[ARC5] Burned out R-78 on BC-457-A

[Dennis Monticelli]

Probably the cap, but there's another possibility.

If the excitation to the PA fails while the cathode circuit is completed,
then the 1625's will pull excessive plate and screen current due to an
absence of grid bias.  It will do in the tubes and could take out screen
resistors too.  You noted only one such resistor burned so it was probably
the bypass cap on that tube.

Oh yeah, the triple 0.05's.  They're never in good enough shape to take
500V.

Dennis AE6C

On Tue, May 26, 2020 at 11:37 AM J Mcvey via ARC5 <arc5 at mailman.qth.net>
wrote:

>
>
> Not too many choices on that one!
> I believe the bypass is a mica type. I don't remember changing one, but it
> might be bad.
> The other choice is some kind of tube meltdown that I have never seen
> either.
> The good news is that it's not hard to fix!
>
> Do change/ re-stuff the .05 X 3 cap in the rear of the transmitter as a
> "given".
>
> On Tuesday, May 26, 2020, 02:17:31 PM EDT, Bill Cromwell <
> wrcromwell at gmail.com> wrote:
>
>
> Hi,
>
> I don't have any of the command transmitters of any flavor. I have had
> the screen bypass get leaky enough to burn the screen resistor. Check
> C-64 (screen bypass?) at operating voltage for leakage or a short. It
> might be possible for the 1625 tube itself to have an internal short at
> operating temperatures. We don't know what sort of abuse it took during
> handling and shipping.
>
> Good luck with your transmitter.
>
> 73,
>
> Bill  KU8H
>
> On 5/26/20 1:43 PM, Dave Merrill wrote:
> > Since many of us don't have much to do these days, would anyone care to
> > speculate on why R-78 in the 1625 screen supply is charred?  Seller has
> > no clue, calls it a 'diode.'  Can't tell for sure from the photos, but
> > most of it looks original.  Someone has labeled the rear power connector
> > so that might be an indication of tampering.
> >
> > Possibly shorted C-64?  Anybody ever find a bad one?
> >
> > https://www.ebay.com/itm/164218971890
> >
> > 73, Dave N9ZC
> >
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>
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