[Collins] 51J-5

[Dr. Gerald N. Johnson]

On 4/14/2010 8:14 AM, kirklandb at sympatico.ca wrote:
>
> Not sure what is meant by the pass band tuning working properly but I am guessing
> they had "problems" because of the mixing arrangement in the 51J series.
>
> Asides from bands 1-3, the 51J uses one crystal for every even/odd pair of bands.
> This is done by using the PTO to on the low side of mixer frequency for one band, and on
> the high side of the mixer for the other frequency.  This is why the KHz dial is calibrated in
> both clockwise and counter clockwise directions.

Nope. You are confused. The 51J-3 in my rack tunes clockwise to raise 
the frequency on all bands except bands 2 and 3. And all bands have 
black scales except bands 2 and 3. Bands 2 and 3 are the IF bands hooked 
directly to the antenna. I checked with HP8640B signal generator to 
maker sure. MHz + 100 KHz, came the same place on the 1 MHz coverage on 
all bands but 2 and 3 where I had to tune 200 kHz backwards as the 
slider scale indicated the same mechanical position was the MHz minus 
100 kHz.

The sideband selection IS opposite on even and odd bands. To check that 
I set the BFO to center (pointer straight up) and tuned a 7.100 signal 
from the signal generator to zero beat. Then I turn the BFO knob 
clockwise 30 or 40 degrees. I had to tune down in frequency on the main 
tuning to get back to zero beat. On 8.1 it was opposite. Or the other 
way around. Any way definitely opposite.
>
> As I recall from a 75A4, pass band tuning is done by literally mechanically shifting the PTO (ie. rotating it).
> Obviously a shift of  +1KHz on one band will result in a shift of -1KHz it's paired band.  This is also
> what makes USB/LSB operation so much fun on the 51J series - you have to alternate the BFO position
> between bands to stay at USB (or LSB).

In the 75A-4 pass band tuning is accomplished by tuning BFO and PTO at 
the same time and rate. The PTO was mounted on trunions so the case was 
turned, instead of making a complicated linkage to the shaft. This with 
the double use of crystals as done in the production 51J family would 
cause great difficulty, unless the two variable IFs and the double use 
of crystals was abandoned. E.g. one crystal per MHz and only one IF 
range. Though it would not be so impossible to have used double 
conversion on half the bands and triple conversion on the other half of 
the bands to make the variable IF move a MHz or two while using crystals 
for two bands. The added conversion would make it more difficult to have 
uniform receiver performance because, especially in tubes but true in 
solid state as well, the mixers are the weakest stages in the receiver 
injecting the most noise, having the lowest gain per active device, and 
limiting most easily to limit dynamic range at both weak signals and 
strong signals.

In the 51J-5 passband tuning would have a chance of working, but which 
direction it tuned would be reversed band to band IF the crystals were 
used like the production 51J which isn't possible to have been limited 
only by alignment. At least not easily. A differential could reverse the 
direction but would introduce backlash unlike the bronze strap in the 
75A-4. But even then likely the USB/LSB indicators on the pass band 
tuning might have to be changed each band.

The 51J-4 used 500 KHz mechanical filters so that collection would have 
been handy for the 51J-5.
>
> Bands 2,3 of the 51j series use the PTO to directly mix with the incoming RF to get a 500 KHz IF.
>
> Band 1, uses triple conversion.
>
Reverse tuning of the IFs could have been set to tune the IFs the same 
as the RF, but likely the birdies from harmonics would have been far 
more severe and probably on some bands would be all across the band, not 
nice for listening.
73, Jerry, K0CQ, Technical adviser to the Collins Radio Association.