[Hammarlund] AADE DFD1 for SP-600 question

[Philip Atchley]

Hi,

First, let me say that I've already checked the Hammarlund archives and 
kept a copy of all applicable back messages regarding this item. And no, 
I haven't yet written Neil about it as I thought I'd tap the resources 
of the Hammarlund list first!  I may be very glad that the unit was 
given to me and that I didn't pay for it!


Recently I was given an unbuilt AADE DFD1, Hammarlund version for my 
SP-600JX14.  It was quite easy to build, but it turned out that the LCD 
display had a big crack on the right side.  However, while waiting for a 
a replacement display (expected to arrive any day now) I went ahead and 
built the interface to connect it to my SP-600.

The interface I built is a FET source follower using a U310 FET.  Here 
are my results.

1.  FIRST I tried tapping the plate circuit of the oscillator, first at 
the plate of the tube itself, and then at the wire that comes out to the 
VFO/Xtal oscillator switch.  Various capacitors from 1pF to 5pF were 
tried. Tapping the signal at either of those points seemed to produce 
the same results.  A usable display up to about 43MC.  HOWEVER, the two 
lowest bands read accurately at the low end of the dial, but by about 
midscale (900KC on band 1) the readout suddenly jumped to a high reading 
and it appeared that it was actually reading the 2nd harmonic of the LO!

2.  I then tapped the Grid of the oscillator tube (Pin 6) with a low 
value capacitor.  Trying various capacitors, 5pF seemed about optimum 
with a trade-off between dial accuracy and the counter operation.  This 
worked well all the way up to 54MC (top end of band 6).

HOWEVER, while the display was 'usually' accurate, it would sometimes 
give "squirrelly" results.  EXAMPLE:  Tuning in WWV at 15MC, it may read 
exactly 15MC on frequency, tune off just a hundred CPS or so and the 
reading may actually jump by 5KC, go another 100CPS or so and it may 
return to what is presumed to be the correct value.  Sometimes the 
inaccuracies may be as great as 10KC.  In other words, the count is 
unpredictable as I could make 15MC read almost anything I wanted it to!

NOW, if I use my HP-5245L counter, the count is stable and accurate at 
all times.  It appears that I have close to 1 Volt signal level as the 
counter works well on the 1 Volt range.  I then connected the Video 
amplifier output of the HP 5258A sensitive pre-scaler to the DFD1 input 
(using the HP plug-in to "condition" the signal) I still get unreliable 
readouts on the DFD1 while the 5245L is reading correctly!

I kind of suspect that the LO in the SP-600 may have some distortion on 
the LO signal that is affecting the DFD1, BUT I don't have an O'scope to 
see what the signal looks like.

Here are some other things that I've tried. . .

1.  I DID try feeding the signal directly from the grid and plate of the 
LO tube to the DFD-1 (With a capacitor in line) WITHOUT the Emitter 
Follower interface.  NO change, other than the frequency pulling from 
the DFD1 was very evident in the dial of the receiver.

2.  I DID try replacing the 6C4 oscillator tube twice, with no change in 
results.

FINALLY, another friend of mine (Cecil Acuff) told me that other folks 
have had problems getting a stable "lock" on this unit.  Anyone here 
have really GOOD results with this unit?  If so, how did you do it?


73 de Phil  KO6BB
My DXing tools (RECEIVERS), all fully overhauled:
    Hammarlund SP-600 R-274C/FRR, 18 tubes (Circa 1952)
    National NC-60 Special, 5 tubes (Circa 1960)
    Heathkit Mohican  (Circa 1962)
    Icom IC-751 Digital Transceiver  (Circa 1986)
    Grundig S350 Digital Portable (Circa 2004)

THE BEACONEER'S LAIR:  http://www.geocities.com/ko6bb/
QSL GALLERY: http://photobucket.com/albums/y123/KO6BB/
Merced, Central California,    37.3N  120.48W  CM97sh

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