OK, in anwser to my own question, I have discovered the
following:
I) Only the input matching circuit need be modified.
Output tank setting appear to be -- at least in the
sample I've seen -- intact for 10m.
II) Disassenbly procedure follows.
1. Unplug the power cord from the back of the unit. Do
NOT attempt this modifcation with AC power applied!
2. Remove the cover screws, then take off the top cover.
3. Remove the 1 top (center) and 2 bottom screws from
the front panel. Leave the remaining 6 screws in
place.
4. Remove the three front panel knobs using a hex
wrench -- 2 places each knob.
5. Remove the nut and washer arround the threaded shaft
of the bandswitch.
6. Carefully, using a thin shaft flat bladed screwdriver,
back off (1-2 turns) the brass screws on the brass
coupling holding the shafts of the Tune and Load
capacitors -- just INSIDE the front panel.
7. The front panel can now be pulled outward -- until
the Tune and Load cap shafts disengage their
couplings -- then let forward and down gently. NOTE:
The front meter panel connector may be unplugged from
the power supply board to allow the front panel
assembly to lie down closer to flat.
III) Modification procedure follows.
1) Carefully inspect the last clockwise setting (10m
position) on the bandswitch, both front and back
wafers. On some units, the fixed contact for the 10m
setting may have raised, so as to prevent it from
making contact. If so, carefully restore it to its
operating position. Use an inspection mirror to view
the back wafer switch, as its contacts -- on some
units -- face the rear shield wall of the swich
enclosure.
2) Add the single coil, below, across the front and back
contacts of the 10m position on the bandswitch. That
is, from the input -- front wafer -- of the band-
switched matching filter to its output -- rear wafer.
3) Add a 133pF capacitor, below, from the FRONT (input
side of filter) bandswitch wafer to ground. Use the
15m position's ground terminal, to the immediate left
of the 10m position.
4) Values:
Band Input C Coil Comments
--------------------------------------------------------
10m 133pF 6t WIDE spaced Pull apart to ~5/8" Long
Coil, above, is: #18 ga, 0.50" dia
Input C is 133pf:
100pF and 33pF in parallel, 500v dipped silver mica.
IV) Reconnect and close the amp by reversing the steps
above. In re-attaching the Tune and Load capacitor
shaft couplings, remember that the Tune cap is fully
meshed at 100 and the Load at 0 setting on their
respective dials.
Thanks to Alan Grey, W3BV, for
his help on this modification.
>
>Hello,
>
>I just purchased a B&W PT-2500A recently and am trying
>to get it to operate throughout 80-10m. Can anybody
>tell me what the PT-2500A 10m modification consists of
>or know where to get it? Am I correct in assuming that,
>unlike on the Kenwood TL-922A, the output tank
>bandswitch connections are already in place, and that
>only the input matching network requires restoration?
>
>Thanks,
>
>Fred, WB9LSC
>
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