This is a transcription of a letter sent by Captain Paul Lee (W3JM) to a ham, dealing with the 51J series of receivers. I encourage reading it, as it shows the different models, their characteristics and the applicable and recommended mods.
Here is a copy of the article from April 1961 CQ, which I wrote. I did this to two 51J2 receivers and still have one of them, which works fine. The AVC action is very good.
To give you the dope on the various models. The 51J and 51J2 were early models, some of which had no antenna trimmer. They have r.f. input circuits which use r.f. transformers, with an antenna primary winding which can be connected either for balanced line input or coax input.
The 51J3 (R-388 military designation) is like the others, but has no r.f. input transformers. The input consists of coils L101 thru L106, with no primary windings. The antenna, if you trace the circuit, goes to the grid of the first r.f. tube thru a small fixed capacitor. This model was a Signal Corps procurement designed for use with a short whip antenna.
The 51J4 is like the 51J3, with no r.f. transformers, but it has the mechanical filters built in. It also was intended for use with a short whip.
The 51J, 51J2 and 51J-3(R-388) can be converted, by means of a filter kit which Collins sells for a couple of hundred dollars, to use mechanical filters. The mod is to remove the first i.f. stage and replace it with the kit.
Now, in the case of the latter models with no r.f. transformers, it has been found that if you are near a b.c. station, or other stations which put out a high r.f. field in your area, and you are using an antenna of appreciable size, cross-modulation will occur in the r.f. stage due to the lack of r.f. input selectivity. This is their one fault. I don't know what antenna you have, but if you use a long wire, inverted V or something of that sort, and have b.c. transmitters near you, you will probably get some 'birdies'. Also, if there is a nearby ham with a strong signal, you may find yourself unable to reject his signal completely due to this. On the other hand, if you are going to work 10, 15 and 20, and use a beam, you will probably be okay. I don't mean to discourage you, but you should know the facts about your set. The 51J-2 and 51J earlier models are okay in this respect - I have never had any problems with front end overload or cross-modulation with mine. The tuning is very sharp.
One other mod which will help the audio quality is to remove C212 across the primary of the output audio transformer.
I recommend the 3.1kc filter if you get the mechanical one. The 2.1kc filters sound horrible in any set I have heard with one in it - this goes for most ham gear. My xtal filter mod is about 3kc bandwidth.
See April 1968 CQ for the xtal filter and dual triode mixer mods.
Captain Paul Lee, USN