Jul 7 22:25:55 2016 From: w0rw1 at msn.com (Paul Signorelli) Date: Thu, 7 Jul 2016 20:25:55 -0600 Subject: [Motorola] Flaming FHTRU, Final Report Message-ID: I have a hunch that this battery case I have with the Squelch pot is not from the original FHTRU. I suspect that when I turned the pot CCW I just ran +65V down to ground and the pot exploded. Probably just a wiring difference between models. Not motivated to take this investigation any further. The FHTRU works great. Paul w0rw