JDB, it was this statement of yours that prompted me to ask you about the VCO alignment:
"With mirror board still in, AM chan 20, freq. cntr on TP3
16.50985 with mic gain off
16.586 and drifting with mic gain full open, no mod."
your VCO alignment cannot be "spot on" if this is the case. if your freq counter starts bouncing around, or doing anything but sitting there solidly displaying 16.5100 mhz then you have a problem with the VCO.
from that statement, it would seem that you were testing it in TX mode, which is where you are having a problem.
the chances of those two things not being related is low.
so if you tell me that the VCO alignment has been checked and re-checked, and you are considering it to be good; then somehow the situation you were having before that i quoted above is now not happening?
please give me some details on that so we will be on the same page.
lets try something else real quick. first set the radio up like this: ch. 20, AM mode, clarifier centered, mic gain all the way down.
now remove the mirror board in the back, and connect your freq counter probe to the center pin of the antenna jack, negative lead to radio chassis. (freq, is AC so it doesnt matter if its PC board ground or chassis ground)
now key the mic and read your actual transmit freq.
it should be 27.2050 mhz, and should not vary at all.
let me know what you find there.
as for the voltage measurements, remember that a capacitor will not pass DC voltage, only AC voltage.
if you see a series capacitor in a path you are following, you can be sure that it is a signal path, and not a DC voltage path.
Q18 and Q19 are part of the receive mixer section and do not provide any DC voltage to this section. you can leave them be.
if you look at the schematic, you can see that pins 11 and 13 are connected together through the secondary of L43, and there is no DC connection for them to be pulling any DC current from.
the DC voltage you measure at the pins is generated internally.
you actually have the greatest source of answers sitting right in front of you!
you have a working DX959 that you can sit right next to the suspect one, and compare voltage readings.
if i were you, this is what i would be doing.
you can also swap parts that you are unsure of as a way to find your culprit(s).
i dont want to go much further until i get more answers from you on the VCO thing, and the other tests.
good luck, i'll be checking back over the next couple of days.
LC
