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 Post subject: Collins 51J-4 vs R-648 receivers
PostPosted: Oct Fri 23, 2020 5:00 pm 
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Joined: Jan Tue 07, 2020 1:41 am
Posts: 3567
Location: Fenton, MI 48430
I bought a very good condition 51J-4. I also used a Collins R-648. I am interested in which receiver is the better performer. One member stated the 51J-4 and especially an R-390A are superior to the R-648. I have no idea of the R-390A as it been to many years since I owned one and are not side by side. I can state the the 51J-4 and R-648 are about equal receivers on 75 meter weak AM signals. Both have the same mechanical AM filters, but the R-648 is very narrow on SSB at 1.4 kHz whereas the 51J-4 is 3.1 kHz. It is hard to beat a dual RF front-end in the R-648 and it is very quiet. A little superior noise wise to the 51J-4, however the 51J-4 is so good it does not make much difference. The R-648 has a far superior noise limiter, the 51J-4 is effective, but not on lightning strikes like the R-648.
Per Collins radio the R-648 was designed off of the 51J-4 and is of later design. I prefer the 51J-4 due to having a crystal filter and full coverage from AM broadcast to 30mHz. The R-648 covers about 150kHz to part of the lower AM broadcast band (not specified for any AM broadcast) and 2 to 25mHz. Either of these receivers are far superior to the average ham radio receiver. I will use the Collins 51J-4 as my main receiver. I did sell the R-648 to a serious collector and that makes me pleased to share vs hoard rare equipment. Both radios are expensive. 51J-4 receivers show up occasionally starting about $600, same for R-648 with dynamotor. I never seen a R-648 converted to 120 volts AC, but I converted my R-648 and had an extra dynamoter to return to factory stock. Here is the 51J-4 pix and not even cleaned up yet.


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 Post subject: Re: Collins 51J-4 vs R-648 receivers
PostPosted: Nov Sat 07, 2020 10:29 pm 
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Joined: Jan Tue 07, 2020 1:41 am
Posts: 3567
Location: Fenton, MI 48430
I found a 9.4kHz and 6kHz filters. I like the 9.4kHz and the 6kHz or 3.1kHz hearing one AM sideband helps with SSB signals as close as 2Khz away. to AM frequencies. This receiver is far superior to conventual Ham tube type receivers.

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 Post subject: Re: Collins 51J-4 vs R-648 receivers
PostPosted: Nov Wed 18, 2020 7:24 pm 
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Joined: Jan Tue 07, 2020 1:41 am
Posts: 3567
Location: Fenton, MI 48430
Rebuilding the Collins 51J-4 is easy. It has seven .1uF @ 600 volt 'bathtub' caps, two 20uf @ 150 volt 'bathtub' caps, and two 8uF 350 volt 'bathtub' caps. I used .1uF @ 400 volt, 22uF 200 volt and two 10uF 400 volt caps. It has about 36x .01uF bypass caps, but being ceramic every cap measured over 50 meg ohm thus do not need replacement. After my rebuild the AVC worked correctly.

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