[R-390] A Few R390A Issues Zero Adjust

[Flowertime01 at wmconnect.com]

In a message dated 11/16/2007 11:09:23 AM Eastern Standard Time, 
wmarvin at hickorytech.net writes:
> 
> I think the Dial Zero adjust is the problem.  I only have a quarter turn CCW
> or CW adjustment right now.
> 
> How do I release the DZ adjust without causing damage?
> 
> Many Thanks
> 
> Bill  KB9IV 
> 
Bill,

You say you only have a 1/4 turn on the zero adjust shaft.

The Knob has been jammed back against the bushing.
Remove the knob, move it out on the shaft.
The zero adjust should have about three turns or more.
The shaft is threaded and the panel bushing is a threaded nut inside.
The shaft screws in to apply pressure to the clutch release pins and open the 
clutch.
The shaft screws out to release the pressure and engage the clutch.

Its just like the clutch peddle on manual transmission vehicles.

The zero adjust should disengage until the disk on the zero adjust shaft is 
free of the clutch pins. This saves wear on all the parts, reduces friction, 
(wrist injury from dialing receivers all day is no joke, Its right there with 
finger injury from pounding mills) [mills are typewriters with no lower case]. 
{mills are not electric}.

Someone engaged your zero adjust until the knob skirt bound on the bushing.

If you have original knobs, the knob has a tab inside the skirt, On the 
midsize knobs the tab hits a knob stop. The knob stop is a special washer that goes 
between the panel bushing and the front panel. On the small knobs the tab 
stop hits a knob stop. There is one under the limiter knob as an example.

So the tab is in every knob. The zero adjust screws in. The tab runs over a 
corner of the hex bushing. It takes a bit of force. But the corners have been 
rounded off because this has happened often in the receivers life time. So 
getting the dial lock jammed is easy.

Now the knob and shaft are locked with the angle of rotation limited to 60 or 
120 degrees. More or less depending on clearance.

You cannot just crank the knob CCW over the tight spot because the burr from 
forcing the knob CW forms a nice square corner. You get a ratchet effect.

You need to remove the knob and reset it.

The zero adjust shaft really sticks out when the dial lock is fully 
disengaged.
So folks like to set the knob as close to the front panel as they can.
Thus as the zero adjust is rotated CW to disengage the dial lock,
The clutch just slips when the knob and shaft are screwed in to where the 
knob skirt smacks the panel bushing.  A bit of file on the tab in the knob will 
let you get the knob a bit closer to the front panel and reduce how far the 
knob hangs out when the zero adjust is disengaged.

A good clean set up only needs about one turn of the zero adjust.
One turn in with the knob against the clutch pins with enough force to let 
the clutch slip. One turn out with the knob leaves the knob hanging free of the 
front panel. The knob is looking good, while not looking like the knob is 
about to fall off the shaft. The zero adjust shaft disk is clear of the zero 
adjust clutch pins. You can spin the zero adjust out a couple more turns but it 
just leaves the shaft and knob sticking out.

Some times a chunk of rubber hose is cut as a washer to fit the zero adjust 
shaft between the disk and the bushing. Split the hose to slip it over the 
shaft without doing a whole front panel disassembly to get it installed. This 
washer then limits how far the zero adjust shaft can be screwed out.

Now you want to look either down into or up into the receiver behind the 
front panel and check the zero adjust shaft operation.

As you rotate the zero adjust knob on the front panel, the shaft screws 
through the front panel bushing. The inside end of the shaft has a large fender 
washer disk. This disk pushes three pins in the zero adjust clutch to release the 
clutch.

The test you want to conduct is to ensure that the disk comes free from the 
pins when the zero adjust knob is used to rotate the zero adjust shaft (CCW) to 
the dial unlock position.

If this test fails, some adjustment of the zero adjust knob on the shaft may 
be in order. Knob skirts will catch on the bushing and you cannot get enough 
turns on the zero adjust shaft to get a full clutch disengagement. The knob 
gets jammed against the bushing.

If this test passes then you need to go with Tim's input that the clutch is 
gummed up and slipping.

Or you have a broken loose clamp.

Turn the receiver up on end, get out the interrogation lights, get mean and 
go looking.


When you turn the KC knob, The VFO shaft (oldham coupler) and dial counter 
should both run in exact no slop relation with the KC knob.

There are clamps, gears, and other things that can come loose in the 
mechanical links between the KC knob input and the two observable outputs. There is 
also a link to the many cams. 

The VFO is direct to the KC shaft. There should be no slip between the KC 
knob and the VFO.

Turn the receiver to CAL, BFO on and find a cal point at 100KHZ some where.
As you rotate the KC knob the BFO pitch should change. No slop. No backlash. 
No binding. If you have a problem here, then you are looking for missing 
oldham coupler spring, binding VFO, loose or broken clamps.

As you rotate the KC knob the dial counter should move. Gear clearance in the 
bevel gears can allow a small play. This can be adjusted to minimum.

If you have about a  90 degree (or less) of slop between the dial and the 
knob as you change the direction of knob rotation then you have a slipping zero 
adjust clutch.

Mostly this is solved by cleaning the clutch, or adjusting the zero adjust 
shaft and knob to get it located properly.


In the last year, we are starting to have reports of broken rivets in the 
clutch assembly. These can be fixed.  Parts are available if you need them.

Mostly this problem just needs a good eyeball and understanding of how the 
mechanism is supposed to work.

You may want to drop the front panel to do some of the cleaning and 
inspection.

The term is drop the front panel, because the panel slips off the antenna 
shaft and two frequency select shafts to swing down on the wire harness. You pull 
the panel forward and it drops.

There are a couple harness clamps on the chassis that need to be unbolted to 
help provide some extra harness slack. The IF shafts are best separated at the 
shaft couplers like you were going to remove the IF deck. 

I hope this helps.

Roger AI4NI




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