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

[jgedde at optonline.net]

I think I fixed the 8 kHz issue but I'm not sure why it worked...  I
isolated the problem to the 3rd or 4th IF amp or T503.  I checked everything
associated with these stages except for the big capacitors and I found
nothing.  I had this silly idea that one of the two tubes in those stages
didn't like to operate at the gain set by the BW switch to compensate for
the increased bandwidth.  So, I simply swapped the tubes from the 1st and
2nd IF stages into the 3rd and 4th stages.  Problem disappeared!  Putting
the tubes back they way they were brought the problem back.  I just replaced
all the tubes in the IF with some NOS JAN Sylvania's from 1969.  

Now I have another problem where .1, 1, 2 and 4 kHz are all about -10 dB
low.  But, at least I have a good method to isolate the problem...  I inject
a signal into E210 from my spectrum analyzer's tracking generator.  I set it
for a 20 kHz span with a 455 kHz center frequency.  Then using a JFET active
probe (very high impedance), I look at the grids and plates of each stage in
succession to see where the bad actor lives.  That's how I identified the 8
kHz issue. 

So, what is the common denominator between .1, 1, 2 and 4 kHz bandwidths I'm
wondering.

John

-----Original Message-----
From: Jacques Fortin <jacques.f at videotron.ca> 
Sent: Wednesday, June 5, 2019 8:37 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

I forgot the first 2.7 ohms resistor: R507 !
These old eyes, you know...

73, Jacques, VE2JFE

-----Message d'origine-----
De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
part de Jacques Fortin Envoyé : 5 juin 2019 20:09 À : jgedde at optonline.net;
'R-390 Forum' <r-390 at mailman.qth.net> Objet : Re: [R-390] R390 Low Gain at 8
kHz BW Importance : Haute

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