Re: {Collins} [Collins] 51J4 Alignment Question

----- Original Message ----- From: "Grant Youngman" <nq5t@xxxxxxxxx>
To: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
Cc: "Collins@xxxxxxxxxxxxx" <collins@xxxxxxxxxxxxx>
Sent: Wednesday, August 21, 2013 10:04 AM
Subject: Re: {Collins} [Collins] 51J4 Alignment Question

Just to follow up on my question. My receiver is an early one, and does not have the IF gain control.

Finished the alignment of the receiver, and measured the gain from antenna terminal to IF output. 3 uV RMS in at the antenna terminals yields 250 mV RMS at the IF output port. The alignment spec for receivers with the IF gain control is 270mV out. I figure that's close enough not to worry about. I think someone also indicated that the IF output signal was loaded down by the detector and was badly distorted and not very sinusoidal (?). I did not find that to be the case.

Which still begs the question … why did Collins feel the need to add the IF gain control in the first place? I can only suppose it had to do with the fact that some contracting officer put it in a specification somewhere along the line.

Grant NQ5T

I've been reading over the alignment instructions for the 51J4 getting ready to touch mine up. In manual section 5.3.17 (1962 manual), it gives instructions for setting IF gain. My question is whether the input (antenna terminal) and output (IF output) voltages indicated are peak or RMS. I presume it doesn't actually matter as long as both measurements are taken the same way, but thought I would ask. I think I've also seen a comment in a posting somewhere that it isn't worth worrying about and that the IF gain should just be set to maximum. Any input on this?

Its hard to know without having definite documented information. One thought is that it had to with compensating for losses in various mechanical filters but that doesn't quite hold water since there were three filters in the 51J-4 and I don't think other receivers with mechanical filters had gain trimmers. The other thought is that it was added to allow matching gain in receivers used in diversity set-ups. The 51J-4 had the other connections for diversity; AVC and detector load so were evidently sometimes used for this. In diversity its necessary to "best" the receivers so that the receiver with the highest gain does not dominate. The range of the trimmer is evidently pretty wide but the gain trimmers on the SP-600-JX17 and RCA AR-88F also have a lot of range. This is all speculation, perhaps someone on the list knows for certain.

Richard Knoppow
Los Angeles

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