NRI Professional Signal Tracer Model 33

NRI Professional Signal Tracer Model 33

Another piece of test equipment that is its own radio

The NRI Professional Signal Tracer Model 33 was sold in the 1950's by NRI, the National Radio Institute. NRI offered correspondence courses in radio and electronics. They also sold some decent test equipment. The NRI Model 33 includes a four-band TRF radio with 2 tuned stages and an audio amplifier. Both sections are similar to the audio and RF-IF channels of the Rider Chanalyst.

The four bands of the NRI-33 cover 170 KHz through 11.4 MHz in four continuous ranges. This includes the commonly used IF frequencies as well as broadcast and short-wave. Tubes are 6E5 eye tube, two 6SK7 for RF amps, 6SQ7 as detector & first AF, 6K6 for audio out and 5Y3 rectifier.

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NRI Professional Signal Tracer Model 33 after cleaning and repair
The unit pictured is actually playing a broadcast station on the built-in speaker. It does not even require a piece of wire for antenna. The RF probe (center), the AF probe (left) and the ground wire clip (right) are permanently attached. Connecting the RF probe to an antenna allows even weak broadcast and lots of short wave stations to be picked up. The eye tube closes only on relatively strong signals, enabling a good indication of stage gain in radios undergoing tests.
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Tuned into local station and touching the RF probe
Touching the RF probe with a finger while tuned to a local broadcast station will increase the signal to the point I have to turn down the volume control. The eye tube also indicates a fairly strong signal.

Cleaning this unit revealed a set in excellent cosmetic condition. I replaced the power cord and most coupling and bypass capacitors. Replacement was mostly with inexpensive ceramic caps. All controls and tube sockets were cleaned with deoxit spray. Dial calibration was relatively accurate. Alignment was just tweaked a bit. I do not have a manual or schematic for this unit although one is hardly needed. The circuit is fairly well laid out and straightforward.

The Meissner Analyst 9-1040which is another piece of test equipment that is its own radio, was the previous item on the bench.

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