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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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31st May 2020, 6:08 pm | #1 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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RCA AR88D L49 & L50 resistance?
Hi Guys
I noted that a rewound L49 /L50 is on eBay, the seller states that the resistance is 741 Ohm while in my Ar88 D the mesaured resistance is between 380 and 400 Ohms. What is the correct value on these filter chokes? 73 Jan |
31st May 2020, 7:12 pm | #2 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
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Re: RCA AR88D L49 & L50 resistance?
Hi Jan,
Your measurement is right- around 400 ohms. 741 ohms will get rather hot- whoever rewound it needs to address what they are doing. Having a solder-sealed base-plate, I'd be a little concerned about how they got into it, too- perfectly possible with a bit of care but I'd expect "care" to apply to winding specification too! Occasionally, one comes across an assertion that the first choke, L50, has a resistance of 800 ohms- this is incorrect and may have come about through someone's misunderstanding of circuit operation, Colin |
31st May 2020, 8:42 pm | #3 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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Re: RCA AR88D L49 & L50 resistance?
Hi Colin
Thanks för your reply, which means that my chokes are OK and I should stay far away from the one on Ebay! right? Jan |
31st May 2020, 10:37 pm | #4 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
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Re: RCA AR88D L49 & L50 resistance?
I don't want to comment too explicitly on that choke, having found the listing in question, but digital multimeters can get misled by reactive components and give odd results when measuring resistance. Are you uncertain about the condition of the chokes in your set? A resistance check, even with an analogue meter, may not reveal the whole story if a choke has shorted turns, an inductance measurement would give a better indication of condition. ISTR I measured the chokes in mine at 13H each out of circuit with no DC component. Alternatively, if you have an oscilloscope with DC isolation capability to at least 500V, this would give an indication of ripple amplitude at each stage of the filter. Ages ago, I actually went through this aspect of the set when I was trying to teach myself about PSU filtering, I clean forget what values I found and didn't note them down but they did correlate encouragingly closely with theory.
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31st May 2020, 11:30 pm | #5 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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Re: RCA AR88D L49 & L50 resistance?
Thanks for the feedback, I don't have any problems with my chokes as of now the radio works very nice, but I was thinking of buying it as a spare for the future. Will do an inductance test sometime in the future and also check with an oscilloscope. But it will have to wait till after the Corona problem is gone.
Jan |
31st May 2020, 11:48 pm | #6 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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Re: RCA AR88D L49 & L50 resistance?
Colin
Which unit do you mean: Milli Henry or Micro Henry? Jan |
31st May 2020, 11:51 pm | #7 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
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Re: RCA AR88D L49 & L50 resistance?
Just Henry, Jan, whole proper undivided ones!
Colin |
1st Jun 2020, 8:58 am | #8 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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Re: RCA AR88D L49 & L50 resistance?
OK thanks
Jan |
1st Jun 2020, 1:21 pm | #9 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,152
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Re: RCA AR88D L49 & L50 resistance?
The smoothing chokes are not crucial components for the receiver to work.
They can be replaced by resistors. The voltage regulation and smoothing will not be as good. Ripple filters or voltage regulators can be added to improve the power supply. |
2nd Jun 2020, 4:27 pm | #10 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
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Re: RCA AR88D L49 & L50 resistance?
Uneasy confession- 25-odd years ago and kicking heels on weekend standby work, I did make up a series regulator block that dropped in place of the second choke, L49. Eliminating the greedy VR150 shunt-stabiliser for a gently-sipping 150B3 comparator reference saved around 20mA HT current, the mains transformer runs noticeably cooler and main HT is nailed to 240V irrespective. I made sure to use existing holes and it would be invisibly reversible to original in minutes. I can't claim miraculous performance enhancement but it was a harmless diversion.
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