[R-390] Scale varies band to Band

[djed1 at aol.com]

If I remember correctly, the R-390A spec says that the receiver must be within 300 cycles of frequency when the set is calibrated at the nearest 100 Kc point.  It also says that the band to band variation must be less than 4 Kc.  So there never was a requirement for close band to band matching.  As noted, it was a cost reduction design to a spec.
Ed  W2EMN

 

 

 

-----Original Message-----
From: Roger Ruszkowski <flowertime01 at wmconnect.com>
To: R-390 <R-390 at mailman.qth.net>
Sent: Mon, May 13, 2013 8:52 pm
Subject: Re: [R-390] Scale varies band to Band



Gordon,

Your receiver is operating exactly as designed.

You have to remember what is going on within the receiver.
Under 8 Mhz there is an extra conversion.
In the R390A its all offset by 17 Mhz.
In the R390 each of the 8 Mhz under 8 offset with
a different crystal frequency.

Above 8 you have a crystal for each Mhz band.

No one ever said that any one of those crystals was
exactly on frequency or even has an advertised output level.

The crystals in the second oscillator deck are only advertised
to be close enough to get the signals within the zero adjust range.

The receiver has a zero adjust. It has a use. Use it.
 Receivers never had a set of crystals that would place
each band close to zero on every band.

I would be common for a couple bands to be way out on the 
margin. It depended on what the receiver was being used for.
Even the spooks did not tune every receiver to every band all
the time.

If it got critical to an op who needed to do a couple band swaps
to chase his dits and the crystals made getting zeroed an
annoyance, we would swap some crystals between receivers
to bring his problem under control.

You can swap crystals to get a set that's closer than 
the current set. It depends on how many receiver and spare crystals
you have to swap around. It is a hit and miss exercise.

As every crystal in the second oscillator deck could be at any 
frequency give or take some you never expected to get an
exact zero from band to band. The trim cap may pull
a crystal a bit but its function is to peak output. We do not care exactly
what the output frequency is as long as its within say 2 KHz.

If you can get the band to peak up within the range of the zero adjust
and do not have to actually reset the VFO coupler or are not actually
against the zero adjust limits with the band the crystal was considered with 
speck
frequency wise. As long as the output level is high enough to get the
signal to noise ratio (better than 20:1) for the band, the crystal is considered 
to
have sufficient output. The fact that its like 4 KHz off from the band beside it
or else where has nothing to do with the acceptability of the crystals 
performance.

The TM alignment procedure says reset the zero adjust with every frequency 
change 
as you go through the alignment procedure. The process expects each crystal in 
the
second oscillator to be not on exact zero.

Consider the cost of making sure every crystal on all the frequencies in all
the many manufacturing runs of these receivers had to be within some small
zero beat of matching.

Just letting a crystal be within a couple KHz on a band is big cost saving
from a production stand point.

There are reasons there are many part numbers that look as if they will
get you a crystal on a frequency and in a can that looks to fit the socket.

Some are transmit crystals on frequency. Some are receiver crystals
sort of on frequency and lots of other things like output level and harmonics
and operating temperature. There is a trade off in cost for each feature.


We would never build a receiver to day that acts like an R390.

Roger AI4NI.





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