Hi Pete,
Got a few minutes today.
Discovery #1: the post-mixer JFET amplifiers were all shut off because
they were not receiving any DC. Culprit was a bad inductor (not a
soldering problem) - L39, a 100 uH that was supposed to feed 13.6V to the
drain of the 4 transistors Q6-Q9 through the 1K resistor R55. The inductor
measured completely open circuit when removed from the board. Hard to see
how an inductor suddenly fails, but it did at some point. We're replaced
that with a regular non-ferrite inductor for the moment which restored
functionality but I'll have to get a ferrite inductor in there eventually
once the rest of the troubleshooting is done. Now the BPF board on those
higher bands that work (see below) has the expected ~6 dB effective gain
from RF in to IF out.
By the way, you weren't kidding about not bothering to match the
transistors - the current draw as measured by the voltage drop across each
100 ohm resistor varies quite a bit as the voltages go from 1.39 V to about
1.5 V, depending. But still, this is an approximately correct biasing
current as far as I can tell.
After this point, I rechecked all bands:
Discovery #2: All the BPF appear to be tuned OK and working right, as
checked at the test point "TP" with a high impedance scope: feeding in a
reasonably strong signal, that signal is seen well at the RF pin of the
mixer.
Discovery #3: The LO levels though are definitely not the same on all
bands, and the lower bands as a result now appear to be almost completely
deaf (40+ dB worse). I need to characterize them further in amplitude and
frequency, but 40, 80, and 160 meters have much lower LO voltages measured
into 50 ohms on connector 26 (disconnected from the board) as compared to
the higher bands, and at least one of them has a distorted waveform. This
is troubling as it might mean that the XO/LO board has problems, and I was
hoping to avoid dealing with that.
I'll return when the LO values have been characterized. Thanks for the
excellent suggestions so far.
73
Phil W1PJE
On Sat, Jan 26, 2019 at 2:15 PM Peter Bertini <radioconnection@gmail.com>
wrote:
> Phil, there are bandpass filters on the LO Mixer board.
>
> If the LO signal falls low enough, the DBM losses will increase. The post
> mixer amp provides a proper impedance terminal for the mixer, and makes up
> for some of the conversion loss.
>
> As I recall, those LO mixer filters covered more than one band, did you
> check all bands to see if the problem is on more than one band? It
> appears that 40 and 30 meters share the filter on the 81596 board.
>
> I would at this point assume that the JFET amps are working, and measure
> the LO by removing the coax cable going to J26, and measuring the LO
> voltage directly on that cable, using a 50 ohm terminating resistor. That
> will show you the approximate drive levels from the LO mixer board.
>
> Pete
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--
----
Phil Erickson
phil.erickson@gmail.com
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