---   The Modulation-Monitor Oscilloscope   ---




Back in the good ol’ days, radio hams cared a great deal about their signal quality. It was a matter of considerable pride. A critical signal report resulted in a sincere “thank you,” and more often than not, an immediate investigation into the necessary corrective action. Unlike today, when a critical signal report is more likely to earn a rebuttal of harsh language, an extended and upraised finger, or the bored intonation of "Whatever!" a modern-day oral equivalent of “Who gives a damn?”

In those “good old days,” it was common for operators of the widely-used plate-modulated AM rigs, especially rigs of higher power, to pay careful attention to the quality of their modulation. Insufficient modulation and/or nonlinear modulation-response of the final amplifier, caused a less-than-optimum signal and communications-impairing audio distortion.*     Excessive modulation too, causes a host of problems, including the aforementioned distortion, an excessively-wide signal with adjacent-frequency interference, and worse, a possibility of radiating out-of-band splatter components.

*        Many hams spurn reports of audio distortion because they think distortion is only an unimportant irritant to the listener. Those hams fail to understand the significant reduction in intelligibility and QSO-effectiveness that audio distortion can cause to their signal.

The unhappy results of poor modulation convinced many a ham to find ways of monitoring their modulation parameters to ensure good operation, and the well-equipped hamshack used a fancy gizmo called an oscilloscope, to do exactly that. National Corporation, purveyor of what many consider the the finest communications receivers of their era, offered to the market, a simple, relatively inexpensive, and small (2 inch), electrostatically-deflected oscilloscope. It was called the National Model CRU, (Cathode-Ray Unit?) and was offered in a rack and panel configuration called the CRU-P. The CRU allowed direct connection to its deflection plates, and a ham could simply sample his RF output, and connect it to the ‘scope’s vertical deflection plates to view the RF waveform under modulation. The slightly more ambitious ham would not only use that RF sample, but also supply a sample of the modulator’s audio-output signal to the ‘scope’s horizontal deflection plates. The resulting pattern, shaped like a geometrical trapezoid, and mysteriously called a trapezoid pattern or a trapezoidal pattern, provided even more information about the modulation characteristics of the transmitter than the pattern resulting from a simple RF sampling.

Well, that’s why RigRig was equipped with a National oscilloscope. When I gave the transmitter to my Chicago pal Orlando, W9RX, in 1962 or thereabouts, I decided, for now inexplicable reasons, to keep the ‘scope myself. It had a different 40-year history than the transmitter. Whilst I indeed kept it myself, I long long ago lost track of it. I don’t where or how it moved about, but only a very few years ago, well before having any thoughts of restoring BigRig or even knowing that the rig still existed, one evening I saw my old ‘scope on the hamshack shelf of a local ham I was visiting. I didn’t even know that ham when I gave the transmitter away in the 1960s. How he came into posession of the 'scope, I have no idea, and I have no recollection of his having been able to tell me. It must have passed from ham to ham, and there it was!

I unceremoniously demanded its prompt return and subsequently repossessed it, probably by threatening to bill him for 40 years of rental fees. The threat proved unnecessary and he was happy to give it back to me. Hence, the CRU is on my restoration list.

The ‘scope was in working order. It had no electrolytic capacitors to have dried out, only oil-filled bathtub caps for filtering, and so powering it up was not risky and was immediately successful. The silly thing still worked! The display on the green P1 phosphor was a mite broad and a mite dim, so I launched an immediate attack on eBay, and hard though it may be to believe, there was an NOS never-used RCA 2AP1 CRT in its original box! A twenty-dollar bill had it heading my way.

The rack-panel had long been discarded, so I ordered one from a distributor. It interested me that a 5 1/4" steel rack-panel is shown in my 1954 ARRL Handbook for 93 cents, and so that same panel probably cost about 75 cents when the transmitter was built. The present price of the panel is slightly over $12.00. Yes, I know, incomes have risen too, but . . . .

I’ve completely disassembled the ‘scope and I’ve layed-out and drilled the new panel. Fortune smiles on we good people, and one of the only two Greenlee punches that I saved from my extensive collection of 50 years ago, was the 2 1/4" punch needed for the ‘scope panel. It’s been kicking around in my various toolboxes for 50 years. Don’t ask me why.

Today, the ‘scope enclosure and the new panel are off being powder-coated. I should have them back in a few days, and I’ll begin re-assembling the ‘scope. The re-building doesn’t appear too onerous, with only a couple of hard-to-turn pots and a couple of paper-dielectric bypass capacitors needing to be replaced. Oh yes, I did order 4 brand-new rubber mounting feet for it. Have you ever squeezed a 60-year-old rubber foot?


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