[Collins] 75A-4 BFO

[Dr. Gerald N. Johnson]

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.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer