[R-390] R390 Dave N3DT, ask and then what is the input Z of the IF?

[Roger Ruszkowski]

Dave, We do not know the Z. We have some engineered answers that work well on odd Mondays. The last mixer of the RF deck has a magic transformer tuned near 455 KHz and is adjusted for maximum signal transfer in the plate circuit. The load side of the transformer couples what ever it perceives as its loading resistance right back on the transformers primary. Transformer ratio is designed for maximum voltage gain because it is only the voltage on the grid of V501 that makes the difference in receiver output. I can accept an input grid impedance of a couple meg ohms. The 6C4 mixer has a plate resistance under a 100K. The RF deck to IF deck coax is less than a relative fraction of a wave length long and its characteristic impedance is of little real indication to the impedance for the input of the IF deck. The coax is simply a conductor and shielded to keep the signal from being intercepted else where. The shielding does not prevent signal loss. The center conductor looses the signal. The shielding shunts the lost signal to ground. Thus the lost signal is not radiated.  Some repeatable test setup values ignore any match between the generator and the input impedance of the IF deck. We can see a 150 micro volt signal on the oscilloscope and set the system loop gain to a DC voltage reading taken at a select point in the IF deck circuit. We jam a signal into just one side of two on the IF deck and do an alignment. So even the IF bandwidth switch setting changes the circuit.  In the school house we never considered there to be a relevant Z in the circuit. The circuit in operation has no external inputs or outputs and thus has no reason to have it's exact Z specified. We jack the modules open on the coax connector and jam a signal in for testing. If those BNC connectors were not such convenient inputs we would never consider the Z between mixer plate and amplifier grid. The design test was to inject 455 KHz into the grid of the third mixer and monitor it through all the circuits of the IF deck. The mixer tube grid is a high impedance input. You may want to pull some of the RF deck tubes  to reduce noise on the grid of the third mixer from the second mixer. Or the second crystal mixer tube can be removed. The VFO needs to be cabled up but can be unplugged on the power and filament (a side effect is no BFO if the test chain extends that far along the IF deck circuits).  This is not the Z point you are looking for. However, the engineers who designed these circuits offered up their rational in peer review and sold it to the Military. When the military ask for a number they had something imaginary but Arabic in mind. What Collins engineered delivered was more of a song and dance vaudeville number. Starting with standard part values in the supply chain, using no special parts, design a circuit that best matches a 6C4 plate to a 5759 grid at 455Khz. Connectors and coax in the middle were some concession to manufacturing and maintenance requirements. 455 KHz was and is just a standard wicket to be played against by everyone. Dave, If not for the modularity of the receiver we would never even consider the Z of the match between the third mixer and the first IF amplifier. And we just know 50, 52, 70, 72, 75, 120 are not right answers. Respectfully, Roger AI4NI