Hi Pete,
Lunchtimes are useful for making incremental progress. I've examined LO
drive into a 50 ohm load on the cable labeled 26 which leads to the BPF,
where radio is set in "CW" mode. Using an Agilent counter, frequencies are
exactly what they should be given the tuning knob, so it's not a problem
with unlocked PLLs.
Here are the equivalent LO levels grouped into "works" = I get the
correct IF level out of the BPF board, and "deaf" = I get no IF out of the
BPF board at all until I jack the RF in way over to maybe -30dBm. V
amplitude for LO signals is measured into 50 ohms and dBm = V**2 / 50. IF
level out of the board is measured with a spectrum analyzer.
Works:
freq, MHz LO drive, dBm
28.00 10.2
24.89 15.0
21.00 9.9
18.00 7.7
14.20 8.4
Deaf:
freq, MHz LO drive, dBm
10.10 10.3
7.03 6.1
3.58 7.0
1.80 8.0
I see no real difference between "Deaf" and "works" here. HOWEVER,
this is with LO cable unplugged into 50 ohms. When actually hooked to the
board, it gets amplified by Q10/Q11 and then fed to the mixer. Using a 10
meg probe and scope, I looked on either side of R47, one of the resistors
comprising the LO amplifier, and with that information, the signal provided
for mixer consumption is actually quite a bit different depending on band -
for example, it's a solid 13 dBm for the "works" bands but barely 7 dBm for
the "Deaf" bands. [Note that I have verified the RF is getting to the mixer
by examining that signal right at the "TP" point just before entering the
mixer. That looks strong at the right level across all bands - "Works" and
"Deaf" alike.]
So I'm stumped. Is the problem weak LO drive not engaging the mixer
diodes? Is the problem with the LO amplifier impedance in or out just
pre-mixer? Is the mixer itself behaving in a strange frequency-dependent
way? No idea. Perhaps a reading from your bench on what a properly
working LO drive input level should be, measured in the same way, is the
next step if you are still willing. (What to do about it would be another
question.)
73
Phil W1PJE
On Tue, Jan 29, 2019 at 8:57 AM Peter Bertini <radioconnection@gmail.com>
wrote:
> Life's lessons:
>
> 1: the tiny 100uH ferrite chokes used by TT make great fuses!!!
>
> 2: any TT inductor is a candidate for wiggling and eventually fracturing
> its solder joint at the PCB pad.
> _______________________________________________
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> TenTec@contesting.com
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>
--
----
Phil Erickson
phil.erickson@gmail.com
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