David,
Note that all the RF voltage values at the buffer output and after are
much lower on 20 meters than on 40 meters. That points the finger at
the Buffer stage.
Even though the MIX output is lower than expected, for right now, I
would suggest you concentrate on the major difference in the BUF output
between the 2 bands. The buffer is frequency sensitive - note that
capacitor C16 in series with R11 makes the gain change with frequency.
The gain should be higher on higher frequencies, and in your case it is
not. So check the component values at C16, R11, and R12 as well as the
soldering for those components (they determine the gain of U9).
In addition, do DC voltage measurements at U9 pin 2 during both transmit
and receive. If Q14 is doing its job, you should see a low voltage at
pin 2 during receive and about 6 volts during transmit.
Yes, those values indicate that the PreMixer crystal for 20 meters is
probably OK (reserve judgement on that for later).
73,
Don WFPR
On 3/19/2013 7:46 PM, David L DuPuy wrote:
Don, here are the values for the Transmitter Signal Tracing. Results are
shown for 40m (all okay except maybe MIX), and on the right are results for
20m, as measured on a 60 Mhz Tek oscilloscope. The problem seems to start
between OSC and MIX, i.e., at U8 Transmit Mixer/Osc.
signal tracing
40m 20m
shunt DRV to gnd
PRE 0.09 0.06
ATTN 0.05 0.05
OSC 0.06 0.07
MIX 0.05 0.05 LOW
BUF 4.3 0.9 LOW
TR1 2.3 0.5
BPF 0.4 0.2 LOW
TR2 0.4 0.2 LOW
TR3 0.01 0.04 HI
remove DRV shunt
DRV 1.3 1.1
PA 40 5.4 LOW
ANT 40 5.0 LOW
RFD 2.1 0.59 LOW
I'm having trouble understanding how the waveform at PRE and ATTN is the
same amplitude on 40 and 20, but very different past U8. The signal at PRE
and BF is not a clean sine wave, but shows a modulation... true for both 40
and 20. Is that normal? The PRE and BUF 20m signal appears to be double
the 40m frequency. I presume that means both X1 and X2 filter board
crystals are operating?? I really appreciate your continued help!! (you
are very patient!) David AC4BN
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