[Collins] 75S-1 Problem (update)

[GBabin73 at aol.com]

In a message dated 2/18/2005 8:05:04 A.M. Pacific Standard Time,  
geraldj at ispwest.com writes:
Tube elements are known as cathode, control grid, screen grid, suppressor  
grid and plate in a pentode going from cathode to plate (anode) in that  
order. There's no mica on the screen grid, its on the control  grid.
    
    **It was pretty late when I wrote  "screens."  Yes, you are quite 
correct, it would be in the control  grid. 

There's  not much effect on the tuned circuit from that grid coupling 
capacitor  because at the extremes its just connecting and disconnecting 
the few pf  of the tube grid. Be sure its not the tube changing.
    **That would make sense. It would not affect  frequency all that much. 
I'll try swapping tubes. I have had it running for  the past 2 or 3 hours and so 
far, it's rock stable.

There  were two types of postage stamp micas, and they date from half a 
century  before dipped silver micas. If their case is tan or brown they are 
made  from sheets of mica and sheets of foil. They depend on the pressure of 
the  molded case for stability. If the case is red, they are silver mica 
where the  
electrodes are painted on the mica sheets which means virtually no  
uncontrolled air gaps between the electrode and the insulator which  
generally means better stability. The can still depend on the pressure of  
the 
molding to make contact between stacks of silvered mica.

The  more recent construction of dipped silver mica depends on brass clips to 
 
hold the stack together and to make contact for the wire leads (and those  
brass clips are what make the ends a little fatter than the middle). Then  
the 
assembly is dipped in the coating, without applied pressure. Sometimes  the 
dipping process is too rapid and air bubbles end up inside and that  make for 
capacitors with jumps in incremental value.
    **It is red. Come to think of it, all of the  ones I have had problems 
with were brown.

Often an  intermittent can be forced by alternated heat and cooling though 
it  doesn't begin to act like its temperature sensitive. Intermittent 
connections  
can be sensitive to vibration or to minute length changes of components  
and that's where heat and cooling can force those minute dimension  
changes. Its a lot easier on a circuit to run a few heat and cold cycles  
than 
to unsolder and replace parts.
    **That sounds like the best course of action.

My 9th  edition 75S-1 manual shows 1K for the IF Gain potentiometer. It shows 
some  parts changes in the PTO on an addendum sheet.

My Rockwell 32S-3 manual  shows that 200 pf to be a dipped mica. A 
DM15E201F-300WV 1%  tolerance.
    **My '59 edition shows a 500 ohm and the  actual pot was 500 ohm. There 
must have been a change somewhere down the  line. The 1K pot gave me much more 
latitude. I had the same problem with the  audio gain in my 75A-4, except the 
stage gain increased with resistance and  still rattled the windows with a 
dead short, but that's another story unto  itself.

There is  no ALC in a receiver. That stands for Automatic Load Control in a  
transmitter. The receiver has Automatic Volume Control or Automatic Gain  
Control.
        **Yes, you are quite  correct (I should stay off the computer after 
midnight). ALC (automatic level  control) =TX and AVC (automatic volume 
control) =RX.

Watch  out for that audio coupling capacitor to the output tube grid. When 
it  leaks (and if its still a black beauty, IT LEAKS) it counteracts the 
fixed  bias 
applied to the output but and makes it run even  hotter.
    **I replaced all of the BBOD (black beauty of  death) caps with "orange 
drops" when the power supply filter caps went  south (not to mention the rest 
of the old electrolytics). Bad juju.

I  suspect one of those IF tubes wasn't a 6BA6.
    **They were both 6BA6's.(at least they were marked  as such). Maybe one 
of them was a Monday morning/Friday afternoon special.
 
Thanks for all the good info, Jerry. There is nothing more difficult than  
diagnosing problems without having the piece of equipment in question present.  
You are obviously quite knowledgeable and have been a great source of  
assistance.
    73! DE N5MCJ, Jerry