[Collins] 75A-4 BFO

[wabate]

Glen, Jerry and Bill,

Thanks for your input.  I don't think I have perceptible backlash but I 
will check into it.  The frequency does not jump and the change in audio 
frequency is very repeatable so I don't think its mechanical.  The cal 
signal tuning is very smooth and repeatable as well with the pto tuning. 
  Sure wish it were mechanical.

Jerry, I checked my HR index but could not find anything on bandpass for 
the A4.  The end point is pretty close and the number of turns between 
100 KC points appears right on in most cases.  Its only this first and 
last 100 KC or so that is off.  Its that darn compensating stack.  Its 
only on 10M that it comes into play.  So I don't think I want to get 
into that, at least not right now.   So it looks like I will attack the 
bandpass alignment.  Bill, thanks for letting me know that at least one 
other person has done it!  Might not be able to get to it right away. 
Got work and then grandchildren coming over.  Mixed blessing! HI HI..

73,

Bill, K3PGB

Dr. Gerald N. Johnson wrote:
> On Sat, 2008-07-05 at 15:18 -0400, wabate wrote:
> 
>>First thanks to those who helped me with my earlier question on my A4 
>>meter.  Looks like the meter is stock but the clip that holds the pilot 
>>light is missing.  I fabricated one.
>>
>>The next order of business is the passband tuning.  It does not track 
>>the main tuning well.  The spec I read says that the frequency should 
>>not change more than 50 HZ when tuned throughout the range.  Is that 
>>realistic?  Mine is much worse than that.  Probably a few hundred HZ. 
>>I've read the alignment a half dozen times.  I think I follow it OK.  I 
>>tried recentering the knob but that did not help.  I tried adjusting 
>>L-31 as the simple approach and that did not help so it looks like I'm 
>>in for the "difficult" approach.  I don't have the 6 KC filter so I will 
>>use the 3.1 and limit the tuning range to 1.5 instead of 2.5.  I will 
>>hook of a frequency counter to the phones terminal and start tweaking.
>>
>>Anyone done this before?  Tips?  I have not seen any write ups on this.
>>
>>Thanks,
>>
>>Bill, K3PGB
>>
> 
> I think there was a write up in ham radio magazine long ago about this.
> First thing I'd check and fix is the endpoint spread on the main PTO.
> This IF shift works if the tuning rates on both PTOs are the same per
> degree of shaft rotation. Of course bandpass tuning rotates the main PTO
> case. Next thing I'd check would be the snugness of the tuning links
> between the bandpass shift and both oscillators.
> 
> And I'm sure Glen Zook is right that the main tuning friction needs to
> prevent the tuning knob and shaft from moving. Perhaps the main PTO
> drive and screw needs cleaning and lubrication. Many a PTO has needed it
> once decades ago already.