[R-390] Re Gain drift

Sun Oct 21 22:52:57 EDT 2018

Hi everybody  , Regarding gain drift and bursts of noise  all the Symptoms 
Describe a fault ,  that I've come across on multiple occasions , In my case 
it was caused by the  Foam  dematerializing  in one or other of the 
Mechanical filters becoming slightly conductive as the set warmed up , If 
you look at the Circuit  the outputs from all the filters are Commoned  at 
the AGC  Line .......... I found the FAULT By disconnecting  all at the 
common point and Meggar testing each  with a Insulation tester  the most 
common Culprit was the AM filter, but I've also found the 2KHZ filter to 
have this problem too.  I used to buy these radios at our gov. surplus 
auctions  and after selling to people I've had many come back for repairs 
over the years  , And Yes I remember one receiver had so much gain  AGC Lost 
control  and was very Noisy too ...........somebody had cut the resistors in 
the IF can , The man who owned it was a 6meter fanatic  using it with a 
converter , It took a bit of finding  Regards  Bernie VK2ABN

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Sent: Monday, October 22, 2018 8:33 AM
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Subject: R-390 Digest, Vol 174, Issue 21

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Today's Topics:

   1. Re: Gain drift (Roy Morgan)
   2. Re: Gain drift (ROSS HOCHSTRASSER)
   3. WJ/CEI RS-111-18-17 (dog)
   4. Re: Gain drift (Jacques Fortin)

----------------------------------------------------------------------

Message: 1
Date: Sat, 20 Oct 2018 21:41:47 -0400
From: Roy Morgan <k1lky68 at gmail.com>
To: dog <agfa at hughes.net>
Cc: R-390 Forum <r-390 at mailman.qth.net>
Subject: Re: [R-390] Gain drift
Message-ID: <A5DB3E1D-1C9C-486C-82F9-92B554F2DBEB at gmail.com>
Content-Type: text/plain; charset=utf-8

>On Oct 20, 2018, at 4:58 PM, dog <agfa at hughes.net <mailto:agfa at hughes.net>> 
>wrote:
>
>Jacques,
>
>Yes, those T502 resistors are still in there. So far I haven't heard the 
>gain drift since I resoldered the joints but I need to give >it some hours.

Dog and others,

There was a BAD modification published a long time ago - folks were advised 
to clip those Q-spoiling resistors.  This increased the gain of the set, 
making it ?much hotter?.  This of course was a big mistake.

Don?t do it.  If you find a radio with that mod applied, put the resistors 
back into the circuit.  Then do an alignment on the affected stages, and set 
the IF gain.

By the way, I recall discussions about the use of modern carbon film 
resistor (some with spiral tracks of resistance material on ceramic tubes), 
and how they exhibit unwanted inductance in RF circuits.  I have long held 
that many uses of these resistors do NOT involve RF voltages on the 
resistor, such as in bypassed screen dropping or bypassed cathode use. And 
blindly assuming that these modern resistors will act badly in all 
situations is a mistake.

BUT, these resistors in the IF (or RF?) cans of the R-390A really do 
experience RF voltages.  (If I remember correctly, the R-390/URR - the 
?non-A? - may use such resistors in different stages than the R-390A.)

HOWEVER, it is another matter whether or not the actual inductance that may 
be present if you use modern carbon film resistors matters much if at all.

If anyone can point us to measured data on the inductance of such resistors, 
please do.  I remember that in the past, such data was posted or referenced 
on the R-390 list.  (I may have it but can?t locate it now.)

A final note: the technology involved in 30 megacycle IF amplifiers used in 
WW-II era radar systems involves the use of resistances to set the Q and 
stagger tuning the various stages to achieve much desired bandpass and phase 
characteristics.  This is covered in Valley and Walman ?Vacuum Tube 
Amplifiers? of the MIT Radiation Laboratory Series.  (The math involved is 
not for the faint of heart.)

Roy

Roy Morgan
k1lky68 at gmail.com <mailto:k1lky68 at gmail.com>

------------------------------

Message: 2
Date: Sun, 21 Oct 2018 10:21:32 -0400 (EDT)
From: ROSS HOCHSTRASSER <bavarianradio at comcast.net>
To: Roy Morgan <k1lky68 at gmail.com>, dog <agfa at hughes.net>
Cc: R-390 Forum <r-390 at mailman.qth.net>
Subject: Re: [R-390] Gain drift
Message-ID: <828507635.253212.1540131692821 at connect.xfinity.com>
Content-Type: text/plain; charset=UTF-8

It probably wouldn't be a bad idea to replace these resistors with either 
more modern carbon or maybe even metal film resistors. It is possible that 
these resistors have drifted far enough out of value that their 
effectiveness has been compromised. The audiophile world has realized that 
carbon comp resistors do contribute noise of their own , especially if 
excessive heat was used when "modifying" the circuit.It may also be 
advisable to re-tune the coils after replacing the resistors. Very 
interesting thread!! W1EKG
> On October 20, 2018 at 9:41 PM Roy Morgan <k1lky68 at gmail.com> wrote:
>
>
>
> >On Oct 20, 2018, at 4:58 PM, dog <agfa at hughes.net 
> ><mailto:agfa at hughes.net>> wrote:
> >
> >Jacques,
> >
> >Yes, those T502 resistors are still in there. So far I haven't heard the 
> >gain drift since I resoldered the joints but I need to give >it some 
> >hours.
>
> Dog and others,
>
> There was a BAD modification published a long time ago - folks were 
> advised to clip those Q-spoiling resistors.  This increased the gain of 
> the set, making it ?much hotter?.  This of course was a big mistake.
>
> Don?t do it.  If you find a radio with that mod applied, put the resistors 
> back into the circuit.  Then do an alignment on the affected stages, and 
> set the IF gain.
>
>
> By the way, I recall discussions about the use of modern carbon film 
> resistor (some with spiral tracks of resistance material on ceramic 
> tubes), and how they exhibit unwanted inductance in RF circuits.  I have 
> long held that many uses of these resistors do NOT involve RF voltages on 
> the resistor, such as in bypassed screen dropping or bypassed cathode use. 
> And blindly assuming that these modern resistors will act badly in all 
> situations is a mistake.
>
> BUT, these resistors in the IF (or RF?) cans of the R-390A really do 
> experience RF voltages.  (If I remember correctly, the R-390/URR - the 
> ?non-A? - may use such resistors in different stages than the R-390A.)
>
> HOWEVER, it is another matter whether or not the actual inductance that 
> may be present if you use modern carbon film resistors matters much if at 
> all.
>
> If anyone can point us to measured data on the inductance of such 
> resistors, please do.  I remember that in the past, such data was posted 
> or referenced on the R-390 list.  (I may have it but can?t locate it now.)
>
> A final note: the technology involved in 30 megacycle IF amplifiers used 
> in WW-II era radar systems involves the use of resistances to set the Q 
> and stagger tuning the various stages to achieve much desired bandpass and 
> phase characteristics.  This is covered in Valley and Walman ?Vacuum Tube 
> Amplifiers? of the MIT Radiation Laboratory Series.  (The math involved is 
> not for the faint of heart.)
>
>
> Roy
>
> Roy Morgan
> k1lky68 at gmail.com <mailto:k1lky68 at gmail.com>
>
>
>
> ______________________________________________________________
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------------------------------

Message: 3
Date: Sun, 21 Oct 2018 12:35:17 -0400
From: dog <agfa at hughes.net>
To: 'R-390 Forum' <r-390 at mailman.qth.net>
Subject: [R-390] WJ/CEI RS-111-18-17
Message-ID: <71913b6b-2a3a-eb21-9ad5-a6a3dfdc8702 at hughes.net>
Content-Type: text/plain; charset=utf-8; format=flowed

I have a nice RS-111-18-17 in operational order with original book. 30
to 1000MHz in 4 bands. I haven't had it on for maybe 6 months, but I can
check it out pretty thoroughly, but if it needs some fixing up it may be
hard to repair. Let me know if anyone is interested in this item. I'm
not giving it away at the moment.

I remember these things or similar were used at the listening post in
Berlin. Used at Watergate too, I don't know for what though.

I worked at the CEI division in Rockville in the 70's and I remember
tuning the CRT displays for these 'receiving systems'.

Dave N3DT

------------------------------

Message: 4
Date: Sun, 21 Oct 2018 17:32:55 -0400
From: "Jacques Fortin" <jacques.f at videotron.ca>
To: "'ROSS HOCHSTRASSER'" <bavarianradio at comcast.net>, "'Roy Morgan'"
<k1lky68 at gmail.com>, "'dog'" <agfa at hughes.net>, "'R-390 Forum'"
<r-390 at mailman.qth.net>
Subject: Re: [R-390] Gain drift
Message-ID: <[email protected]>
Content-Type: text/plain; charset="UTF-8"

Hi Ross,

As the internals of the R-390A IF transformers have been MFP coated (at 
least in the ones I have) I do not believe that the ambient oxygen had a 
chance to go thru the carbon composition resistors cases used within.
If the MFP coating is still intact, the resistors should still have the same 
value than when they were manufactured.
And they are Allen-Bradley parts, known to resist "aging" better than 
similar parts from other manufacturers.
But... in case of doubt, CAREFULLY remove the original resistors and replace 
those by the variant of your choice.

Audiophiles pretend that the carbon composition resistors are more noisy 
than other types of construction, and it is true to a point.
When DC current passes thru those, the path followed by the electrons within 
is not always the same, causing a type of "partition noise" at the microvolt 
level.
The carbon film and metal film variants, on which less "mass" of resistive 
material is involved, creates less of this "partition noise" for the same 
current value going thru.

For the "inductive effect" of the spiraling of the carbon/metal film 
resistors tracks, some testing is in order, especially at very high 
frequencies.
However, the tests I already performed on low-value 1W parts proved that the 
dominant factor is the inductance of the connecting wires, and way much than 
the resistive element within.
I do not believe that bad behavior can be created by using replacement 
carbon/metal film resistors within a R-390A.

73, Jacques, VE2JFE

------------------------------

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