A Very High Performance 75A-4
In this photograph, you can see evidence of some of the modifications. Look closely at the left side of the chassis. All of the tubes have added labels indicating the new type of tubes used. At the lower left are the mechanical filters. You'll see a label placed over one of the filter slots. That's because a 2.1 KHz mechanical filter is mounted below chassis. The other two installed filters have bandwidths of 3.1 KHz and 1.5 KHz. Thus, three ideal SSB filter bandwidths are available, 3.1 KHz, 2.1 KHz, and 1.5 KHz.
Drawing your attention the right side of the chassis, you can see a square plate just to the right of the large electrolytic can. Mounted on this bracket is a 3.1 KHz mechanical that is always in the circuit. Replacing an i.f. transformer, it significantly improves the ultimate attenuation of the filter system and creates far sharper filter shirts than would be possible with only one filter in the i.f. strip. A very nice feature of this particular 75A-4 is the 3.1 switchable mechanical filter paired up with the 3.1 KHz mechanical filter permanently installed father down the i.f. strip. The combination of two filters produces a bandwidth of about 2.8 KHz at -6 dB and a 4.8 KHz bandwidth at -60 dB. This permits a wider -6 dB bandwidth to be used even under adverse band conditions. The result is much improved audio from received stations, while QRM is still eliminated by the sharp filter skirts. Of course, it is common to use two or more filters in cascade in modern transceivers.
That is about all of the modifications that are visible from the top side of the chassis. Most of the work is done on the underside.
If you are unfamiliar with the 75A-4, part of its magic is the mechanical coupling between the PTO (the variable frequency oscillator which sets the frequency) and the BFO. You can see a long shaft running from the BFO "can" to the front panel. The BFO "can" is just in front of the power transformer. Notice the brass strap which links the BFO shaft to the PTO. When the BFO frequency control (normally called a BFO pitch control in other receivers) is rotated, the PTO is mechanically "rocked" in its mounting. The mechanical tracking is nearly perfect, it's within 50 Hz. When the BFO shaft is adjusted upward 1000 Hz for example, the PTO is moved exactly the same 1000 Hz but in the proper direction to insure a zero change in the "pitch" or frequency of a carrier. Thus, mechanical passband tuning is achieved. It was done first in the 75A-4 and is a feature of every quality rig ever since. The fact is that this mechanical system is more effective than many receiver that use two or more mixer stages to effect the same action electronically. It's just one more feature that makes the 75A-4 exceptional.