DX-959B 10.240 VCO Refference Circuit

[Robb]

I don't know if this still hold true, but every crystal oscillator circuit usually has a capacitor in parallel with the crystal. Is there something missing here?
http://electronicdesign.com/analog/fundamentals-crystal-oscillator-design

A different load capacitance will produce the wrong frequency.

4. The load capacitance is the capacitance “seen” across the crystal’s terminals, exclusive of the internal shunt capacitance.

Two caps in parallel sums the amount of total capacitance. Even tho the varactor is a diode, it is acting as a variable capacitor when the voltage to it is reduced or increased. That circuit I posted above is the Loop Osc, which means that is the circuit that will shift freq up or down with the clarifier control pot. Right?

So if that one cap is removed, there will be less capacitance transferring loop RF. Guess the point that Xit13 is making, is that by removing some capacitance and reducing the amount of capacitance with a different varactor diode, there is less capacitance to move the freq up or down.

 

[KD8HMO]

Well i learned something new today. The 10.240 vco was not off. I had used my MFJ 259c's freq counter to measure it and it is off. I found out today that my little Elenco signal generator also has a counter (gotta RTFM i guess). I checked all the other PLL test points with it, and they were spot on, so I reset the vco and now everything seems to be perfect now. When i get my new Fluke counter back from the calibration shop, I am going to have to find out how to trim the counter in the MFJ.

 

[Robb]

I'd just use the Fluke; that is its intended use and accuracy is for . . .

 

[KD8HMO]

If the PLL generates the frequencies for the radio, what do these adjustments in the transmit section do?

TX Frequency:
OffsetSet radio to CH 20 LSB TX mode.
NOTE: If the radio has the clarifier mod done skip this step. Connect frequency counter to TP3
VR7
16.50750 MHz ±20Hz.
AM OSC:
Set radio to CH 20 AM TX mode. Connect frequency counter to TP5
L23
10.69500 MHz ±20Hz.
USB OSC:
Set radio to CH 20 USB TX mode. Connect frequency counter to TP6.
L24
10.69250 MHz ±20Hz.
LSB OSC:
Set radio to CH 20 LSB TX mode. Connect FC to TP6.
L25


10.69750 MHz ±20Hz.

 

[Yeti Communications Ltd]

part of the pll loop

 

[KD8HMO]

Does anyone know of any updates to this board? Test point "6" is marked, but no pin sticking out, just a hole there.

 

[KD8HMO]

I just found out from a post on another board that TP6 was eliminated by Ranger, and R160 is the new TP6.

 

[Ranch55]

Well i learned something new today. The 10.240 vco was not off. I had used my MFJ 259c's freq counter to measure it and it is off. I found out today that my little Elenco signal generator also has a counter (gotta RTFM i guess). I checked all the other PLL test points with it, and they were spot on, so I reset the vco and now everything seems to be perfect now. When i get my new Fluke counter back from the calibration shop, I am going to have to find out how to trim the counter in the MFJ.

When you find / figure out how to trim the frequency in the MFJ 259C, please share it with me. I have had mine open to do this and have not had any success yet.

 

[Robb]

You cannot use a 259 as a bench freq counter. It does NOT have any where near the resolution that is needed for alignments. You need a counter that displays eight places; not five places.

 

[loosecannon]

kd8hmo, that IF section can be a b!tch to align due to the low signal level. some freq counters will lock on it, but many of the lower priced consumer versions won't.

when i align these radios, i set the VCO offsets to dead nuts correct. (clarifier perfectly centered) very important to get them perfect. then i inject a 1khz tone into the mic. it's also very important that this tone be exactly 1000hz. you can measure this with your freq counter and adjust your tone gen accordingly.

then put the radio in SSB mode on channel 20. clarifier perfectly centered, (set it before doing the VCO adjustments and dont move it) remove the bias board so you arent actually putting out any power, and key the mic.
put the tone through the mic and tune the appropriate IF can in LSB for 27.20400 mhz.
in USB mode, tune for 27.20600 mhz.

you will be dead on freq. as long as your freq counter is calibrated.
LC

 

[KD8HMO]

Well, my Fluke 1920A counter arrived at the repair center in Trenton, NJ Friday. I cant wait to get it back and try it out.

 

[LeapFrog]

That 'counter' in the MFJ wasn't meant to be used for definitive testing or alignments. It does not have enough resolution for alignment work. It displays only '27.385mhz' (for example); and not 27.385.000mhz - which is what you need to align radios.

IOW, it cannot display enough freq detail. The Service Manual calls for +/- 20hz accuracy for aligning the radio, which is just not possible with the MFJ unit. Keep in mind, that 27.385mhz is read as 'twenty-seven million three hundred and eighty-five thousand hertz'.

Sooo, if you adjusted the 10.240mhz (which is actually 10.240.000hz) osc xtal with the MFJ, you are probably still way off the +/-20hz tolerance that it allows for. You cannot see that part of the freq because that counter doesn't have enough resolution to display it.
View attachment 20064

I am doing a 2547 I have 10.240, but if i put ground of counter to crystal it is low, should i use pc ground instead?

 

[loosecannon]

look at the schematic.
neither side of the crystal is grounded.

connect the positive lead of the counter to IC3 pin 8, and the negative to PC board ground or chassis ground.

remember that you are measuring an AC signal, so it's ok to ground to the metal chassis of the radio.
LC

 

[LeapFrog]

look at the schematic.
neither side of the crystal is grounded.

connect the positive lead of the counter to IC3 pin 8, and the negative to PC board ground or chassis ground.

remember that you are measuring an AC signal, so it's ok to ground to the metal chassis of the radio.
LC

That makes sense why the crystal had no continuity to pc ground, I can get it now.
Thank You Loosecannon
73 -LeapFrog