[R-390] 2nd osc again with T401

[Roger Ruszkowski]

Dave, Larry pointed out"T401 should be adjusted so that all the trimmers will peak correctly.  Remember, when T401 is changed alltrimmers need to be adjusted." The science has not changed and the statement is still true. The 70 year old manuals offer a tuning procedure for T401 that is now a 70 year old procedure. Ops someone let age loose on reality. T401 is usually just peaked against the crystal calibration tones on each megahertz.This makes everything relative and you can not compare output of crystal A to crystal B because the circuits are not the same.You re tuned the RF deck when you  twisted the megahertz change knob.If the cap for the megahertz has two peaks then,T401 is in range.The cap has sufficient change to cross the peak coupling on both sides.The cap is not set to max.The cap is not set to minimum.The cap has range to peak the circuit.The crystal, cap, transformer are all good to go. In step one of peaking the second crystal oscillator deck for each megahertz tune up and downthe whole 31 Megahertz of choices. Remember the poorest 5 megahertz.Check the poor five for use on more than one crystal harmonic.If one of the poor 5 is used on more than one megahertz then each of its harmonicsmust also be checked. So you may have your 5 poorest crystals servingon 15 different megahertz. Not like likely, but it could happen.No it can not, there are only 15 crystals in the deck. Do not try and start this recovery process by peaking T401 to thesorriest crystal in the deck and working back to the optimum performance yourreceiver will yield. This step is a gut feel.Know what you want to hear.Of your poorest 5 megahertz which one do you have the most interest in listing to? On that megahertz. You are going to run a series of test.Spin T401 down two turns. {I said spin down CW to some, this is a screw job not a frequency change or tune.}Spin the cap for the megahertz and peak. If T401 is in range the cap will have two peaks points.The peak points may not be equal so remember which side is up. If T401 will peak on the two points this is not the crystal you are currently seeking.Just move on to the next candidate of 5 megahertz to inspect at this point in the process sequence steps. If T401 is not in range, adjust T401 1/2 turn CCW.Spin the cap for the megahertz and peak. If T401 is in range the cap will have two peaks points.The peak points may not be equal so remember which side is up. You are going to repeat this search step on each of the 5 megahertz bands you rated as the receivers poorest 5 megahertz bands.T401 will keep coming up and crystals will each peak twice. Do not look back at this time. By the time you reach the bottom with the poorest megahertz the receiver will have 1 of 3 conditions.Condition 1 Good news T401 is setting at some slug position.Bolt length means nothing because we can not see the ferrite core hanging in a field.Ferrite core density way not be as uniform or random as it was years ago. Now just spin the receiver across the megahertz bands and peak the caps. I liked to spin the Kilohertz to 500 KHz.You get a feel for how far each megahertz is off at mid band.If the cap on each megahertz peaked twice and you left the cap tweaked to the best of the two peaks, the second oscillator deck is good to go.On those megahertz where you align an RF octave, then use that test setup and get the cap on peakwith the best meter in use. On the other megahertz, take the time and use the generatorrather than just the calibration tone to peak the second crystal oscillator deck. Condition 2 Almost Good News.There will be one or two megahertz where the cap did not offer two clear peaks.Once you get to testing with a signal generator you may find the megahertz cap has two pointsvery close together. This indicates this cap is at the end of its min or max range.The receiver is OK on this point. Condition 3 More work ahead.There will be one or two megahertz where the cap did not offer two clear peaks.Once you get to testing with a signal generator you may find the megahertz cap still does not have two peaks. This indicates this cap is at the end of its min or max range.The receiver is NOT OK on this point. T401 is where T401 is because a broad band alignment procedure found it a place to be.We settled one of two variable in the equation.(Using a shot in the dark process that has served well over time and not worth reinventing.)We peaked the second of two variables against the first variable.1 of 15 crystals just barely accepts T401 where it is.13 of 15 crystals will accept T401 where it is.1 of 15 crystals is still standing off. If you looked back in this process, you keep finding the outlier and never let T401 settle,you are testing against your last change. Whatever the megahertz where T401 was last twisted is one end of the range for T401.One (or more) caps just will not peak up with the preferred double peak.  The problem can go three ways here also. Adjust it correctly up, Adjust it correctly down, Isolate a specific problem.Remember where T401 is currently adjusted to.Set up the  failing megahertz for test and  peak the cap.Move the cap ever so slightly off peak.Run T401 1 turn up and 1 turn down.If T401 peaked clean some where in that range, you have your second limit.The first setting of T401 is one limit goal in bounds.The current setting of T401 is the second limit goal in bounds. T401 is an impedance transform between a plate and a cathode. Hope fully there is a L some where in T401 that will let the cap for each megahertz peakthe circuit up somewhere within the range of the cap C for each crystal frequency.We know C is in range because the mechanical design of C has two points of theoretical equal value.Which we find not to be exactly true in the real world. The circuit peak is simple the best impedance match .With the input at the antenna, and meter on the diode load, thetest is not close to impinging on the dynamics of T401. You arrived here with 14 of 15 happy crystal circuits.number 15 is not that far out of range. Number 15 may in fact be working on its other harmonic ('s). So the general setting of T401 was not close enough for every megahertz.But you have two limits for T401.You know which two specific harmonics on which megahertz are on the opposite ends. Can you find a place on T401 where you can remove the technician and resume normal receiving? Now finish the alignment procedure on the entire receiver.Just because a specific megahertz cap has only a sort of soft peaky point almost kind of is not necessarilythe long pole in the tent. The signal to noise on the megahertz may be excellent even as we knowthe oscillator alignment was not all that crisp. The flip side is you do not have a good clean 20 : 1 all across the megahertz and you know the second oscillatordeck has the weak peak then you know some serious additional work is ahead. In the last quarter century we have on yet observed any failure patterns in the crystals.Moving a receiver and cracking the plating to the quartz from a mechanical shock is the normal loss.Mice and weather corrosion are more mechanical and repairable. Over the years in my tear down for the picnic table bathing I have found loose bolts in the oscillator chassis. I have had to take more than one deckfurther apart than I wanted (required soldering) to tighten upground lugs. I had one loose bolt that shorted B+ to where else, ground. Dave, The 6AK5 experience and the antenna trim are normal.Here again with the tubes pick the one that gives you the best end to end signal to noise ratio.The first oscillator hums 17 megahertz with out switching wires.The second oscillator has 15 choices and additional wires.6AK5's prefer 31 Megahertz.The first oscillator was engineered to keep a 6AK5 humming at 17 Megahertzwith a more measurable voltage transform (less sensitive to measurement). Cherry picking tubes for signal to noise is what brings these receivers fromwell aligned to very well aligned some where over 20 : 1. Tune the receiver to 31 MHz Swap the 6AK5's into the second oscillator where you can get your fingers on the little things.Pick the best one for signal to noise ratio and install it into the first oscillator.Pick the second best one for the second oscillator.Mark choice 3 as spare #1. Why do signal to noise on the 6AK5 oscillator tubes at the high end of the receiver? Because Dave, reminds us by test and experience, as frequency goes up the 6AK5's usable output goes up.Engineering tells us noise goes with efficiency. If the tube is low noise at 31 MHz it will be quite below that frequency.A quiet 6AK5 at 17 MHz may not be as quiet at its second harmonic on 31 MHz This is a cherry picking exercise with tubes.Part of the pick is knowing where to pick. T401 is knowing where to pick a point where all capacitors cross.Complete an analog solution in two adjustments.   I agree with Larry, Thank you Dave for the report. Respectfully, Roger AI4NI