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



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








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