The AM Forum

Hi,

I have heard there are some good mods out there for the 51J-4, but Google has let me down on this one.  I really can't find much.  Anyone here know of any worthwhile mods for this fine RX?

73,
Mike WB0SND

First Google hit - Lots of great 51J info, mods, updates, etc:
http://site298.webhost4life.com/barryhauser/archives/HSN-Hammarlund.pdf

Oh, I did find that one, thanks.  I guess I should have been more specific.  I heard somewhere there were some Colonel somebody or other mods which included something like the Pullen mixer mods on the 75A4.  I was looking for those mods.  Sorry for not being more specific.  I think I was up a little too late the night I posted this!

Did the Bill Orr product detector mod. on mine years back.  Its a good one, IMHO.

Since it's such a "fine RX", there probably wasn't a need for improvements other then the product detector.

CQ and QST published a range of mods in the 60s and 70s for those rigs.
I've found a few articles on the subject... they range from changing RF tubes
through detectors.

Today the biggest bang for the buck mod has to be the Softrock or similar I/Q board taking the 455kc IF or 500kc IF from before (or bypassed) filter and running SDR software...

                         _-_-bear

I should probably just leave it alone.  Seems to be a bit noisy so I thought maybe a Pullen type mixer would help.  I have installed the Treetop Circuits product detector which is a very worthwhile addition for very little money or effort.  SSB/CW reception seems noisy to me, though.

The J4 was anything but an ideal receiver as that wasnt its intended use.

Besides the product detector the AGC isnt good for CW/SSB, the audio is comm quality marginal, it overloads easy and its noisy.

What do you want to use it for?

Carl
KM1H

Ha Ha!  I guess you're really right.  While the product detector improves the radio greatly over stock for SSB/CW, it's still not very good for SSB/CW compared to the 75A4.  I don't know why I want to put much effort or money into it.  I probably should leave it be.  It's a great general coverage RX.  I use it for AM mostly as in order to use my 75A4 for AM I have to pull it out and change the filters.  My 75A4 is filtered up for CW, so until I build a filter switchbox for the 75A4, I use the 51J4 for AM.  It actually works pretty well for that application.  And besides, it looks good with the rest of the Collins stuff (KWS1, 75A4, SC101, 32V2 & 51J4)!

I havent had mine on a real antenna in decades as its stock and has horrible overload.  Its used in the shop on AM BCB most of the time with a 20' basement wire. It does get used for finding signals from whatever Im working on; sort of a glorified frequency meter/selective voltmeter.

Carl

When I saw the Bill Orr mod in HR back in '78, I tried calling him at Eimac (Ithink that's where he was working) to discuss it and was surprised to find out how easy it was to do so, and also how willing he was to talk about Ham Radio.  He said the values in the circuit were more of a starting point and to experiment with what worked best for you.

We also discussed getting the DC plate voltage of V301 off the mechanical filters input coils. Breakdown from age would kioll the filter. The agreed to fix was to shunt fedd the DC to the plate with a 1 to 2.5 mH choke and use a .01uF coupling cap. It was also agreed to isolate the output side with again a .01uF cap and the 1Meg shunting the grid of V302 to ground. This eliminates the annoying loud clicks when changing filter selection.

The front end over load and blocking problem is due in part, to the use of the 6AK5 for the front end amp. Though is has high gain for reduced noise, it is a sharp cut off type and easily overloads. There are a number of remote cut off types one can experiment with, but you want to have high gain. Collins uses the 6DC6, a semi remote tube, in other radios. Just note that changing out this tube will affect AGC and therefor S meter readings. The stock radio tracks dB scale very well. When I lived in Southern NJ, a local about a 1/2 mile away would poke holes in my 75 meter audio with his 80 meter
CW. Was the wierdest effect.
I have not messed with the stock AGC, but have noticed that power supply hum on the AGC negative supply voltage modulates the received signal. If the power supply is not filtered well enough, there could be some audio ouput getting back to the RF/IF signal and modulating it....

Since I have now paired up the 51J4 with the "new" Valiant I will probably want to revisit some rework. The link provided by Pete has some interesting ideas in it.