{Collins} Dial slip and power supply hum...

[Jim Hendershot <wa6vqp@xxxxxxxxxxxxxxx>]

Hello to the group!

Well, a happy ending to both of my S-Line issues...

First, my 75S-3 dial slippage problem is cured.

Ummm...

Nothing like reading instructions to get a person back on track.  The  
problem started with a sticky VFO.  Lots of dial slippage, lots of  
backlash.  So I got a set of Bristol wrenches, took everything apart,  
cleaned and lubed the VFO, and presto!  Backlash gone, slippage gone!   
Well, sort of...

It didn't take long for the slippage to reappear.  So I adjusted the  
offset nut behind the tuning knob, which fixed the problem for a  
little while.  Emphasis on little.  What I should have adjusted was  
the nut on the forward side of the tuning knob!

So yesterday, I finally cracked open the unit, this time with the very  
helpful assembly and disassembly instructions found on the website  
(thanks, guys!) and discovered the problem.  Well, two actually...

1)  The previous owner, who also had trouble with the VFO backlash,  
had helpfully sprayed some kind of silicone into the mechanism.  A  
gentle cleaning with warm water and dish soap helped immensely.  The  
whole time I was doing it I was scared to death, not knowing how well  
the silkscreened dial markings would hold up.  I'm happy to say they  
held up nicely.  The bonus:  The dial looks much better now that it is  
clean!

2)  The previous servicer (me) didn't read the instructions on  
assembly/disassembly, and the mechanism was not put together  
correctly.  What happened was that the shaft assembly was not  
straddling the dial ring, but merely pushing against it.  Depending on  
the alignment of the chassis, phase of the moon, or whatever, the dial  
would slip, or not, making operation erratic.  The correct re-assembly  
procedure addresses the problem quite nicely, and all now works well,  
and, I must say, at the minimum tension setting of the offset nut.   
The unit is now truly a joy to tune!

The moral of the story:  Read the instructions, even if it isn't the  
manly thing to do!

Now onto the more mysterious issue, the small AC hum in the power  
supply when the unit is turned off.

First of all, thanks to the many of you who suggested ideas on where  
to look.  Very helpful!

What did it turn out to be?  Well, disconnecting the rig from the  
supply killed the hum, so I figured that there was something going on  
there.  Turns out the power supply, which I modified to add a relay to  
save the switch (which I had to replace, and believe me, that  
unobtainium stuff is REALLY hard to find, hence the relay) was  
humming...

at the relay. Yup!  The relay has a slight hum.  An investigation  
revealed that there are a pair of 0.01 uF caps on the switch, and that  
just enough leakage (about 280kohms at 60 cps, if my math is correct)  
to make the coil hum, ever so slightly.  So the hum turns out to be a  
non-issue, and all is well.  I gave some thought to removing the caps,  
but the only way to notice the hum is to put your ear on the cabinet  
when the shack is completely quiet, and so I think I'll leave them in  
for now.

Again, thanks to all who responded to my question on the issue.  Your  
responses were most helpful!

73,
Jim