[R-390] R: R-390 Ant trimmer question

Sat May 2 06:02:35 EDT 2015

Roger,   ... Thanks to all!

I just quote a part of my feed backs to the various kind answers.

As I said, it takes time to me to comprehend the relevance and pertinence of
the suggestions. I'm working on them...

Quote

"Hi, Cecil! 
Thanks for your infomations.
By the way,  I have connected the RF generator to the unbalanced antenna
input (50 ohm) via an attenuator/impedance matcher, which has a 50 ohms
resistive output, almost exact.

But, your suggestion about mica caps failure may be right.
Indeed, in a first restoring  stage, I had problems of relevant variation of
sensitivity in the 16-32 Mhz range (both via calibrator and via antenna). I
finally solved this former problem by substituting both the mica capacitors
(5 and 18 pf) of the first RF coil with current ceramic NPO capacitors.
After that, at least in calibrator position, I could reach a CARRIER METER
reading of 40-45 db also in the 16-32 bands, and that in a rather stable
manner. The residual problem, as said in former mail,  is "only" an
unpredictable, slow variation of sensitivity via antenna, in the range of
5-10 db (i.e a sensitivity variation/reduction in the range of 1: 2  to 1:3
, at least)"

"Hi, Jim!
Thanks for kind interest

About the calibrator readings (from 39 to 50 db, from band to band) , I can
say that:
- the Carrier meter indicates exactly 0, when on AGC position and the RF
gain knob  is completely counterclockwise
- the gain adj. of the IF module has been set to give an output of -7,0 Volt
at diode load, when a 100 microvolt Rms un-modulated signal  is injected in
the module via the IF output socket (50 ohm impedance)
- with no signal and "0" RF gain,  in AGC, the AGC output varies from -0,05
to - 0,1 volt (it is more negative with a new tube in the AGC circuit, about
-0,25 volt,  and becomes less negative after ca 10 hours "aging")
- with a  43 Db reading in calibrator position, the AGC output is about -5,2
Volt

- Ok, oil may reduce the insulation of the antenna trimmer shaft (I have
also dismantled the trimmer itself!).  But, in my case, the overall
resistance toward ground, measured on pin 1 of V201 (control grid of 1st RF
tube), is about 1 MegaOhm, almost exact: this should be an indirect test of
the AGC circuit insulation (or, at least, of no extra losses toward the
chassis).

- Anyway, your message is correct and helpful, i.e.: I have to take into
consideration the possibility of some influence of ineffective AGC and bias
related to my problem, in addition

I'll let you know"

Unquote

Franco Mengacci

-----Messaggio originale-----
Da: R-390 [mailto:r-390-bounces at mailman.qth.net] Per conto di Roger
Ruszkowski
Inviato: venerdì 1 maggio 2015 23:29
A: r-390 at mailman.qth.net
Oggetto: Re: [R-390] R-390 Ant trimmer question

Jim,

Thank you for this post.
I did not know the coax would give us problems.
The coax is getting old.  We can think to check it if the noise floor is
coming up of we have the noise pops and can not find a bad tube or other
cause.
Roger AI4NI

Some things to consider:I've never seen 40db carrier readings on either ofmy
390a's except when AGC wasn't functioning fully. If the AGC lines are
beingshunted somehow the radio "appears" to be more sensitive, i.e high
carrier levelreadings.What can impact AGC action?  Any caps anywhere on the
AGC circuitsin IF and RF decks.  If you lubricated the antenna trimmer shaft
duringrestoration, the AGC will be affected because the phenolic shaft
absorbs thelubricant.  The trimmer is on the AGC line and the phenolic shaft
is supposed tobe an insulator.  Likewise if you sprayed the band switches
with cleaner such asdeoxit, the chemical makes the wafers no longer be high
impedance.  Theimpedance of the wafers could be reduced just enough to
affect agc or tuning. Same for tube sockets.  These may dry out eventually,
or just spray them withsomething like Big Bath.I agree with comments about
silver mica caps in theRF stages.  I recently had to recap the RF and tuned
IF stage coils in a SW
 . Any cap that directly touches B+ will be suspect because the high voltage
tendsto cause failures in those caps.  If failure is isolated to one band
this issomething to look at.   Unfortunately many are across coils inside
the cans soyou have to remove the cans, open them up, and replace the caps.
If the failureaffects all bands the same way, then it's something common to
all bands.Nowthings get interesting when you think about the coax cables in
the radio.  In myold SW, I was plagued with clicks, pops, etc.  Finally I
discovered that thedielectric of diode load coaxes out of the IF module was
breaking downinternally.  Had to replace them all with fresh coax.  Doesn't
sound like whatyou're experiencing.Hope this helps.JimN4BE

______________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:R-390 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html