Richard Sears, Vintage Electronics

AN/USM-24 Synchroscope

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Another navy synchroscope. In this context, a syncroscope is a triggered oscilloscope fitted with an internal trigger generator that can be used to trigger both the timebase of the oscilloscope and the external circuit under test simultaneously. I imagine this would have been useful for working on both sonar and radar circuits where the ability to trigger a portion of the circuit independently must have helped fault diagnosis. Here it is displaying the internal calibrator, the photograph has not done justice to the display:

My enthusiasm for this stuff has waned and I had this unit sitting on my bench along with a re-wound power transformer since January (2012). I finally got up a big enough head of steam to take it on. While well made it is extremely hard to work on. I do NOT recommend this instrument to a casual collector or anybody hoping for a useable oscilloscope.

I have two manuals for this instrument, one dated November 1952 from Waterman Products, Philadelphia and the other dated December 1956 from Model Engineering and Manufacturing, Huntingdon Indiana. Both the manuals are horrible to use and considering that they were written after Tektronix had first shown the world how to write manuals, are a shame. The Waterman instrument has a slightly different and faster Y amplifier from that of the Model Engineering instrument, the first being 3db down at 8MHz and the latter 3db down at 6MHz. My instrument is the later model and I can attest that it will easily display and trigger from 6MHz signals.
The timebase sweeps at rates from 0.5μS/in to 0.05S/in and includes a variable delay.
The trigger generator provides 1.6μS pulses from 50/S through 300, 800, 2000 to 5000/S.
The marker generator intensifies the beam at 0.2μS through 1, 10, 100 to 500μS intervals.

It is a clumsy and hefty unit being 15″ high by 17 1/8″ wide by 13″ deep, odd proportions giving a large frontal area. It weighs 48lb without the front cover and accessories. It has 28 valves plus a 3JP1 post-deflection acceleration CRT that, operating at 3kV, is very bright and well focussed, however it is not flat-faced.
The front lid houses a remarkable range of accessories including a AM detector probe and a Cathode Follower probe that uses a 5179 sub-miniature triode:

Of the 4 valve-era naval oscilloscopes that I have, three have had failed power transformers. In one case, the problem was obviously due to dried out electrolytic capacitors, however in this case all the capacitors were fine. I had the transformer re-wound by Gary at Transformer Rewinding Services (xfmrs@roadrunner.com) and even though the cost of $250 relects my dedication to keeping this one in a demonstrable condition, I cannot see how Gary made any profit. Gary told me that the failure was in the HV winding which fits the scenario. Nevertheless, I was extremely careful to test each circuit out as I installed the rewound unit. There were no problems other than one section of the 4 parallel 6203 dual rectifiers having a failed heater. The unit was drawing a little too much current, I finally traced this to a leaky coupling cap (C52) in the synchronisation amplifier circuit resulting in the gate generator being biased on too hard. I found this while fault finding the gate generator, I didn’t like the performance and noted that the signals in the gate circuit were about 40% low in amplitude. Replacing this capacitor resulted in correct gate function and correct supply voltages everywhere. In the Post-Script, you may read about an adventure I had due to several other coupling capacitors failing due to heat when the unit was re-installed in the case.

The time base is of the triggered multivibrator type that employes a gate generator to turn on (sweep) and turn off (flyback) the sweep generator. The stability of the multivibrator can be adjusted from astable (synchronized free-running) to monostable (triggered). However unlike the Tektronix topology, there is no separate trigger multivibrator (Schmitt trigger). There is a positive and negative synchronization amplitude control; however the function it provides should not be confused with the trigger level control of the Tektronix circuit that allows the user to adjust the point at which the trigger fires to any desired level on a rising (+ slope) or falling (- slope) edge.) The control on the USM-24 oscilloscope does not do that; instead, it allows the user to adjust the amplitude of the negative going trigger signal delivered to the synchronization amplifier from zero to maximum that in turn, delivers a large positive pulse to the gate generator. The clever thing is that it delivers the required negative going signal whether you choose to trigger off a rising or a falling edge. Said another way, the synchronization control allows adjustment of the amplitude of the synchronization signal from either a positive or negative going edge as opposed to permitting the user to set the point on the edge under examination at which the timebase triggers. This circuit, along with the that of the Tektronix 511 and 511A, is significantly inferior to the first “modern” time-base designed by Dick Ropietquet for the Tektronix 315. All true analogue time-base designs since the 531/535 have followed the Tektronix lead, with good reason. In conclusion, it should be noted that the -32 and -24 designs are both synchroscopes and as such, work very well; they are simply less user friendly then a general purpose laboratory oscilloscope.

Here it is displaying a signal using the marker generator set at 100μS; from this, it can be inferred that if the marker rate is accurate, the signal has a frequency of 10kHz and in fact that was correct:

Here is a top view, on the left is the Y amplifier and on the right is the time-base and X amplifier. In addition to the outer shield, the CRT also has an inner shield. Note the small cooling fan on the left. The case of the unit has a small vent in the front and a vent door behind the fan; an interlock switch is provided to prevent the unit from being turned on if the vent door is closed, there is a bi-metal thermal switch inside to control the fan. This unit is not intended to emit much RF radiation!
Looking at the underside, the re-wound power transformer is obvious. It is a shame because the original had the same beautiful epoxy finish as the choke:
The back view shows the tough, braced construction. On the right below the fan, the push vent door interlock switch is visible and if you look carefully, you may be able to see the round thermal switch (that looks a bit like a trimmer capacitor) just to the left of the X brace:
On the right is the time-base on top and the marker and trigger generators on the lower chassis:
On the left is the Y amplifier on top and the power supply rectifiers on the lower chassis:

Post Script:
The case is designed to minimise RF radiation from the unit and has just has two small vent grills and a thermal switch controlled exhaust fan. When I returned the ‘scope to the case it failed with many burnt resistors in the synch amp circuit together with a completely burnt up 12AT7! What happened is that the heat inside the case showed up a number of other leaky paper-in-oil coupling caps causing high bias. I have now replaced all the coupling caps in the ‘scope. Also, the fan which is controlled by a thermal switch did not run. I crudely tested the thermal switch, it switched on with my hot-air gun on low and 18″ away, so I guess it is functioning as intended and that the internal case heat is simply too much for old components. I placed a diode across the thermal switch to provide half-wave current so that the fan is always running, just not at full speed. The ‘scope now seems to work properly, in the case as well as out of it!
Here it is in the case showing the small ventilation grill on the front.:
Here is the vent door showing the tiny exhaust fan grill:
It was necessary to remove the CRT yet again so I took a picture of the revealed circuitry, top to bottom, HV circuitry, fan rectifier, negative rail and 500V rail rectifiers. I installed silicon diodes across the 500V and negative rail rectifiers with series resistors to restore the correct voltages. The slim grey unit on the left is the delay line. The small fan unit is partially visible on the top left:

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