[Hammarlund] Old Hammalunds

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "David Wise" <David_Wise at Phoenix.com>
Cc: <Hammarlund at mailman.qth.net>
Sent: Wednesday, April 13, 2011 10:35 AM
Subject: Re: [Hammarlund] Old Hammalunds


> On 13 Apr 2011 at 9:41, David Wise wrote:
>
>> Yep, I sweep-aligned my SP600.
>> I drove it with an HP 8601A Sweeper, and
>> hooked a scope to the IF OUT jack.
>> The envelope height was different for each of the
>> three LC bandwidth settings, but the area was the same.
>
> According to an old Elmer I had when I was young (Woody 
> Davey W7CJB)
> who was the head of electronics for the FAA in Missoula, 
> Montana, as the
> passband is narrowed, the "S" meter should read higher 
> since the S/N ratio
> is improved.
>
> Ken W7EKB

     SNR has no effect on the signal stength meter plus the 
AVC compensates for any change in gain of the stages with 
change in bandwidth.
     The SP-600-JX, in common with the earlier Super-Pro 
receivers, uses a true variable mutual inductance coupling 
for a couple of the IF transformers, the others being either 
critical or under coupled. That means none are stagger 
tuned. When at minimum bandwidth there is a single peak at 
the IF frequency so a simple peaking at that frequency 
should result in correct alignment. A sweeper may help in 
slight tweeking to get the maximum bandwidth as symmetrical 
as possible but the curves supplied for both series of 
receivers indicates its not quite symmetrical. The IF chain 
should be adjusted with the AVC off and at a fairly low 
signal level. The same for the RF stages. The reason is that 
the change in bias to reduce the gain tends also to change 
the tuning slightly especially were series fed circuits are 
used. Since presumably you want the best selectivity for 
weak signals to maximise noise and interference rejection 
the IF and RF stages should be optimised for high gain.
     The SP-600 procedure asks for a low-level (30%) 
modulated signal and peaking is done on the recovered AF 
signal.
     Note that the meter on the SP-600 can not be used when 
the AVC is off since it is disabled. This meter reads 
carrier strength at the output of the detector. The 
variation is due to the imperfection of the AVC. If the AVC 
were perfect the carrier level at the detector output would 
be perfectly constant at all input levels. In many receivers 
the S-meter reads some combination of tube currents which do 
vary as the AVC bias changes so there will be a variation 
even if the signal delivered to the detector is constant. 
Actually, in theory there MUST be some variation albeit 
small, for the AVC to work since it is a sort of negative 
feedback circuit. It can perhaps even be analysed as a servo 
mechanism. There must be some residual error.
     So, swept alignment is helpful for receivers that have 
either fixed overcoupled transformers (like the HQ-120, 129, 
etc or the Collins 51J and others) or have stagger tuned 
IF's like wide-band FM or TV receivers but may not make much 
difference in a set with critically or undercoupled 
transfomers typical of most conventional radio receivers.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com