I just wanted to confirm that I believe my previous assumption is correct.
The LLD subsystem has to be putting out about 10w.
The final amp in the Omni D is exactly the same as that of the old Model 540
Triton 2, except the final pair of transistors are MFR 458, whereas the
Triton had PT978. As a result, the resistors on the base of the final
transistors are slightly different; everything else is the same. The first
set of transistors in the PA, the PT3647's in Push Pull driver stage are
EXACTLY the same as in the Triton.
So we can assume the input power to the PA must be the same as the Triton,
or about 10w.
Since you have only 2w, then you definitely have a problem in the earlier
This does not preclude also having a problem in the final stage.
2w of drive should drive it to more than 4w of output.
You may find that one transistor is blown in the final amp board; it still
amplifies, but not correctly.
So, I highly suggest that you disconnect the +12v from the final stage, plug
the LLD directly to the LPF board (the cable is too short, you will have to
extend it), and then trouble shoot the earlier stages until you have the
transceiver putting out 10w.
Keep in mind that the ALC circuit also controls the output power from the
Try the following; turn the power control down all the way, and then check
the base voltage on Q2.
It should be 0.8v in transmit and receive.
If it is not, then you probably have a problem with the ALC circuit.
BTW, Q4 is the same. Q2 and Q4 control the level of amplification of Q1 and
Q5 has no ALC control.
If you are lucky, the worm is in that circuitry. That stuff costs pennies.
I'm still siding with Murphy and saying the finals (at least one of the
MRF's) are shot.
Gud luk, Scott.
From: firstname.lastname@example.org [mailto:email@example.com]
On Behalf Of firstname.lastname@example.org
Sent: Saturday, February 11, 2012 5:03 PM
Subject: [TenTec] OMNI D ser B low power output--still no joy
I am still not getting much power out of the omni...so far I have
disconnected the input to the PA and measured direct out from the Low Level
Driver (LLD)...almost nothing to measure even on the 5 watt scale...yet,
when going thru the PA and adjusting the final PA bias on the control board,
I can get close to 4 watts which seems to indicate the PA is taking the very
very low input and bringing it up to that level...problem is I am drawing
about 8 amps to get 4 watts!
I tried replacing the mrf476 on the LLD with an equiv 2166 and no diff...I
also replaced the two other transistors on the LLD with a 2n3503 (or
I even went further back to the OT board (the stage before the LLD) and
swapped out those transistors...no change at all.
When I transmit into a dummy load for a few seconds the transistors on the
low level driver do get warm, yet I again, cant measure anything really from
the LLD output...
Any suggestions? I am thinking perhaps switching out the diodes on the LLD
next but I am really taking a shotgun approach here. As always, your
collective wisdom/comments are welcome...If TEN-TEC has time perhaps they
can shed some light as well next week.
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