[R-390] R390 Low Gain at 8 kHz BW

[jgedde at optonline.net]

HI Jacques, I replaced R519 thinking that was it (it did measure high).  I
have checked all the 2.7 ohm resistors and they are all fine.  I checked
them but I didn't really suspect them since the passbands were all good (see
second para).  No obvious Q issues.  I will check again tomorrow.  I'm beat
and it's getting hot in the lab from running this beast.

I haven't checked the resistance of the coils yet.  I checked all the IF
bandwidths using a spectrum analyzer with tracking generator feeding E210
and measuring off the grids of each stage with a JFET probe into the
spectrum analyzer.  All the bandwidths are perfect, just 8 MHz is low.  I
also see it in the diode load voltage.

When I get this straightened out, I'll post the plots of each IF filter like
I did with my R390A (yes A).  The passband is smoother with the R390 but the
skirts are better with the R390A as is to be expected.

John

-----Original Message-----
From: Jacques Fortin <jacques.f at videotron.ca> 
Sent: Wednesday, June 5, 2019 8:09 PM
To: jgedde at optonline.net; 'R-390 Forum' <r-390 at mailman.qth.net>
Subject: RE: [R-390] R390 Low Gain at 8 kHz BW
Importance: High

Hello John,

12dB down ? WOW !

I have one or two ideas:

1_ Check all the 2.7 ohms resistors in the IF deck: they are only used when
the 8kHz BW is selected. (R514, R 524, R529, R534) check also the internal
IF coils continuity (the ones selected for the 8kHz BW).
2_ Check R519, which was a selected manufacturing value to set the global IF
strip gain when the 8kHz BW is used. Remove it or measure value. If
doubtful, replace with a modern resistor of the same value.

73, Jacques, VE2JFE