Hello, I have a jb150 that has higher than I like swr on the input side. This is between the radio and jb150. When it is out of line I have a 1.1. When I put it in line it is around 2.0 - 2.5. I have done the normal things like changed jumpers 3x etc. I dont have to turn on the jb, just have it in line. Is there a way I can tune it down to lower swr? Maybe adding components? I cleaned the relay contacts, but it did not seem to help. Im thinking it may be folding power back, because I am only seeing about 50 or 60 watts with a 6 watt carrier from the jb. I have the carrier set to 3 watts from the cobra 2000. I realize the 8417 could be soft....but I dont want to tell myself that. I still dont like the swr like that. Thanks
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wawasee black cat jb150 swr high on input
- Thread starter troyota
- Start date
troyota,
Having owned a few of these in the past, what you are seeing is normal in both input match and power output.
I do recall that changing/adjusting the tuning coil to set the meter to the "set Line" was necessary. It may change your input swr, if so set it to the lowest input swr you can ignoring the "set line".
After re-reading your post, I am not sure that you know that the "dead key" setting on a Black Cat Modulator should be less than 5 watts. Your audio should make it swing to 50-60 watts-maybe more depending on the tube condition. These were made for a very low input (less than 4 watts) just enough to close the relay and a very low unmodulated output.
I am not aware of any component change you could make to lower the input match and raise the output but maybe others will chime in with some suggestions.
73
David
Having owned a few of these in the past, what you are seeing is normal in both input match and power output.
I do recall that changing/adjusting the tuning coil to set the meter to the "set Line" was necessary. It may change your input swr, if so set it to the lowest input swr you can ignoring the "set line".
After re-reading your post, I am not sure that you know that the "dead key" setting on a Black Cat Modulator should be less than 5 watts. Your audio should make it swing to 50-60 watts-maybe more depending on the tube condition. These were made for a very low input (less than 4 watts) just enough to close the relay and a very low unmodulated output.
I am not aware of any component change you could make to lower the input match and raise the output but maybe others will chime in with some suggestions.
73
David
Thanks Dmans. I tried setting the meter pot last night. i did not see any difference in the swr. But i was only setting it to the line. I will try again to see if it makes any difference. I was thinking that the JB12 was the one that done around 50 watts. I have the JB150, I thought i had read that they would do 125-150 peak. I am keying about 2-3 watts into the JB150. About the DK watts out of the 150 i have heard everything. The number that i have read about the most was not to go over 10 watts DK out of the JB150. I want to think I read that here in another thread. but dont hold me to that. I have read a lot about this in the last few days. Thank you very much for your input.
The input circuit that feeds the radio's drive power into the 8417 tube was not designed for low input-side SWR, but to allow you to set the carrier power.
As a result, the input SWR will rise and fall as you turn the small 'carrier' knob on the rear panel.
It was designed to use with base radios that had a tube for the final amp. Tubes tend to be more forgiving about high SWR than transistor finals.
There are two ceramic-disc capacitors that serve to resonate that input coil, one on the coil's input wired across the 47-ohm 2-Watt resistor. The other one is connected from the input coil's secondary winding to ground. The capacitance values used were not always the same from one production batch to the next.
You could 'tweak' the capacitance value of either or both of those disc caps to reduce the input SWR, but this will tend to reduce the effectiveness of the tuning knob as the coil's slug moves in and out of the windings.
The coil serves to reduce the carrier drive to the tube by introducing a mismatch between the radio and the grid of the 8417 tube.
Decades ago I wired a trimmer capacitor in place of each of those two disc capacitors. Found that when I tweaked the trimmer caps for lowest input SWR the knob wouldn't reduce the JB's carrier power very effectively. Diddled with it until I had a compromise between a 'moderate' input SWR and the ability to turn down the carrier. Took the trimmers loose, measured the capacitance and wired in fixed caps close to the values measured on the trimmers.
If the radio has a carrier control, you can use that to keep the JB below 10 Watts carrier output. Odds are you can find a setting on the input-coil knob that gets you down to 2 to 1 input SWR or so.
Probably a better compromise than trying the trimmer-cap 'tweak' method.
Kinda reminds me of a long-selling book called "How to Keep Your Volkswagen Alive. A Guide for the Complete Idiot". Was written around 1970 or so, and detailed how to repair your air-cooled VW, and told you which tools you would need and what to watch out for to do just about any repair you would need.
Except for changing ball joints. Not a word about that. Decades passed, the original author died, and the publisher continued to publish updated editions. Eventually the 12th (or 15th?) edition included a procedure to do this. The book's editor noted they had received numerous requests for it, and finally found a writer to contribute a procedure.
At the end of that section was the question: "There, don't you wish you had spent the same time collecting aluminum cans, turned them in for the cash and hired someone to do this for you?"
For me that answer was "NO". I was glad to have the ball joints replaced and took the opportunity to grease the torsion bars and replace the torsion-arm seals.
But by then I understood what the author meant.
73
As a result, the input SWR will rise and fall as you turn the small 'carrier' knob on the rear panel.
It was designed to use with base radios that had a tube for the final amp. Tubes tend to be more forgiving about high SWR than transistor finals.
There are two ceramic-disc capacitors that serve to resonate that input coil, one on the coil's input wired across the 47-ohm 2-Watt resistor. The other one is connected from the input coil's secondary winding to ground. The capacitance values used were not always the same from one production batch to the next.
You could 'tweak' the capacitance value of either or both of those disc caps to reduce the input SWR, but this will tend to reduce the effectiveness of the tuning knob as the coil's slug moves in and out of the windings.
The coil serves to reduce the carrier drive to the tube by introducing a mismatch between the radio and the grid of the 8417 tube.
Decades ago I wired a trimmer capacitor in place of each of those two disc capacitors. Found that when I tweaked the trimmer caps for lowest input SWR the knob wouldn't reduce the JB's carrier power very effectively. Diddled with it until I had a compromise between a 'moderate' input SWR and the ability to turn down the carrier. Took the trimmers loose, measured the capacitance and wired in fixed caps close to the values measured on the trimmers.
If the radio has a carrier control, you can use that to keep the JB below 10 Watts carrier output. Odds are you can find a setting on the input-coil knob that gets you down to 2 to 1 input SWR or so.
Probably a better compromise than trying the trimmer-cap 'tweak' method.
Kinda reminds me of a long-selling book called "How to Keep Your Volkswagen Alive. A Guide for the Complete Idiot". Was written around 1970 or so, and detailed how to repair your air-cooled VW, and told you which tools you would need and what to watch out for to do just about any repair you would need.
Except for changing ball joints. Not a word about that. Decades passed, the original author died, and the publisher continued to publish updated editions. Eventually the 12th (or 15th?) edition included a procedure to do this. The book's editor noted they had received numerous requests for it, and finally found a writer to contribute a procedure.
At the end of that section was the question: "There, don't you wish you had spent the same time collecting aluminum cans, turned them in for the cash and hired someone to do this for you?"
For me that answer was "NO". I was glad to have the ball joints replaced and took the opportunity to grease the torsion bars and replace the torsion-arm seals.
But by then I understood what the author meant.
73
Nomad, that was exactly what I was looking for. I do have variable power on the radio. I agree just using that would be the best. I have many tube rigs I can use it with and was wanting to use it to modulate a Sonar fs23, so I will not be changing the caps if it will affect the carrier setting on the JB. You have helped me many times before with tube rigs I have. I rebuilt a browning mark 4a with your help. Plus many others over the years. We were in the middle of trying to fix another JB150 I have on CB tricks forum when it crashed. Never finished it. That one has a 7581 in it. Maybe I will start a new thread for it here. Anyway, thanks again
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