Bob
If the tantalum went immediately it was either in
backwards or there is a power supply that is
supplying a negative spike. I have popped
tantalums a number of times just because the
marking are so inconstant. Replace with another
tantalum and have a scope across the cap when you power it up.
Gran
At 10:31 PM 1/21/2008, you wrote:
At 1/21/2008 17:49, you wrote:
>Bob,
>
>I'd check C15, the 470 pF mica capacitor that's on the same power supply
>lead. It might be a good idea to replace both of the other caps that
>parallel C16.
I tried replacing C16 with a 4.7 µF 35 V tantalum, which immediately
exploded upon applying power. I checked the rest of the 4.7 µF caps in my
parts bin; all are bad. I remember buying these at Dayton a couple of
years ago, & at near-retail prices too. Although the 6.8 µF caps check OK
at DC, & think they may not be handling the high operating
temperature. All the caps in that area run fairly hot due to heat
conduction from the PC board.
I put a 1 µF 50 V non-polarized cap. in place of it & so far it's holding
up. I also checked the PA output on the spectrum analyzer, both directly &
looking into the flatpack duplexer & with & without the extra capacitor: no
spurs.
Bob NO6B
>73, Eric Lemmon WB6FLY
>
>
>-----Original Message-----
>From:
><mailto:Repeater-Builder%40yahoogroups.com><mai
lto:Repeater-Builder%40yahoogroups.com>Repeater-Builder@yahoogroups.com
>[mailto:[EMAIL PROTECTED] On Behalf Of
><mailto:no6b%40no6b.com><mailto:no6b%40no6b.com>[EMAIL PROTECTED]
>Sent: Sunday, January 20, 2008 10:02 PM
>To:
><mailto:Repeater-Builder%40yahoogroups.com><mai
lto:Repeater-Builder%40yahoogroups.com>Repeater-Builder@yahoogroups.com
>Subject: [Repeater-Builder] Problem with C16 on RFPA board in VHF HB MVP
>
>I've had a 6.8 µF 35 V capacitor burn up on the GE VHF HB MVP. This is a
>filter cap. for the final PA DC voltage, in parallel with 0.1 µF & 150 pF
>caps. I replaced it just before Dayton & noticed that the replacement got
>rather warm (too hot to touch), so I did a 2 hour burn-in. It survived
>just fine, so I concluded that the cap. was simply conducting heat up from
>the PA board. The radio worked fine at Dayton, & I've been doing some more
>testing at home in preparation for an installation in about 3 weeks.
>
>This evening after TXing for only ~5 minutes, smoke began to pour out the
>back of the radio. After carrying it to the garage to open it, I found C16
>charred once again. My first suspicion was (very) low frequency
>oscillation of the RFPA, but if this were the case I think I'd be
>experiencing desense, RF feedback & other oddities during testing. I'll
>check it on the spectrum analyzer tomorrow (after it's somewhat
>"deodorized"), but I doubt it's spurious.
>
>Any other ideas as to what would cause this cap. to repeatedly fail?
>
>Bob NO6B
>
>
