15-11-2016, 10:56 AM
(This post was last modified: 15-11-2016, 11:03 AM by Terry.
Edit Reason: AF detail added
)
Is the AF feedback supposed to be positive, I wonder?
Perhaps a production error reversed the connections to the primary of the driver transformer, switching the desired negative feedback to positive!
As for the pot in the IF circuit, the diode is intended to switch the 680Ω effectively in parallel with the primary of IFT1 to cause damping in the event of particularly high signal levels being received. The pot adjustment is, to me, the unusual bit.
Interesting to note that, in two otherwise identical IF designs, the position of the pot moves from secondary to primary! In both cases, however, it determines the point at which the diode conducts. From a DC point of view, the diode is between the two collectors but the KB circuit adjusts the collector voltage of TX1 whilst the Regentone alters the collector voltage of TX2 by adjusting its bias!
If you can reliably date both designs it should be a pointer as to which version works best!
Update: I didn't notice the two AF circuits but have now looked at them. I suspect that the KB design came first but when the design team decided to bring the first AF stage into the loop they forgot to tell the PCB designer (assuming it is on a PCB!) to reverse the phase of the transformer ...
Perhaps a production error reversed the connections to the primary of the driver transformer, switching the desired negative feedback to positive!
As for the pot in the IF circuit, the diode is intended to switch the 680Ω effectively in parallel with the primary of IFT1 to cause damping in the event of particularly high signal levels being received. The pot adjustment is, to me, the unusual bit.
Interesting to note that, in two otherwise identical IF designs, the position of the pot moves from secondary to primary! In both cases, however, it determines the point at which the diode conducts. From a DC point of view, the diode is between the two collectors but the KB circuit adjusts the collector voltage of TX1 whilst the Regentone alters the collector voltage of TX2 by adjusting its bias!
If you can reliably date both designs it should be a pointer as to which version works best!
Update: I didn't notice the two AF circuits but have now looked at them. I suspect that the KB design came first but when the design team decided to bring the first AF stage into the loop they forgot to tell the PCB designer (assuming it is on a PCB!) to reverse the phase of the transformer ...
