Ameco Electronic US navy Frequency Meter FR 5A/U with 35mm films inside - Page 1
Forum section: electronics / testgear

author: oz2cpu
author_rank: Frequent Contributor
timestamp: January 03, 2024, 11:58:30 am
content: this is most likely the most wierd frequency adjustment i have ever seen,using a very look reel of 35mm film with numbers and a huge book to look up and convert this to harmonic frequencies,I also found two extra spare 35mm films inside the unit.10MHz to 100MHz in 3 bands (+harmonics up to 300MHz)it's a detector / frequency measure unit, by beat tone in headphones,made using 2 main oscillators, mixer and audio beat osc,I was able to find the schematic in the large book of harmonicsthat is used to manually calculate the measured frequency.weight 33kg, 88-105 Watts at 220V, depending on oven power on/off27 tubes :OA2, OB2, 3 x 6BN6, 2 x 6X4W, 3 x 6AK6, 6BA6W,5 x 6AU6WA, 6C4W, 2 x 6AH6WA, 5814A, 7 x 12AY7WAall tubes start with the military code : JTL, JG, JRC, JRPand all tubes also got year and month codes 1959Ameco Electronic US navy Frequency Meter FR 5A/U 1957-59 test teardownvideo link:https://youtu.be/pJetze-Kdao

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2024, 12:10:38 pm
content: I find it strange to have a master copy of the film in the instrument. Unless the film is really individual to the instruments, this does not make that much sense as making a copy of the film is still quite some undertaking - nothing to do in the field. A master copy would also be better stored separate, or with the manufacturer to than offer spares if needed.The long list of numbers is also strange. It should be a reasonable simple formular, like N*base frequency +- constant. Even in the old days this may be doable as math and not a long lookup table, especially if the constant part is relatively simple (e.g. 3.6 MHz or ideally even simpler). In stead of printing on paper, at least the next harmonic could also be on the film.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: January 03, 2024, 12:29:56 pm
content: exactly what i had in mind, that look up table is crasy complex, math was invented, but maybe engineers did not have a pocket calculator in 1957 :-)I imagine the film in a unit is specific to that unit, so you cant swap film with another unit, then it will be way off,it must be their way of "fixing" it to be accurate,maybe the shipment document said a thing about taking out the master film and store it somewhere nice and safe,and let the spare copy follow the instrument so it could be fixed in the field if needed,but they just forgot to do it, that will make sense to me that way.maybe someone have ever seen something like this before ??

author: alm
author_rank: Super Contributor
timestamp: January 03, 2024, 03:20:44 pm
content: Nice teardown! What an unusual instrument.Quote from: Kleinstein on January 03, 2024, 12:10:38 pmThe film canister appears to be marked with a serial number, so my guess is they are indeed specific to that one instrument. HP used a photographic process to print the scale of analog meters to improve linearity. I imagine every navy ship and facility would have a photography lab back then that could have handled the duplication. Making the film custom to the individual instrument would have improved accuracy / linearity. Similar to correcting the linearity of an ADC with a table or curve. Must have been an expensive instrument.Keep in mind that back then slide rules were only good to a few significant digits, and for higher accuracy books with tables likethiswere commonly used. So a table like this would likely be very natural for any engineer, and the only reasonable solution for this kind of accuracy.

author: iMo
author_rank: Super Contributor
timestamp: January 03, 2024, 05:14:00 pm
content: My guess would be it can measure with, say, 1Hz (or 10Hz) resolution.Now the tech guy on the ship has to check out, for example, a 78489.345 kHz frequency .So he needs to know roughly where to set all the oscillators and multipliers/dividers, and then he has to find the zero beat by finetuning, and then he has to get somehow the precise freq reading back. And you have to do it fast. No MCUs, PLLs, DDSes, TTL dividers and such fancy things inside the box.. I doubt it would be doable at that time without all that stuff seen in that meter, however..