Hi Again All, At the specific suggestion of Garey, I went ahead & completed a 3rd stability test on my Drake T-4X transmitter PTO...
I essentially repeated everything that I did in the very first test, i.e. I used the ICOM 751A as an umpire, both it and the T-4X were started at the same time (i.e. "cold"), and the ambient basement temperature was 59F---but this time I LEFT THE AIR EXTRACTING COOLING FAN ATOP THE FINAL AMPLIFIER OF THE T-4X CAGE TURNED OFF. Again, I measured the frequency a total of SIX times, i.e. (1) immediately at the start of the test, (2) 3 minutes into the test, (3) 15 minutes into the test, (4) 30 minutes into the test, (5) 1 hour into the test, and finally (6) two hours into the test. The actual PTO frequency itself was monitored with the 751A in general coverage mode... The results with this third (and final---I promise!) test were most interesting (NOTE: for comparative purposes, I have square-bracketed---[ ]---the changes observed at the very same time intervals in the FIRST test, at the end of each observation line). Here goes: (1) 5455.0-KHz (cold start)---[0.0-Khz]; (2) 5455.0-KHz (unchanged)---[0.0-KHz]; (3) 5454.3-KHz (downward drift of 700-Hz from cold start)---[500-Hz]; (4) 5453.7 KHz (downward drift of 1.3-KHz from cold start)---[1.0-KHz]; (5) 5453.2-KHz (downward drift of 1.8-KHz from cold start)---[1.2-KHz], and, (6) 5453.2-KHz (unchanged)---[0.0-KHz0 The end result in terms of PTO drift with the fan off was 1.8-KHz, versus 1.2-KHz with the fan on. Interesting! That's a difference of 600-Hz, or HALF of the drift experienced with the fan running! It's obvious that proponents of utilizing cooling fans for their Drake transmitters are on to something, alright---but the benefits are NOT restricted merely to prolonged tube life: PTO drift is reduced by one third, as well. My guess is that the cooler environment afforded by the fan to the entire "works" of the transmitter hastens the PTO in achieving its "ambient operating temperature." Because its environment is more directly controlled with the fan, the PTO has less opportunity to diverge in as great a frequency swing as it normally might, as may be evidenced by the results when no fan is used. Anyway, I'm still attempting to wrap my head around all of this, and I certainly do welcome any & all feedback & comments from the readership. One final point that bears repeating, as was mentioned to me by no less than two subscribers to the Reflector: the Drake manual specs for stability are quoted for THE TRANSMITTER AS AN OPERATING WHOLE, and NOT just for the PTO alone. The final transmitted frequency is a mix of many different frequencies, from self-excited oscillators, & from crystal control. As the TV advertisements state, "RESULTS AT HOME MAY VARY," Hi Hi. This has all been a most interesting exercise, nonetheless---I only hope that my ramblings herein have NOT inspired readers to unsubscribe from the list..! ~73~ de Eddy VE3CUI - VE3XZ _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
