[R-390] Mystery Component

Mon Oct 14 12:40:11 EDT 2024

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
>
> -----Message d'origine-----
> De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De
> la part de Barry Envoyé : 13 octobre 2024 23:43 À : Jacques Fortin
> <jacques.f at videotron.ca> Cc : R-390 Mailing List
> <r-390 at mailman.qth.net> Objet : Re: [R-390] Mystery Component
>
> Hi Jacques,
>
> One end is connected to R560 and the other end is connected to Z503.  I'm not sure but I think it's R554 and was likely changed from 2.2k to 470 ohms.  Either that, or R554 was mislabeled from the beginning.  I do see where that resistor is still 2.2k for the R-390A/URR so not sure how it ended up being 470 ohms in these radios.
>
> Thanks again,
> Barry - N4BUQ
>
>
>> Hi Barry,
>>
>> Eeer... Is that "470 ohms" resistor connected at the same post than
>> the 56K in the picture ??
>> Where the other end of it is going ?
>> And if your R560 measures 68K, change it !
>> It will affect the Carrier Meter calibration and the AGC time constant in "slow"
>> mode.
>>
>> 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 Barry Scott Envoyé : 13 octobre 2024 23:03 À : Jacques
>> Fortin <jacques.f at videotron.ca> Cc : R-390 Mailing List
>> <r-390 at mailman.qth.net> Objet : Re: [R-390] Mystery Component
>>
>> I'd originally started looking at R560.  It's about 68k in mine.  Not
>> really sure how critical that is and I think at most it might have
>> some effect on the AGC time constant.
>>
>> I'm pretty sure now, though, that the "mystery" component is a 470 ohm resistor.
>> Another list member sent a picture of a couple of his IF modules and
>> one is clearly 470 ohms.  That last band is brown instead of orange.
>> Mine is checking a bit over 500 so probably good enough as-is.  Odd,
>> though, that they used one of the "older" style resistors there.
>> That's what threw me off more than anything.
>>
>> Thanks,
>> Barry - N4BUQ
>>
>> On Sun, Oct 13, 2024 at 8:41 PM Jacques Fortin <jacques.f at videotron.ca> wrote:
>>> Barry,
>>> That "mystery" component is R553.
>>>  From your picture, a 1W, 47K ohms, 10% IRC brand resistor (maybe
>>> not the original one, but who knows ??).
>>> It connects the 180V supply to the screen grid of V509.
>>> R560 (56K ohms) can be seen in the same picture.
>>>
>>> 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 Barry Envoyé : 13 octobre 2024 18:15 À : R-390 Mailing
>>> List <r-390 at mailman.qth.net> Objet : [R-390] Mystery Component
>>>
>>> Maybe not really a “mystery” but I can’t identify the dark brown
>>> cylinder with one yellow and two brown(?) stripes in the AGC
>>> time-constant compartment in my R-490/URR. A 410-ohm resistor?
>>>
>>> If those are actually violet stripes then I’m even more confused.  Anyone know?
>>>
>>>
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