Over The Air > Ham Radio Circuits and Repair

President HR 2510 CPU/PLL issues

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sonoma:
have you checked out Rogerbirds site.

http://rogerbird.tripod.com/

Lazarus:
IC306-1/2 (actually the b half of the chip according to the pinout) is half of a dual op-amp acting as an active loop filter (active low pass filter), its job is to take the phase error pulse train signal coming from the phase detector (A) output (pin 13) of IC301 (pll chip), and turn it into a smooth dc control voltage for the vco tank made up of D306, D307, and L317 (best i can make out, its blurry),

i highly suspect that the reason for its output being so high (as high as possible i suspect), is because of the too low freq going into the Fin pin 9 of the IC301, its putting out max voltage in an effort to raise the freq going into that pin up to where it should be,

there are many things that can go wrong in a pll system, especially one as elaborate as this one, but a maxed out loop filter voltage is almost always due to an either missing or too low Fin freq,

NOTE: the other half of IC306 is also an active loop filter for the VCO tank made up of D303, D304, and L302 (best i can make out, its even more blurry), its associated PLL chip is IC302, its associated VCO output, after being divided by 10 while going through IC303, eventually
winds up going into IC304 to be mixed with the previous PLL/VCO, then this newly down mixed signal looks to be what feeds the Fin pin of IC301, so looks like you have your work cut out for you narrowing down where the fault is,

and i just noticed, looks like the first VCO goes through yet another mixer (IC305) before being mixed with that signal, wow there's sure alot of mixing going on in this PLL system,

anyway hope that helps a little, just holler some more if you need anything further, my hope is that my giving a little detail on what things are doing will help you to narrow the problem down,

:)

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