Topband: Did you say Collins 75A4

[Herb Schoenbohm]

Carl, The Collins 75A4 was amazing on TB with their excellent mechanical 
filters.  One feature of  the "A4" was that on 160 meters it used single 
conversion and double conversion on other bands.  It was able to 
producea better copy on weak signals in noise than any other amateur 
radio tube type receiver at the time.  The only drawback was that the 
tuning on 160 was backward, i.e. to go higher in frequency you had to 
tune the dial to the left.  Another engineering marvel at the time for 
this unit was the PBT or pass band tuning method.  The PBT control was a 
mechanical strap that actually moved the VFO-PTO body so the pass band 
revolved around the signal rather than the signal aond the passband.  
This way it was possible to move the edge of the passband "right to the 
very edge of the cliff" and drop the interference into the deep notch on 
the edge.  The drop appear not a slope but a sharp cliff straight down 
for 70-90 db while the signal was readable resting on the edge of that 
cliff.

I don't know how better to describe the phenomenon nor have I ever used 
a receiver since that had those signal slicer capabilities.  A refurbed 
75A4 today cost as much as your car and was designed and built over 60 
years ago in Cedar Rapids, Iowa for amateurs by amateurs. (As a teenager 
I lived only 25 miles from the plant in the old Cherry Burrell plant 
where Collins used to open up the wholesale parts area on Saturday's and 
you could buy almost any part not meeting QC specs for just pennies, 
including filters with a scratch or a 4CX250B with a bent fin for a 
quarter.)   I just wonder why no one has built a retro solid state 
version.  Maybe those hand made mechanical filters would drive the price 
to high.   But it sure was a great radio.  I wish I still had mine!

Herb Schoenbohm, KV4FZ




On 3/27/2012 11:32 AM, ZR wrote:
> I have found that the cascaded 800Hz mechanical filters in my 75A4 to have
> less ringing and IMD induced noise than a TS-940 or 950SD with any of the
> 250, 400, or 500Hz selections.
>
> Rather than reducing the RF gain pot I use the front end attenuator and have
> an external one for the A4. None of the above are even close to ideal and
> each has its better nights.
>
> Id also suspect the 50-80KHz LC filtering of the vintage boatanchors to be
> superior but reducing the front end IMD might be a chore without the
> extensive mods similar to the A4. While my R-4C's had extensive filter, AGC,
> product detector and audio mods, nothing was done to the front ends and it
> was never a pleasant radio to use for extended times.....with all its own
> faults the TS-930 that replaced it was less tiring. For about 6 months I had
> one of each.
>
> The NC-300/303 and HQ-170/180 come to mind
>
> Carl
> KM1H
>
>
>