Phil,
I spent several years working with one in production work in 1978-80.
The Hp675A sweeper is ancient Hp technology, all transistor. But
nothing wrong with it. It needs optional crystal oscillators for
markers, and I found the International Crystal made nearly identical
modules called OX- something, which could be modified to fit into the
675A. You could do without, but its like using a Hallicrafters or
Hammurlund RX near the band edges and turning on that 100 KHz marker.
Puts nice blips on the detected waveform. Without this, there are no
digital markers or pointers that show what the frequency sweep
display scale represents.
It (or the 676) has detector input, so that you can hook up a
'rhotector' or VSWR bridge, and get demodulated RF back to display on
the scope. I suppose the 676 is recommended to have, since that
tracks the RF sweep. (cannot remember if narrowband tracked
detector?). A directional coupler works as a detector also, but it
should be a reasonably good one, with better than 20 dB directivity
in my estimation, if you are trying to measure good matches < 1.1:1
etc.
Its a big unit, and you benefit having the Hp TESTMOBILE to place it
on. But nothing fundamentally bad about having those old clunkers.
Nice room heaters too.
[An aside on boat anchor test equipment: I have an Hp3570 automatic
network analyzer, that sweeps from 50 Hz to 13 MHz, with a stepping
Hp synthesizer. It is fun to play with, lights of lights and buttons.
It has zero VLSI microprocessors inside, all done with state machine
logic using TTL and earlier technology. There are boards in their
chock full of ICs, just to be the processor board. I would hate to
have to fix it. I cannot believe that it actually has a GPIB bus,
that decodes simple commands from a matching antique Hp desktop
calculator. It never locks up, no matter how many buttons you press
out of order.....]
You need an XY monitor scope to go with it. In a pinch you can use an
old oscilloscope, but the real display screen was nice, since you can
mark with a grease pen the various VSWR's, or dB of return loss, or
gain. No fancy digital readouts on the screen with this one, you put
your own marks on the graticule!!!! The Wavetek 1901 was a favorite
12 inch display for production lines.
Also, find a "Rhotector", or good directional coupler, to do return
loss, VSWR type stuff. And a switchable attenuator, preferably a 0-80
db model with 1,2,4,5,10,20 switches, such as those made by Kay,
ComSonics,Trilithic (Formerly Texscan, formely Wavetek), etc. All of
these can be found at hamfests and surplus dealers. With the advent
of automatic network analyzers with cal kits and incredible accuracy,
these old systems are not being used much anymore. (ANYBODY HAVE AN
Hp3577 or 8753 TO SELL - at hamfest prices?). You will need to
calibrate yourself and the 675/676 to various loss and gain settings.
Also, doing VSWR plots require having a handfull of small type N VSWR
standards, which were made years ago by many companies, such as
Weinschel, Narda, Texscan. If you stick with return loss in dB, you
can get by with a short and an open, a precision 50 Ohm load, and an
attenuator box (see above).
Wide Band Engineering, Eagle and Trilithic make return loss bridges
or 'rhotectors'. Wide Band Engineering has a nice pictorial of the
setup one would use with their bridge. Wilton (now Anritsu/Wiltron)
used to have a lot of application notes on using what they called SWR
autotestors, basically the same thing. I remember a lot of concern
over the phase of reflections affecting the accuracy with a magnitude
only detector like a diode. Used to use tricks like doing a short and
open cal, and averaging the results for the infinite VSWR
condition....
For $130, yours is a reasonable deal - if it works. If it has
problems, then it isn't such a great idea. I would suspect power
supply ripple might need some capacitor replacements by now.
Good luck playing with the technology that was the standard of the RF
industry for years. Of course, for only a couple hundred dollars
more, you can by an antenna analyzer from MFJ or AEA. You loose all
reference to calibration with these, unless you have the standards to
test them, which are the same sorts of devices as with the Hp
675A/676.
73
John
K5PRO
>
To: <amps@contesting.com>
>Date: Sun, 12 Nov 2000 19:56:29 -0500
>From: "Phil (VA3UX)" <phil@vaxxine.com>
>Subject: [AMPS] Opinions on HP 675 & 676
>
>I need opinions on the HP 675 10 Khz - 32 Mhz CW sweep generator, and the
>matching HP 676 Tracking Detector. I have no experience with sweepers but
>I've always wanted one for experimenting with tuned circuits and filters in
>the HF range. This is strictly for non-serious, amateur, hobby use. I've
>been offered the pair for $130, in good working condition.
>
>Should I or shouldn't I ? Feel free to tell me to steer clear of them if
>they have a bad reputation, are notoriously hard to repair or whatever.
>
>Many thanks in advance.
>
>Phil
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