[Boatanchors] Scope Available

Wilson infomet at embarqmail.com
Tue Sep 8 15:12:41 EDT 2015

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.



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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