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Robyn 520D modulating on LSB with no modulation

S

Stuck in the mud

Member
Nov 28, 2020
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Muncie, Indiana
What I have is a Robyn 520D and on LSB key down no modulation it produced 3 watts output.
When in USB key down no modulation it is at 0 output.
When in AM key down no modulation it has a carrier at 4 watts.
I had the unit recapped and aligned. When I received the unit back I can see all the caps have been
replaced and the alignment has drawn in the clarifier to match the TX.
The issue now is the same as before but now in LSB key down no modulation it is producing 9 watts
output. It seams like the new caps have amplified whatever is causing the internal modulation.
Still 0 on USB and 4 watts on AM like it should.
Note:
Has extra channels installed back in the 80’s.
Has clarifier mod done in the 80’s.
Mic and TX circuit change done per Cb tune up manual completed – done the same time as the new caps.
Any help as to where to start or maybe one of you have experienced the same issue already and you
knowledge would be grateful.
 

Stuck in the mud said:
Thanks Big Lou for the reply It does the same thing with a hand mic a D 104 and a Galaxy echo mic.
Click to expand...
The reason I mentioned taking the head off is I had some thing like that happen to.me .If you have a HOT radio and add a hot mic or mics it will pick up back ground noise hand mic or desk mic it happened to me. That,s how I found the source of my carrier . I run a DELTA M2 mic now it has a on /off switch on the head that turns the head off I use it when I check the SWR on my radio,s . Good luck to ya I no you will find the problem .
 
I agree with Lou on this one but it is interesting that it does not have the same issue on USB. On that board there is a VR that is for carrier balance. In other words, in SSB, when you key the mic this adjustment is made to minimize the carrier which hopefully should be 0 watts. Because you have 0 watts on USB I am unsure why you would have something on LSB. It is possible that the radio is tuned to be too hot...
 
Thank you for the reply I had the alignment done by someone else so I am not sure what you mean by to hot? I have adjusted VR4 carrier balance. According to the alignment you are suppose to adjust VR4 in USB on ch. 19. It is adjusted now on USB the RF meter reads all the way to the left. If you flip over to LSB the meter shows the 9 watt carrier . If you readjust vr4 the meter will fall to 5 watts but will not fall any further.
 
Sounds to me as if there is a solder bridge between adjacent foil pads. This would explain a carrier on one sideband only, maybe.

Re capping a radio is one of those 'good intentions' kind of effort. It's a good thing if it doesn't cause more harm than it fixes.

A bright light and a magnifier are your friend finding this sort of issue.

73
 
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Stuck in the mud said:
Thank you for the reply I had the alignment done by someone else so I am not sure what you mean by to hot? I have adjusted VR4 carrier balance. According to the alignment you are suppose to adjust VR4 in USB on ch. 19. It is adjusted now on USB the RF meter reads all the way to the left. If you flip over to LSB the meter shows the 9 watt carrier . If you readjust vr4 the meter will fall to 5 watts but will not fall any further.
Click to expand...
Follow what Nomad suggested. He is on to something here.

As far as "too hot" goes what I meant is that sometimes (many times) people modify their radio for more power and end up making it worse. You will often see a radio be too sensitive to audio/modulation or it will become so misaligned that you get strange results like this. Knowing that it has been recently re-capped I would agree that it is likely a mistake from that effort. I re-capped a GE radio last year that actually had the + symbol on the board incorrectly. When the re-cap was complete it had transmit issues. I knew it worked before but it took me a LONG time to figure this one out. I had to get another radio with the same board and compare things. It wasn't until I saw the cap reversed in the working radio that it became clear. Then I needed to research it because you would not expect the board to be marked wrong. Oh well, it was a great lesson learned for me....

Good luck!!!
 
Thanks everyone for the reply,s Here is the new scenario. I have been over this board with a fine tooth comb with a bright light and magnifier. I can not find any solder bridges or cracks in the board. I have tried readjusting the carrier balance no luck there either. What I have found out is if I touch the ground leg of C193 disk cap with my finger then key up the radio I do not have a modulation carrier and the radio transmits fine and receives fine. If I take my finger off of the leg I have the modulation carrier again. And the radio has the hum back in it. I have replaced FET 7 Xmit Mixer still carrier. I have replaced L38 still have carrier. I have tested R190 R191 R192 all test good. Replaced C193 C 195 C 28 Still Modulation carrier. Anyone with anymore suggestions? HELP Help Thanks Dale
 
I think touching your finger on C193 making the problem go away was just an unrelated coincidence. Go back to troubleshooting in and around the Balanced Modulator IC2. Check all of the DC voltages (shown in parenthesis) and see how they compare to this schematic:
858 PLL Bal. Mod.jpg

Pay close attention to pins 1 and 4 and see if they are equal. On these old radios I've seen trimmer pots including ones for carrier balance become open circuited to one of the end-of-rotation legs. This can cause problems especially on a setting as critical as carrier balance.
 
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