[TenTec] OMNI 6+ and INRAD CW roofing filter

[Pete Hoover]

Chuck & Tommy & others:

    The following is applicable ONLY for the OMNI 6+ (Model 564) & NOT the 
6 (Model 563 - in any version - Option 3, etc.)! And further, only for use 
with the INRAD CW roofing filter(s), NOT for the SSB roofing filter(s).

    With the following mod, the roofing filter can be switched in or out 
using only the "LP" front panel push-button and leaves all other functions 
in standard configurations - i.e. no need to sacrifice usage of N1, N2, 
Processor, and/or Aux Ant. Interested? Read on.

    A bit of explanation: The OMNI 6+ has a DSP audio low-pass filter, with 
cut off frequency setable from a menu (I have mine set at 800 Hz), and is 
"..intended to limit the post-detection bandwidth of the audio path in CW 
mode" to quote the manual. When LP is actuated, it lights a front panel 
LED( NB: on the OMNI 6, actuating the LP switch results in a momentary 
legend that toggles between "LP ON" and LP OFF" - there is no front panel 
LED).

    Since the "LP" is used only in CW mode, I thought that a good approach 
to activate the INRAD roofing filter would be if I could figure out how to 
piggy-back (technical term - sorry) on the voltage that lit the LP 
LED.  The manual was no help as there was no indication of what part of the 
circuitry was activated by the LP push-button. The brute force method would 
have been to take off the front panel, locate the LED, and attach a wire to 
the appropriate point, and route that to the roofing filter board. From 
past experience (don't ask) I know that removal of the front panel removal 
is a real pain in the * and was deemed the last resort.

    A couple of calls to the most helpful Tech Service folk at Ten Tec, Don 
(KF4HHO) in particular, provided the answer: the manual was/is incorrect! 
The voltage that drives the LP LED is readily accessible without removing 
the front panel! It is available on P96 on the Logic Board (see p. 4-19 - 
Logic Board Component layout for location). Only the removal of the top 
cover is needed to access the Logic Board.

    Ready? See page 4-33 - Keypad/encoder board schematic) of the ten Tec 
Manual:  P96 as shown on this page - lower-right hand side - is valid only 
for the OMNI 6 (Model 563). On the schematic, it shows, starting at the top 
of P96, "AV", "DLY", "GN", "MS" and "GND". However, the gospel according to 
Sevierville, the correct P96 for the OMNI 6+ (Model 564) should be: 
starting at the top of P96 "NR", "LP", "MT", "MS", and "GND". In other 
words, AV = NR, DLY = LP, GN = MT, MS = MS and GND = GND. On this same page 
(4-33) it should be pointed out that P95 for the OMNI 6+ is also different 
from P95 from the 6 - alas. If this is interest to you, ping me direct as I 
get the digest.

    Cut to the chase: on the Logic Board, P96 is a five wire plug: GND is 
the wire #1 and closest to the front of the rig; LP is wire #4. With LP 
activated, there is 1.97 VDC on wire#4 of P96 - to make sure you have the 
correct wire, measure the voltage - it is easily reached on the top of P96.

    From either terminal of J3 on the roofing filter run a wire to the 
Logic Board area and splice it into wire #4 on P96. How you accomplish this 
is up to you - I fabbed a quick disconnect using IC header parts. With no 
other mods, when LP is activated there is about .6 VDC at the junction of 
R7 & R8 on the roofing filter board. This is not enough to drive the relay 
- more voltage is needed. Shorting out R7 provides plenty of voltage to the 
relay but drops the circuit voltage so low that the LP LED is not lit - 
bummer. However, putting a 1K, 1/4 W resistor across R7 (this can be done 
on filter board as installed in the rig) -  will provide about 0.72 VDC 
which is most adequate to drive the roofing board relay.

    With this approach, there is no connection to J4 of the roofing filter 
board, S1 1&2 are off, and S1 3&4 are on (depends upon your connection to 
J3). Power connections and J1 & J2 on the roofing board are as per the 
INRAD instructions. Any messing with J4, P16, etc., should be reversed 
and/or returned to standard.

    That's it! Roofing filter turned on & off via the LP push button. 
Success! Happy Dance! Too bad the bands are dead, huh?

    Any questions, etc., ping me direct.

Pete, W6ZH
w6zh at ix.netcom.com