Don't think that the MC1496 version of the 2nd IF/AF card will solve all problems. I have two TR-7s, one with the MC1496 and the other with the diode product detector. After some other "modifications," the MC1496 IF board sounded somewhat worse than the diode detector card. Namely, if one copies a weak CW signal, a good receiver will output to the speaker a weak signal with some noise. However, the TR7s (and my other Drake receivers before "modification") sounded mushy as if the signal and the noise were intermodulating.
The problem with the Drake design of the MC1496 implementation is the removal of the BFO amplifier used in the diode design. The BFO signal from the PBT/Reference Board is only 80 mV p-p (TR-7 service manual data). That is about 30 mV rms. If one looks at the Motorola Data Sheet for the MC1496 in product detector service, the suggested BFO level is 300 mV rms. The curves for modulator operation show a knee at approximately 100 mV rms for the carrier level where the output levels off above 100 mV rms. To me, that means the BFO should be at least 100 mV rms (280 mV p-p) at pin 10 or about 10-dB greater than what is available. For my particular 2nd IF/ AF board with the MC1496, I changed R1172 to 4.7k from 47-Ohms and inserted a ferrite broadband RF transformer with a 6:1 turns ratio between the pin 11-40 BFO input and the MC1496. I don't recall the exact increase in the audio output level (6-dB comes to mind) but the distortion caused by under injection was greatly reduced. The BFO signal might use more increase but what the transformer and R1172 changes afforded are good enough. Any more increase could be one of those diminishing returns things. There are other changes I have made to my TR-7s to make them sound good. I may be 68-years old but I don't especially like the "soft vacuum tube distortion" sound. Distortion is distortion, period. A CW note should sound like "a note" and not some mushy representation of same. The AM detector in the TR-7 and R-4C also need modification. Without the mods, the distortion is around 10% at 1-kHz tone at 60% modulation level. With my mods, the same distortion is around 1%. Then there's the poppy Drake AGC on CW but that's an entirely different story (for which I have "modifications"). Anyone interested in the schematics for the changes, contact me off the list. The schematics are in the Express PCB format (free from the company and easy and intuitive to use). 73, Bob K9JU
_______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
