I have one of these available for a few bucks, plus shipping. Looks fine, but
I’m not going to mess with it.
Please let me know if you are interested. There is no front cover, no
accessories.
Of course let me know if there’s a good place to post it. Might be good for a
museum ship.
Wilson
W4BOH
The second AN/USM scope is a OS-34/ USM-32, developed and made by Dumont. It
was also sold to the civilian market as the Type 301-A. It is a very compact
unit being 7.5 H x 5.5 W x 15 long, excluding the front cover which contains
many accessories including a AM detector probe and triode connected 5702
cathode follower probe. The CRT is a 3WP1 flat faced type that was produced by
Dumont amongst others and is notably used in the excellent Tektronix 310 and
310A. It is around twice as sensitive as the 3RP1 used in the OS-8E/U. This
scope was intended for specialist use in maintaining radar equipment and in
some ways is the most interesting scope in this collection. Note; if you are
considering one of these, know that there is no auto or free run capability so
when using it as a regular oscilloscope, a trace will only be present when a
signal is present (no base line) which is a bit disconcerting.
The design is unusual, for me at least. It features a fairly crude triggered
time base that is not calibrated, despite the use of an active shunt-regulated
power supply. The shunt regulator is also unusual, all the other scope
regulated power supplies I have encountered are series-regulated with the
exception of the DC heater supply in the HP 150A. (Technically,
shunt-regulation is superior, however, it is vulnerable since if the load is
removed, usually the shunt element over-dissipates.) The voltage reference is
also a bit quirky, the 5651 reference tube being located in the ground end of
the CRT divider chain providing -87V. This means that the divider chain is
designed to run at around 2mA which is much higher than necessary than is
necessary to properly bias the CRT. Here is my hand re-drawn time base
schematic, I did this to help me understand it while getting it working. I
found C507 to be leaky, a very similar fault to the one I found in the HP 150A
time base. The waveforms and voltages shown were taken after the modification
described further down this page.
A crude marker generator is included that blanks the beam at the marker
intervals, however it is not crystal controlled so I do do expect any useful
accuracy. (As received, it is actually disconnected, I expect the coupling cap
is leaky and that was the simplest way to get the scope usable at the time…)
The other rather interesting feature is the provision of a trigger generator
that produces trigger signals at intervals from around 25mS to 180μS. I say
around because that is the specification, there are repetition rate marks
around the control but the actual repetition rate did not even come close. I
fiddled with it until I got the end points in the ball-park however, the
increments are still wildly off. I wonder if ageing of the pot track has
changed the rate of the pot as well as the over-all value? The trigger
generator puts this unit in the category sometimes referred to as a
“synchroscope”. The idea is that the trigger generator is not only used to
trigger the time base, but also the external circuit under examination. The
intended purpose of this scope was for use in trouble-shooting triggered
circuits that might be found in radar and since the unit is naval, I guess also
sonar. I am looking forward to getting the unit operating fully and playing
with this feature!
This scope also uses the clever unblanking bistable method that I explained in
the Solartron CT 436 discussion. In summary, the unblanking signal (from the
sweep gate) is ac coupled to a bistable that is riding on the CRT cathode and
switches the CRT grid on and off in response to the leading and trailing edges
of the sweep gate thereby obtaining proper unblanking at low sweep speeds
without DC coupling to gate to the CRT grid.
Here it is as received from via eBay showing the glue used to hold the green
filter that has run down over the filter over the years. The only other picture
of an OS-34 / USM-32 I have found (at www.navy-radio.com/test.htm) shows the
same problem. I simply removed the green filter, it could easily be replaced
with a new one for authenticity.
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