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From: David Brodbeck on 10 May 2010 13:41
On May 8, 9:34 pm, "Phil Allison" <phi...(a)tpg.com.au> wrote:
> If the failed electro really was the cause of Q17 failing, then the unit
> should work after replacing it and Q17.
>
> However, if Q17 still overheats then suspect the SC1019 module is blown.
>
> ...  Phil

Thanks, Phil. That's also the conclusion I've come to, so it's nice
to get a bit of a sanity check that I'm not overlooking something
obvious. :)

Right now I'm working under the theory that the SC1019 module is OK,
because I otherwise can't explain why C54 would be burned to a crisp.
A bad PA module could cause excessive current through Q17 but it
shouldn't have affected that capacitor.

I'll let you know what happens when I put it back together.
From: Phil Allison on 10 May 2010 21:59

"David Brodbeck"


My current working theory is that C54 developed an internal short
(maybe due to age) and cause excessive current in Q17. Seems like
it's pretty much got to be that or a bad PA module, and a bad PA
wouldn't explain why C54 burned. I've never seen a capacitor so
thoroughly immolated. It left a big smoke spot on the inside of the
case, and it just fell apart when I poked at it with a screwdriver.

** Is C54 actually a * tantalum* bead cap ????

If so, no special explanation is needed for its fiery demise.




.... Phil


From: Meat Plow on 10 May 2010 22:33
On Mon, 10 May 2010 10:41:35 -0700, David Brodbeck wrote:

> On May 8, 9:34 pm, "Phil Allison" <phi...(a)tpg.com.au> wrote:
>> If the failed electro really was the cause of Q17 failing, then the
>> unit should work after replacing it and Q17.
>>
>> However, if Q17 still overheats then suspect the SC1019 module is
>> blown.
>>
>> ...  Phil
>
> Thanks, Phil. That's also the conclusion I've come to, so it's nice to
> get a bit of a sanity check that I'm not overlooking something obvious.
> :)
>
> Right now I'm working under the theory that the SC1019 module is OK,
> because I otherwise can't explain why C54 would be burned to a crisp. A
> bad PA module could cause excessive current through Q17 but it shouldn't
> have affected that capacitor.
>
> I'll let you know what happens when I put it back together.

If you have some way to limit current, use it. I have a supply that I can
dial in from 0 to 35 amps and it comes in real handy
From: David Brodbeck on 11 May 2010 14:57
On May 10, 6:59 pm, "Phil Allison" <phi...(a)tpg.com.au> wrote:
> **  Is C54 actually a * tantalum*  bead cap  ????
>
>  If so, no special explanation is needed for its fiery demise.

Could be. It's 10 uF, polarized, which could be either. It was too
badly charred to tell what it used to be, but I didn't find the
remains of an electrolytic can rattling around in the case, so a
tantalum does seem likely.

From: David Brodbeck on 11 May 2010 15:05
On May 10, 7:33 pm, Meat Plow <mhyw...(a)yahoo.com> wrote:
> If you have some way to limit current, use it. I have a supply that I can
> dial in from 0 to 35 amps and it comes in real handy

Will do. I don't have a variable-limit supply, but I do have one with
a fixed 1.5A limit that I plan to use for initial low power transmit
testing.
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