[R-390] R390A RF deck advice

Sat Jan 10 02:25:54 EST 2015

Barry,

I put some other bad numbers in a post.
The IF deck needs to get us 27 28 to 1 for 150 uv at 455.
we will never see a receiver do those numbers end to end.
We would like to see 20 : 1 end to end. 
We can get a bit over 20: 1 but you need to really pick tubes to get much over 20:1 end to end for noise.

I will admit, I seen one receiver get 30:1 end to end in 1974 with a lot of work and only on one octave.

We look at these resistors and their values.
First off we see a resistor that 44% out of range 27 vs 39.
If it was a 100K resistor and 44% over value we would say OMG it needs to be changed.

If was a 27 ohm cathode bias resistor that set the grid bias operating point of the tube stage we would
again consider the resistor to far out of range and change it.

But these are just parasitic suppressor resistors.
What do we really expect them to do in terms of circuit performance?
Is the value critical or would any thing under a 100 ohms work OK?

 And I just do not know about carbon resistor noise over the age of the part.
Maybe these things do get quiet over time and should be left alone.

We do find these things 27 ohm resistors read 39 ohms.

Now we need to get smart and know if this is a meaningful observation and have a part that needs to be changed.
Or is it not so significant and should just be left alone as changing the part introduces more problems than it solves.

I thought that we should go back to the end to end signal to noise and sensitivity test as a guide. 
If the receiver will muster up and pass then leave the parts alone.
If the receiver will not pass then you need to make sure you are going after real problems and not just
grasping straws.
Again the usual suspects (tubes) need to be considered first.
After you do all the easy things like clean contacts and get a set of low noise tubes and still do not have the 
sensitivity or noise ratio, then go back and visit these out of range resistors.

Fellows come along and find these things.
They ask us if its a problem.
We need to give advice that helps individuals solve problems.

Roger AI4NI

-----Original Message-----
From: Barry <n4buq at knology.net>
To: r-390 <r-390 at mailman.qth.net>
Sent: Fri, Jan 9, 2015 2:43 pm
Subject: Re: [R-390] R390A RF deck advice

Roger,

The percentage out-of-tolerance was given as one point to consider as to whether 
to change the resistor(s).  Since there is considerable difference between 30% 
and 44%, I just wanted that clarified.  Again, I said that I might be being too 
picky...

As for whether carbon-comps get better with age, I don't know.  So many 
electronic devices get worse with age but perhaps carbon-comps don't?  Again, I 
don't know.

If noise is the primary consideration, then aren't other resistors a better 
choice than carbon-comps?

Regards,
Barry - N4BUQ

----- Original Message -----
> From: "Roger Ruszkowski" <flowertime01 at wmconnect.com>
> To: r-390 at mailman.qth.net
> Sent: Friday, January 9, 2015 11:57:19 AM
> Subject: Re: [R-390] R390A RF deck advice
> 
> 
> Barry,
>  
> What ever, 44.44% its enough to consider changing.
>  
> The 27 ohm resistors are inline parasitic suppressors.
>  
> After 50 plus years I would think they were getting well aged and quieting
> down.
> Why would you want to install new fresh noisy resistors now? You have just
> gotten these
> well seasoned.
>  
> Their exact value is not the function of the resistors.
> First we need to understand the function of the resistor in the circuit.
> Then we need to understand how it was suppose to provide the features we
> expected to get from the resistor.
> Then we need to look at the whole lot of modern resistors and decide which
> type provide the best performance
> features in the receivers circuit.
> 
> Do the carbon comp resistor get noisier over time and thus we should change
> them all every 20 years or so?
> Or do we think they quiet down over time and should be left alone even as the
> low current DC resistance value
> drifts up or down?
> 
> Are very old new stock resistors less noisy than when new?
> 
> Roger AI4NI
>  
>  
> -----Original Message-----
> From: Barry <n4buq at knology.net>
> To: Roger Ruszkowski <flowertime01 at wmconnect.com>
> Cc: r-390 <r-390 at mailman.qth.net>
> Sent: Fri, Jan 9, 2015 5:07 am
> Subject: Re: [R-390] R390A RF deck advice
> 
> 
> It might be picky of me, but, I think that's about a 44% error, correct?
> 
> Barry - N4BUQ
> 
> Sent from my iPad
> 
> > On Jan 8, 2015, at 10:49 PM, Roger Ruszkowski <flowertime01 at wmconnect.com>
> wrote:
> > 
> > 
> > 27 / 39 is about a 30 % error.
> > Consider your skill level and all that could go wrong. You may just want to
> leave it alone.
> > 
> > 
> > Roger AI4NI
> > 
> > 
> > -----Original Message-----
> > From: ngietz <ngietz at mymts.net>
> > To: r-390 <r-390 at mailman.qth.net>
> > Sent: Thu, Jan 8, 2015 2:50 pm
> > Subject: [R-390] R390A RF deck advice
> > 
> > 
> > Hi all, first I want to thank everyone who responded with advice about
> replacing
> > C227
> > (I think it was) in a general overhaul of this 1963 Teledyne RF deck.
> > Now, I have several more questions:
> > 1)How critical are the values of the 27 ohm resistors on some of the grids?
> > A
> 
> > couple
> > are up to 36 or 39 ohms.  What effect if any would that kind of difference
> have
> > on
> > receiver operation or gain?
> > 2)Some time ago I ordered some replacement slugs for the tuning coils and
> > they
> 
> > came
> > with brass slot head screws rather than the Bristol spline screws I was
> > expecting.  The
> > slugs are still coded with the red and white dots.  Is there a chance I got
> > incorrect slugs
> > or are the brass screw slugs just a late manufacturing change?
> > 3) Does anyone disconnect the heater line on HR202?  Seems to me the
> > crystals
> > and
> > associated components would last longer without all that heat.  That corner
> > of
> 
> > the
> > deck has the worst cluster of out-of-spec resistors etc., and I suspect the
> heat
> > did it.
> > I’d guess there might be a slight effect on stability, but I’m not using it
> for
> > RTTY.
> > Thanks for all your wisdom!
> > Nelson Gietz
> > ______________________________________________________________
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