"The Models 524C and 524D Electronic Counters are precision counters which can measure frequencies to 10.1 megacycles and display the readings in digital form on an eight-place indicating system. In addition to making direct frequency measurements, the counters can measure period, frequency ratio, and total events. The counters can also count random events such as encountered in nuclear work. A self-check feature enables the operator to verify instrument operation for most types of measurements.

The internal oscillator of the 524C and 524D is stable within 5 parts in 10e8 per week. Thus these counters are good secondary frequency standards and in many cases can replace more elaborate installations. To increase the range of measurement of Model 524C and 524D Electronic Counters, seven accessory plug-in units are presently available.

The Models 524C and 524D are identical except for type of readout. the 524C uses eight in-line digital display tubes behind a light filter, and can blank the readout during count time. The 524D uses eight numbered masks, illuminating one number in each mask; the readout is visible at all the times" (hp, Manual 524C, 1959-1964)

BRIEF SPECS

"We wanted to develop a high-speed counter for measuring nuclear radioactivity, and Bagley [Al Bagley from Cal Tech] was assigned to this project. It turned out he was able to develop an electronic counter that would operate at a frequency of up to ten megacycles. This took the counter beyond its original purpose and enabled us to make a frequency counter out of it and add it to the HP product line [hp 524A in 1951], This frequency counter was not very reliable and we assigned one of our engineers, Marv Wilrodt, to provide repair service at our customer's location. The counter was so useful when it did work that our customers tolerated its unreliability." (David Packard, The hp Way, 1996)

The 524C/D is the improved version of the famous hp 524A/B (see above) tube counter. The 524D has 'columnar' readout whereas the 524C has nixies. Reliability was improved dramatically. While the 525B manual has a 142 (!) pages service section, the later 524C manual is reduced to 'tiny' 28 pages. The most visible difference was the change of the fastest decades from analog meters to neon digit readouts. Also the crystal timebase stability was improved from 1ppm/2ppm (short-term/per week) to 0,03ppm/0,05ppm. The monstrum was described in the hp Journal No. 2 in 1951 first. While the left 7 decades are decade counter assemblies (six 100 kHz-type AC-4C and one 1 MHz-type AC-4J) the units counter capable of 10 MHz counting rate is done at the mainframe. For operation a tremendeous count of 97 tubes is required. My counter has the option '-H12', which changed the fan motor from ac to dc operation. This enabled the instrument's use at the 400 cps aircraft power line.

Inserted you see a model 525A frequency converter for measurements up to 100 megacycles. Another welcome effect of this plug-in is an increased sensivity. The plug-ins ranged if the 300 ... 555 $ region. Even in the 1965 catalog a new plug-in (525D; 460 to 1010 MHz) was announced, while the famous 5245L counter was already available for 2,950 $ (w/o plug-ins).