[R-390] Mystery Component

[David Wise]

Why 1W?  Good question.  If it's really 1W in the instrument, there's no technical reason, as its power dissipation is insignificant and noise is not a factor.

David

________________________________
From: Jacques Fortin <jacques.f at videotron.ca>
Sent: Monday, October 14, 2024 10:45 AM
To: 'David Wise' <d44617665 at hotmail.com>; k6fsb at arrl.net <k6fsb at arrl.net>; 'Barry' <n4buq at knology.net>
Cc: 'R-390 Mailing List' <r-390 at mailman.qth.net>
Subject: RE: [R-390] Mystery Component

Hi David,
The "other" question about this R554 value change is: why a 1W part is specified (as the R608 and R632) ??
Is it really the R554 reference "misplaced" in the stock list or an effect of "use a value already there" ?
Maybe there is a field change issued somewhere that left no trace...
Or like the changes applied to their R-390A by the USN, that are not present on other units ?

V509 at AGC voltage: yes, like there is a "local" AGC loop around the AGC Amplifier stage.
Not sure of the intended purpose... AGC characteristic linearization ?
I have already plotted the R-390A AGC characteristic (stock circuit) and from that, I believe that the Lankford mod for SSB reception (diodes bypassing resistors in the AGC circuit) does more harm than good to the whole receiver.
I developed a simple circuit that reacts faster to the instantaneous RF signal changes (fast attack) while preserving the original AGC characteristic (that influences the Carriel Level Meter calibration).
But I still have a lot of testing to do about it.

73, Jacques, VE2JFE in Montreal

-----Message d'origine-----
De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la part de David Wise
Envoyé : 14 octobre 2024 12:40
À : Jacques Fortin <jacques.f at videotron.ca>; k6fsb at arrl.net; 'Barry' <n4buq at knology.net>
Cc : 'R-390 Mailing List' <r-390 at mailman.qth.net>
Objet : Re: [R-390] Mystery Component

I'm with Jaques.  As long as there's enough plate voltage on V509 (6BJ6 pentode AGC Amplifier) to pull electrons through the suppressor grid, the exact plate voltage - and R554's value - is "don't care".  The 6BJ6 is sensitive to cathode current (R552) and screen voltage (R553), but not plate voltage.

There is a complication.  Note that V509's suppressor grid is at AGC voltage.  As signal strength - and negative AGC voltage - increases, the suppressor goes negative, requiring more plate voltage to exceed the "don't care" threshold.  But I think Collins was looking for diode clamp action at zero signal when R558 tries to pull the AGC line positive.  I have personally observed, the pentode suppressor grid makes a lousy diode; the real clamp is the V511 (12AU7 triode AGC Time Constant) grid.

R560 was mentioned.  Personal experience borne out of the Wise/Gitzen Carrier Meter Retrofit mod: it has substantial effect on the Carrier Meter.  Specifically, lower R makes the law steeper, so it reads higher at high signal.

HTH,
Dave Wise
SWL (inactive)

________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Jacques Fortin <jacques.f at videotron.ca>
Sent: Monday, October 14, 2024 8:28 AM
To: k6fsb at arrl.net <k6fsb at arrl.net>; 'Barry' <n4buq at knology.net>
Cc: 'R-390 Mailing List' <r-390 at mailman.qth.net>
Subject: Re: [R-390] Mystery Component

I am still wondering about why the R554 value change have been implemented, and if ever it was an "official" change...
Yes, it is documented as such in the TM11-5820-357-35P of 1964 (see attached) but no other traces anywhere.

If the purpose was to "boost" the gain of the AGC amplifier stage, this is quite futile, as the loading impedance of the Z503 is way over any effect coming from the change of the 6BJ6 plate supply voltage caused by the R554 value reduction.
The 6BJ6 is a pentode, so relatively insensitive to the plate supply value changes...
More (a lot, in fact) gain change would have been achieved by lowering the value of R553.
Or even lowering the value of R552 for that matter...
I already made some trials with Larry Haney in the R-390A IF deck related to the AGC Amplifier gain: changing the 6BA6 for a 6AU6 and changing the cathode resistor value between AM and SSB (BFO ON) modes to provide more AGC primary voltage when SSB is received.
It works, but I am still not satisfied with the results.
Anyways, I will made some tests the next time I can extract the IF deck from my R-390/URR.

73, Jacques, VE2JFE in Montreal

-----Message d'origine-----
De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la part de Renee K6FSB Envoyé : 14 octobre 2024 02:53 À : Jacques Fortin <jacques.f at videotron.ca>; 'Barry' <n4buq at knology.net> Cc : 'R-390 Mailing List' <r-390 at mailman.qth.net> Objet : Re: [R-390] Mystery Component

Barry- 1964 Manual hmmmm  maybe....hmmm..... thanks at least you found the possible source. I do not even recall why or what back then....
I wonder since the R390 was being replaced by the R390A no one bothered to document it fully?
Renée

On 10/13/24 9:19 PM, Jacques Fortin wrote:
> Hi Barry,
>
> I found that this resistor value was logged as 470 ohms in the FIELD DEPOT AND MAINTENANCE PARTS LIST (1964 edition).
> But there is no trace of it in any engineering change sheets (the C1, C2, C3 and C4).
> R554 is a 2200 ohms in the 1953 parts list, however.
> The latest schematics still show R554 as a 2200 ohms part and all the values changes from the C1,2,3,4 are also incorporated in those.
> So where is the bug ? In the latest parts list ??
> BTW, I have nothing in my notes regarding R554 when I restored my R-390.
> Meaning that the R554 fitted in my set when I got it was a 2200 ohms !
>
> 73, Jacques, VE2JFE in Montreal
>