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 Post subject: Heathkit fmo-1 10.7mhz issue, schemo vs construction notes
PostPosted: Sep Sun 19, 2021 6:05 pm 

Joined: Jun Sun 24, 2018 12:57 am
Posts: 43
I have been working thru one i picked up and have replaced the electrolytic caps and the over-value resistors (alot). when received, the unit had an extra micamold cap (.01mfd) in parallel with c21 (.005mfd) at T1 which provides a tone to aid alignment. without this extra capacitance, the unit will not broadcast an audible tone. i've traced out the wiring to and from T1 (color-coded leads), all seems good, except that r16 as found was 22k (correct per construction notes and diagram) and is shown as 39k on the schemo (see attached). the original 22k was in spec, but i put in a 39k per the schemo, and still need the extra cap to get a tone; i thought i would find a mistake or component out that would explain this, but have not. the parts list does have just one 39k resistor on it, and several 22k's. i replaced c21 because it was easy, and still had to add the .01 back.

should i care/does it matter? i can't figure out if i have failed to find a problem somewhere or the circuit just needed some help and i should leave well enough alone.

is there a way to align the 10.7mc (sweep) coil with a spectrum analyzer that has a tracking generator, maybe by just hooking to the coil correctly, power off? i've been trying to learn about an SA by borrowing one to align this. i was able to do the other coils thru the fmo output, and confirm the two xtals were good as marked by hooking to them when removed from the unit. on the fmo 10.7 sweep setting where this coil is used, i can find the pattern with the SA, but it's moving of course. i need a stationary peak i can measure while turning the slug.

i learn a lot here -- thank you all.

complete-single-page-fmo-1-schemo.pdf [376.97 KiB]
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