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AMfone's Online AM Handbook => Receivers => Topic started by: Ralph W3GL on March 28, 2008, 08:43:18 PM

Title: Hammarlund Super Pro Service Notes
Post by: Ralph W3GL on March 28, 2008, 08:43:18 PM
Hopefully the text I salvaged will appear here:



WHEN Hammarlund receivers type SP-100, SP-200, SP-400
and the surplus versions (BC-779, etc.) are operating
properly, the maximum high-frequency oscillator drift
is usually about 50 kc., and most of this takes place
during the initial warm-up.

The oscillator drift characteristics of these receivers
can be materially improved by making a simple modification
suggested by Jack Scheider of Hammarlund's Engineering

First, obtain a 3.3-pf- temperature compensating capacitor
having the highest negative temperature coefficient available.
Three 10-pf-capacitors may be connected in series if the
3.3-pf- job is not obtainable.

Lift the top shield from the main tuning capacitor gang after
removing the mounting screws from the top plate of the shield.
Connect the 3.3-pf- capacitor between the stator terminal of
the oscillator tuning capacitor (the first section away from
the panel) and ground.

Realignment of the oscillator trimmers is the next operation.
Turn the receiver upside down, panel facing toward you, and
remove the bottom cover. Using a signal generator or the
standard frequencies from WWV, adjust the trimmers for on-
the-noise calibration of the test signals. The trimmer control
shafts are accessible through the line of holes running from
left to right across the shielded coil compartment. The line
of holes directly to the rear of the panel are over the
oscillator trimmers. Do not touch any of the other trimmers.
All of the more popular receivers in the group referred to
above have the high frequency band trimmer at the left end
of the line. Move on one step to the right each time the range
of the receiver is decreased to the next lowest band.

Since the shunt capacitance which has been added to the circuit
is only 3.3 pf, it is obvious that each oscillator trimmer will
require only slight adjustment. Only a fraction of a turn of each
adjustment screw should be necessary.

Everyone who has made this modification to his receiver reports
complete satisfaction. One report stated that the drift had been
reduced to less than 200 cycles, and another claimed that the
modified set was excellent for s.s.b. reception.


The above was written by Frank Lester (SK) back in the mid 50's.

Yeah, they DO drift but remember, the things were built to be
run 24/7 so eventually the drift stopped (a day or so...) and are
the best of the communications receivers produced in that era.

Actually, the only change between the SP-200 and the SP-400
series was shifting the IF from 465 to 455 kc.  The tube line-up
is the same!

Good Luck de W3GL

Title: Re: Hammarlund Super Pro Service Notes
Post by: Steve W8TOW on July 14, 2008, 11:45:42 AM
Well, I wonder if anyone has tried this? I did and noticed
no improvement in stability!
any other ideas on SP-200 drift?
73 Steve

Title: Re: Hammarlund Super Pro Service Notes
Post by: w2emn on December 21, 2012, 03:14:12 PM
I've tried adding a temperature compensating capacitor to my BC-779 and it reduced the drift somewhat, but I still had about 35 KHz of warmup drift at 20 MHz.  However, I just ran across an article which claims warmup drift of a few KHz can be achieved.  The article is in May, 1948 CQ, and is written by James Whitaker W2BFB.  The three things he discusses are to
1. Adjust the main tuning capacitor so the rotors are exactly centered within the stators
2. Replace the 6J7 oscillator with a miniature tube
3. Add the temperature compensating capacitor.

The curves he shows claim a drift reduction from 60 KHz to about 5 KHz.

I haven't tried his recipe yet, and will not modify the tubes, but I will try adjusting the main tuning cap.


PS:  I tried adjusting the end play on the tuning cap- I didn't get a distinct change in frequency as noted in the article, and it didn't change the drift (37KHz at 20 MHz, over 3 hours.  After that, it was a KHz or so per hour.  good enough for AM work.

Title: Re: Hammarlund Super Pro Service Notes
Post by: KA0HCP on December 21, 2012, 05:28:52 PM
Does this concept apply to other makes and models of receivers?

Title: Re: Hammarlund Super Pro Service Notes
Post by: Ralph W3GL on December 21, 2012, 05:42:50 PM
Back when I posted the original text here I forgot the impotent variable cap adjusting...  

Yes, this was written-up in CQ and 73 Mags and if done along with the
compensation/tweaking of the osc will make a great improvement of that
series receiver...  The conversion of the tube to a miniature version was an
option but the "bang for the buck" invested  just is not there...

I have an SP 400 out there somewhere that I was hoping would have
those mods/tweaks performed as well.   (Yo Frank...)

The drift problem with the 779/1004 and all that vintage radio series was
well known back in the 40's however the way the gear was used it was not
a serious problem...  

Once the receivers were turned on, after a few hours they settled down and
then were left operating 24/7 in fixed locations, hence the brutish external
power supply's.  

Of course when the receivers migrated to the ham community, no one left them
on this way and when that warn-up cycle took place in the ham shack, it
became a major PIA, hence the published mods.    

Title: Re: Hammarlund Super Pro Service Notes
Post by: WA7DUY on March 27, 2020, 02:56:37 AM
Is anyone still using the BC779?  That was my first receiver as a novice in 1967.

There was some discussion on one of the email reflectors about the BC779 and mods including I think these mods listed here but the tone was not favorable.  Did anyone do these mods?  What was your experience?  Was it worth doing the mod?

Thank you.

Title: Re: Hammarlund Super Pro Service Notes
Post by: Steve W8TOW on April 21, 2020, 02:48:23 PM
I use both a BC-779 and also 1004 (which has 160m coverage!)
These are wonderful receivers and (to me) more appealing than looking at a R390...IMHO
This summer, I'd like to pick up a 2nd 779 or 1004 so I can create a "dual diversity" receiver....
anyone got one in the mid-west they wanna part with?
Steve w8tow

Title: Re: Hammarlund Super Pro Service Notes
Post by: WA7DUY on May 30, 2020, 01:54:45 AM
I wonder if anyone has put in sockets and plugs to make the external power supply a plug in unit rather than a wire in unit on the BC779's?
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