[Penthode]

Run into a bit of a snag with the IF alignment.

Lined up the Audio IF and the last (6th) IF stage with no problem. The first IF stage (6U8) located on the tuner I cannot achieve the RCA published response curve. The coupling transformer between the 1st and 2nd IF stage must provide a wide bandpass. I have stagger tuned the primary and secondary in an attempt to secure the best response. the sweep alignment is supposed to include a 6MHz bandpass with a litle dip in the middle. the best I can provide is a hump in the middle with the low frequency end at 41.25MHz elevated above 45.75MHz.

Using the prescribed input head to couple to the mixer diode and from the testpoint through the detector block through T1, the signal is fed to the 6U8 in a common grid amplifier configuration which then is coupled to the grid of the 6U8 pentode section. The output is coupled to the tuner IF output via transformer T2. The detector block is connected to point T2 at the plate of the penthode section of the 6U8 on the diagram. The response curve is not affected by stray capacitance or moving the cables. It is a good consistent trace. I swapped the 6U8 (with NOS 6U8A's) and there is no change.

I am using for the sweep generator an HP8601a and the marker is it's companion HP8600a. The HP equipment is very precise and perfect for television sweep alignment. The swwep alignment of the detector stage matches exactly the picture in the notes.

With the response I achived so far with this coupling, the poor response adversely affects the subsequent sweep alignment from the mixer to the second IF stage as prescribed. Hence I have gone back to investigate further.

My plan next is to pull the tuner top and have a look at the transformer and components around it. The great thing is I may not even have to disconnect any connections as the complete tuner chassis holding the RF amplifier, mixer, oscillator and 1st IF stage is easily removed from the tuner box by removing a few screws.

Curious note: the rest of the curves depict low frequency to the left and high frequency to the right. You would normally get a reversal when the signal is hetrodyned or mixed with a local oscillator with frequency about the desired incoming. In this case no hetrodyning is done so why do the notes show a reversal? I checked Sams notes and they copied the RCA diagram exactly.

Whether you are familiar or not with this tuner, I would appreciate your comments.