RCI 2950 PLL alignment issues

[Lkaskel]

Hi Everyone,
I hope you all are doing ok. I have lost a few in my community recently to Covid so it makes me realize that we are not over this yet. Please take good care!!!!

So, on the bench is an RCI 2950 1st edition. Orange display and the EPT 295012Z board. To start, the radio had been "hacked" and did not transmit or receive when I began. I mainly just de-modded it and repaired a lot of solder/trace issues. It now transmits and receives. Now to the alignment. I am using the procedure from CBT for the 2970 with the 295013Z board (attached). I did not get far. I set the radio up as per specified, connected the counter to L61 and I do not get a stable frequency. It bounces form 2-16 MHz and everywhere in between. I verified the counter function with a different radio and it is fine. I tried some of the other procedures including the voltage adjustment for IC7. I could only get the voltage down to 1.6vdc if I drove the L21 core to the bottom. I have spent another hour verifying parts and solder to be sure that I have not missed anything and I feel somewhat confident that the radio does not have any parts swapped in the PLL section.

Any thought as to where to begin in troubleshooting it?

Thanks as always!!!

 

[loosecannon]

if you actually are transmitting and receiving, then you definitely have your 10.240mhz reference, so either you are testing for it at the wrong location, your freq counter is not sensitive enough to catch it, or your radio is not actually transmitting and receiving.

you might check that the radio is actually receiving signals on the correct frequencies, and verify with a freq counter that the radio is actually putting out power on the correct freq.
LC

 

[brandon7861]

many of the test points are very high impedance and connecting the probe loads the circuit down - often distorting the signal and reducing it's amplitude. Even a 10x probe can load down radio circuits because, at RF, the probe cable has capacitive reactance. The nasty signal that remains is hardly big enough for average frequency counter to latch on to, which is why the readout bounces around.

You might get lucky by adding a small value capacitor to the probe tip (1pF or so) to prevent the probe from loading down the circuit, but the counter still may not be able to see what makes it through.

The solution to this issue is an active RF probe. I recently made my own for this very reason. There are several schematics online if you want to build one and pre-made ones for under $20 online.

 

[Handy Andy]

There are several Tantalums that are part of its' "low-pass" traps used around all those mixers.

Since you may be having trouble "latching onto one" the older caps may be degrading the signal quality to a point you cannot even scope it to find something there.

Since boards vary, the ones still use the TP11 and TP10 spots but use a cap that might act more like a resistor than a capacitor. (as an example, C106 and C161 if you have the board with the mixer mess. )

One board has the programmable VCO/PLL chip the other one "generates" what the PLL "should do" - but both use a LPF cap on the two test points but to help...

You should have bare leads of 10K resistors too, so then you know you're close to the cap that reflects that signal.


IT's the typical - gotta recap some to restore the original condition so that can get fixed...- (SEE: MURPHY'S LAW)

 

[groundwire]

like its been said, the counter is not locking because the signal is to low for said reasons. you can also use a broadband low signal amp on the probe to compensate. you also can use the sig out jack on a scope to connect the counter too. in effect using the scope as a preamp .

 

[nomadradio]

An unstable counter display may result from an insufficient signal level. You pretty well have to use a "times ten" type 'scope probe to prevent a counter from loading down every circuit you probe. But that raises the needed sensitivity for the counter's input circuit.

And they're not all created equal.

Could also be that the frequency you're probing is genuinely unstable, wobbling randomly about.

I use a counter and probe as the very last resort to measure a frequency. Got tired of the aggravations. I use a receiver that's calibrated to WWV to snoop on the radio's internal frequencies without touching the circuit at all. A signal that's jumping around in frequency will be immediately apparent as soon as you hear it. Zero beat can be tough to hear properly, so I use a 1 kHz tone box. Yes, it's crystal controlled. When the heterodyne tone from the ham receiver matches the pitch from the tone box, you'll hear the beat note between two audio frequencies that are close. You have to add or subtract that 1 kHz from the frequency displayed on the receiver. We use LSB, but it's arbitrary.

A coax jumper to the receiver gets the sleeve backed down from the plug. The center pin makes a perfectly sufficient 'sniffing' antenna for the signal I want to eavesdrop.

Much faster way to put an adjustment within a couple of Hertz than waiting for a counter to update once per second. Can't display one Hertz on a counter faster than one update of the display per second. You can hear that once-per-second beat note right away.

Discovered one bonus. An old trimmer cap that has become "scratchy" like an old volume control is impossible to miss this way. Don't have to have a golden ear to recognize a noisy control. Learned to put a drop of cleaner on this trimer caps, and then twist them around a couple dozen times. When it's cleaned up, you'll hear the smooth rise and fall of the tone pitch as the screw is turned. This is not so apparent on the display of a counter.

Sounds kinda Rube Goldberg, but it speeds things up, and avoids all errors that come from hooking a probe to a circuit.

73

 

[Lkaskel]

Thanks everyone!! I ready do appreciate all of your thoughts!! I use an HP 5335A as my counter and the input sensitivity is 25mV so its plenty sensitive for "normal" adjustments. The gate time is plenty fast as well. I just had it calibrated so I feel good about its use at this point. I actually set that radio aside as a sanity check. I've aligned a few Galaxy's and 2 President Washington's with no counter issues so I think I will put it back on the bench and try again.

I'll admit that I need to study the zero beat suggestion again. I know that it is a very reliable thing to use but the one time I tried to wrap my head around it I think that it got the best of me. I'll report back when I get to spend some more time with this radio. I'm not going to let it beat me!!!!! Now its time to play with the grandkids before they go home....