90501 Secondary Frequency Standard
(Variants 90501, 90503, 90505 and 90507)
| The Millen frequency standard probably didn't see much use in the ham shack as it did it the laboratory environment. It has an adjustable RF output in intervals of 10, 25, 100 and 1000 Kcs with useful signal strength to 50 Mcs. It also incorporates an internal 800 cycle audio generator/modulator and a detector with phone jack. The unit runs from a very stable helium filled 1000 Kcs vacuum quartz oscillator and uses synchronized multivibrator oscillators to create the 10, 25 and 100 Kcs markers. There were several variants of the 90501 and we have as yet not found any in the field that have the marking other than the 90501. It appears from the later schematics that these could be purchased with or without the tubes being installed. The lower two numbered variations were delivered without tubes and the upper two with tubes installed. From an ordering standpoint this makes sense, but I must wonder if an ordered 90505 was actually marked as a 90501 on the front panel. Since once the tubes were installed they'd be the same. The 90503 and 90507 variants were made for a 230VAC (50/60 Hz) primary voltage vs the 115VAC (60 Hz) of the 90501/90505 versions. Output and Input terminals also varied over the years. Most often the ads showed only dual banana jacks used from Millen's 37200 series with bakelite plates. However, later ads also showed a version using a single ceramic (Steatite) bushing posts in their 32200 series. In the above pictured unit there are also two SO-239 UHF Female connectors in parallel with the banana jacks. These may have been added by a later user or perhaps on request through Millen. At the very least those UHF connectors were professionally installed. Owner: Don Buska, N9OO |