Apache TX-1 "RF Hiccup" Puzzle

[Garey Barrell]

Lin -

FBTO comes up with about 4-5 QST references in the 59-60 years,
but none refer to this problem.  Things like drilling small holes
in the cabinet to allow some adjustments without removing TX from
the cabinet, adding a spotting switch, etc.

Just a guess.  I don't have a schematic of the TX-1, but is it
possible that the bias supply, (cutting off the low level stages
on "key-up",) filter cap is bleeding down before the plate supply
filter?  Possibly a "dried out" high ESR filter cap in the bias
supply....  The result would be "keying" the transmitter, which
would quickly dump whatever remaining charge there was in the HV
supply.  Or, worse yet, possibly an open bleeder on the HV supply
causing it to discharge too slowly?

Easy to check, (or at least as ANY task involving opening up the
Apache is easy, see drilling extra holes above! :-),) just
parallel a large electrolytic across the bias supply.  Or check
bleeder resistors on HV.

Let us know what you find.

73, Garey - K4OAH
Atlanta


>>On Wed, 23 Jun 2004 20:54:51 -0700 digital-conjurers at ADELPHIA.NET writes:
>>
>>Have an Apache TX-1 running AM only with an unusual problem:
>>
>>About 250 milliseconds after unkeying, there is a small "RF Burp" or
>>small RF pulse of very short duration (100 ms or less), and small
>>energy, that comes out of this transmitter.  This is especially
>>puzzling, given:
>>
>>1.  The finals seem perfectly neutralized
>>2.  Voltages, tubes, everything checks out as nominal
>>3.  Coax routing seems very orderly
>>4.  The complaint _slowly disappears_  as you tune the final tank
>>circuit slightly off the resonance dip; i.e. if you're dipped at
>>200ma, and slowly swing up to, say, 240ma, the phenomenon goes
>>away.
>>5. SWR is low; even does it on the dummy load.
>>
>>[BTW we're using 6293 industrials instead of convetional 6146's.
>>(The primary difference between these and the "normal" 6146 is that
>>the plate is much "heavier" in its construction. )]
>>
>>So, the sequence is:
>>
>>1. Unkey, the Dowkey releases, all power meters swing to zero, 250ms
>>later there's that >100ms RF "hiccup", then all is fine.
>>
>>Wierd.
>>
>>Ideas?
>>
>>Thanks to the group in advance,
>>
>>Lin/KJ6EF

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