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Old school RCI 2950

Smokinone

Smokinone

WDX-3820
Jun 24, 2020
780
966
103
Southern Nevada
I got this older 2950 out of the cabinet and when first fired up, the receive would cut in and out, I think it had low power but can't remember...been playing with this for a few days.
I replaced all the electrolytic caps, and as stated in another post, I found about 4 totally bad caps, and a few over and below ratings. That seemed to cure the issues with the in and out receive. So on to the next step with the PLL alignment. All went as it should with all the voltages set,coils that needed to be adjusted with the scope all went well. AM alignment spot on, USB spot on. Now comes the problem with the LSB portion. Best I could get was 10.6954, and it should be 10.6975. Thinking maybe the L28 coil, a 12016, I replaced it with a new one I had. Same issue although I think it may have settled in to the 10.6975 for a second...maybe. Like I said I've been working on this for a few days and my mind is in a jumble.
Went through all the steps again today hoping for some over night divine intervention but alas, same results.
I've included some pictures of the coil(s). The first on the left is the original with the group picture and the next to the right is out of the same bog of new replacements. The 3rd and 4th in line were in the bin with the others, at least one came from a 959 if i remember correct.
I was wondering if anyone could help me out with this, I would like to try and check the inductance of the coil but am not sure what pins to check.

The first 2 pics are of the original I pulled with part number and pins.

DSC03597.JPGDSC03599.JPGDSC03600.JPG

On the above pic, the first one is the original, the second one is identical to the replacement I used, and the other 2 are what was in my bin.
I think these would all be the same by the first 5 numbers being the same. The 2 on the right end have been used as there is some solder residue still on them. I don't usually throw stuff away until I can check them, and I don't know where to take the readings from these unfortunately.
Thanks for any and all help.
 

So this style of variable inductor is actually a variable transformer. The main coil is on pins 4 and 6, and adjusting the slug in and out will change the inductance of the main coil, as well as the coupling factor between the two. It's possible that only the main coil is being used in the circuit.

FWIW, unless there is physical damage to the coil, it is unlikely that the inductance will change over time, but stranger things have happened before.
 

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So putting an ohmmeter between 4 and 6 should show some resistance or a short...I assume. Is the voltage from the meter going to overload the coil?
It also looks like the 4 and 6 pins aren't used.
 
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It's also possible that the circuit only uses some combination of pins 1, 2, and 3. Kind of a strange design choice, but perhaps it was done to cut down on the number of unique components on the bill of materials. Either way, it's just a coil of wire, so you should be totally fine probing it with a meter to measure resistance.
 
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I checked the points between 1 & 3 and come up with 0.01mh. I checked all the 12016 coils I had with the same results. Of course I am using a T7 cheapo, but it seems to be able to at least get me in the ball park. Between 1 & 2 or 2 & 3 I get nothing, but like I say, it's a pretty cheapo tester.
I also pulled and checked the D84 and it checked OK as well.
 
Pulled C239, 56pf, it checked within spec, tried a 39pf in it's spot with no change in frequency for alignment.
Just can't get the LSB to get to the 10.6975.
I might just have to send it off or put it on the bay.
 
Are you able to get the output on frequency? As long as the PLL and IF are stable, the exact frequencies are largely irrelevant as long as A+B-C=(intended frequency.) I would check at the top, bottom, and several in-between frequencies to make sure everything tracks properly, but if it's on target, send it.
 
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No, I haven't been able to get it to go on LSB. I am not that experienced on radios really. I have the basic equipment to do alignments and poke around, but not the knowledge of where to look for what in a troubleshooting situation. I have had great support from this forum, and was hoping to get some help on troubleshooting this issue.
It just seems that with all the PLL numbers in place and the AM and USB being able to be set, that the LSB wouldn't be able to go. I put another new L28 coil in just to check and that left me exactly where I was. So I can almost say it's not the L28 coil. I just don't know where to check up stream or down to try and cure the issue.
I don't know if it could be a weak signal from the SSB end of things, but I did peak all the coils that were described in the PLL alignment, and I have all the prescribed voltages, X3 is where it's supposed to be @ 10.6975.
 
Had to do some searching and found this here:
https://www.worldwidedx.com/threads/rci-2950-pll-alignment-issues.263549/


Judging from that thread, I surmised that essentially a 30mv sensitivity on my frequency counter won't pick up the needed SSB signal for alignment even with the VR7 in stated position on this early 2950.
Although I was able to set the 10.240, and the AM, at the D45 test point I couldn't adjust the L28 for LSB and actually the L29 for USB.

I ended up tuning by ear when the skip was rolling, now I'm good to go.
 
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