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Old 19th May 2020, 7:40 pm   #1
JanWra1
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Default RCA AR88 D - how to improve the SSB reception?

Hi Guys

My 1944 AR88 works fine except that the SSB reception is not as good as it should be. Have read that in the AR88 RCA used an electrostatic coupling of the BFO (V12 pin 4) into the grid circuit of the 3rd IF amplifier (V7).
As far as I can see in the schematics the connection is from V12 pin 4 to connector "A" on the base of L43/L44. and Connector "F" is connected to V7 via C105.
Question 1/ Is this point "A" same as point "A" in the schematic?

Question 2/ Is the electrostatic coupling made in side the can holding L43 & L44? In my radio there is new looking blue wire (see enclosed picture) between points 4 and "A". Is this the original way to do it?

Question 3/ Have read abt inserting a small capacitor instead of the electrostatic coupling, would the correct way to do this be to connect "A" and "F" with a capacitor of abt 2pF (as in CR88) 10pF ( as in SR88)

Have seen a number of views on the best value on the new capacitor so I am thinking of adding a trim capacitor instead of a fixed one.

Appreciate some advice on this.

73/ Jan SM5MRQ
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Old 19th May 2020, 9:46 pm   #2
Radio Wrangler
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Default Re: RCA AR88 D - how to improve the SSB reception?

Hmmm

People have been playing with the value of injection capacitance since the 1950s trying to get better SSB demod. You 'll find that there is no good value. You either lose one thing or another.

Sneaking in a proper product detector and leaving the 6H6 for AM would be a lot better AND would let AGC work on SSB and CW.

There have been numerous designs. I once used one from Short Wave Magazine that must have been published sometime from the mid 50's to the late 60's

David
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Old 20th May 2020, 4:09 am   #3
Bazz4CQJ
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Default Re: RCA AR88 D - how to improve the SSB reception?

I think there are quite a few designs for product detectors to work with the AR88 and other vintage receivers, but it's sometimes the case that people don't want to modify and lose originality.

In that case, it's worth thinking about having an external unit, perhaps sitting on top of the receiver, and including the product detector together with a digital frequency meter, possibly some AF processing, or any other features you might like to add to vintage receiver.

B
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Old 20th May 2020, 10:52 am   #4
TrevorG3VLF
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Default Re: RCA AR88 D - how to improve the SSB reception?

I have a rubbish AR77 which I modified because it had been in a fire.
I changed the 6H6 to a double triode and injected the BFO into the grids.
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Old 20th May 2020, 10:56 am   #5
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Default Re: RCA AR88 D - how to improve the SSB reception?

I suppose that in the way that a few Eddystone receivers were "evolved" from 6BA6/6C4 BFO to 6BE6 product detector (same place, same socket), a 6SA7 product detector in the original 6J5 socket could be contrived for the AR88 with appropriate AF gating. It's something that I'd been meaning to try for years, but as Bazz says, there's always a reluctance to make a vintage item something that it isn't and wasn't! At least it wouldn't involve too much irreversible hacking about.

Colin
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Old 22nd May 2020, 11:08 pm   #6
JanWra1
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Default Re: RCA AR88 D - how to improve the SSB reception?

Hi guys, thanks for your comments. Have now copied the schematic from CR88/SC88 and A/ connected both pin 4 and 6 with connector "A" on the base of L43/L44. and then connected "A" and "F" with a 15 pF capacitor. The result so far is that tuning SSB signals is easier and gives a better sound.

73 Jan /SM5MRQ
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Old 23rd May 2020, 10:10 am   #7
ms660
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Default Re: RCA AR88 D - how to improve the SSB reception?

I fitted an overlength gimmick capacitor in the AR88 I had and trimmed to suit, I liked the Hallicrafters idea of having a pot on the front panel to adjust the injection voltage, that worked well.

Lawrence.
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