[R-390] Mechanical Alignment Part 01 Front Panel Bushings (start)

[Flowertime01 at wmconnect.com]

Fellows,
For reading and editing.

Thanks  
Roger.

Mechanical Alignment
01 Front Panel Bushings

Mark Richards asked; is there any tolerance for the front panel control 
bushings?

Once upon a time and some where in all the military specifications you know 
in your heart that some one specified exactly how much clearance every one of 
the bushing must have. Enough to allow the shaft to turn freely and not so much 
as to be excessive. As a shaft is hand rotated, it is unlikely you have a 
bushing with a hole worn to large.

Mark Richards asked; how do you replace front panel control bushings?

The bushing come in two sizes. There is a small bushing for the Bandwidth, 
BFO pitch and Antenna Trim shafts. There are larger bushings for the KC and MC 
change shafts. The small bushing are standard ¼ inch shaft extension bushings. 
A suitable replacement is available for small parts houses. A potentiometer 
can be scraped to salvage the mounting bushing from it as a replacement part. 
The larger bushings for the KC and MC change shafts are somewhat rare. If you 
are short a large bushing, place the one you have on the KC change shaft. 
Several members on the R390 mail reflector know of limited parts. The inventory is 
always changing so ask the current members in a mail posting if you need a 
bushing. The larger bushings are not exotic and a reasonable bushing can be 
machined to fit. The bushing need not be stainless steal. A plastic bushing would 
give years of service.

The bushings have two problems. The shafts get burs on them that prevent the 
front panel from being removed and bushing bind after the front panel is 
replaced.

To disassemble the large KC and MC change shafts bushing in order to removes 
the front panel remove the knobs and then remove the retaining nut from the 
front of the bushings. Let the large bushing remain on the change shaft. If your 
receiver bushings are assembled with the retaining nut inside the front 
panel, reverse the bushing assembly the next time you have the front panel off your 
receiver. The KC and MC knobs have a clamp and sleeve design so as not to 
score the shafts and prevent the bushings from sliding off the shafts when the 
front panel is removed. Burs happen. Use a small file to remove any burs that 
prevent the bushings from sliding off the shafts. Burs are a repeat offence and 
you may encounter one any time you need to disassemble the front panel.

The antenna trim bushing should also be assembled with the retaining nut on 
the outside of the panel. The knob and bushing retaining nut can be removed to 
disassemble the front panel. The shaft has a flat milled on the shaft. The 
antenna trim knob should always placed on the shaft so the knob set screw rest on 
the shaft flat. This practice helps reduce bus on the shaft that hampers the 
front panel disassembly process.

The Bandwidth Select and BFO Pitch knobs and extension shafts are almost 
never disassembled. The standard practice is to loosen the clamp on the extension 
shaft and pull the shaft forward to release it from the IF deck shafts before 
the front panel is removed. The shafts float in the front panel bushings while 
the front panel is removed.  The knob pointers are set by positioning the 
knobs and then tightening the extension shaft clamps onto the IF deck shafts. The 
knobs set screws on the extension shafts generate burs on the shafts. These 
burs make setting the knobs on the shaft for exact alignment a problem. These 
burs also make getting the shafts out of the bushings a problem. The shafts can 
be filed to remove the high burs and allow disassemble. 

Once you have these five major bushings, shafts, knobs disassembled the front 
panel bolts can be removed and the front panel dropped. Remember to also 
disassemble the dial lock before pulling on the front panel. The front panel is 
almost never "removed" as the wire harness to the front panel is still attached 
to many switched mounted on the front panel. However the front panel can be 
dropped. There are several maintenance actions that require the front panel to 
be dropped.

Once the front panel is dropped and alignment is not an issue of the moment 
the bushings likely run free on the shafts. If a bushing is binding some small 
amount of filing or use of emery stone will return the bushing and shaft to a 
free moving condition. You likely find you do not need to replace the 
bushings. The bushing only needs to be replaced  if the threads on the bushing have 
become so fouled that the retaining nut can not be easily seated.

Bushing can be drilled or reamed out. Likely a bur on the shaft has been 
forced into the bushing and "turned" some metal thus causing the bushing to bind 
on the shaft. Cleaning the grim and bits out of the bushings will also helps. 
Some bushing have been squeezed out of round. If one of these bushings can not 
be reamed to run free, then by all means replace it.

All bushing should be assembled with the nut outside the front panel. Thus if 
a shaft binds in the bushing, the nut can be removed and the front panel 
dropped with the barrel of the bushing left on the shaft. With the front panel 
dropped the shaft and bushing can receive whatever maintenance is required to 
remedy the assembly problem. When reassembling the dropped front panel, leave the 
bushing nuts loose on the bushing barrels.

Mark Richards asked; what do we do for the worn bushings between the front 
panel controls (particularly the tuning knob) and the proper operation of the 
gearing?

With the following ideas YMMV and FWIW here are some thoughts.

Joe [name please] contributes 

There are THREE bushings on the KC CHANGE shaft the one on the front panel 
should be left "snuggish" so the other two won't be bound by it.  Also, that 
shaft is the one most likely to be bent, being that it is so far from the 
protective handles.  On a total rebuild of the radio it should be removed and 
centered on a lathe to be checked for run-out, then straightened. This will make it 
run true, not be apt to bind and will help to keep the DIAL LOCK from binding 
and making that awful scraping noise so common on these radios.

Tom Norris contributes

Partially loosen all the front panel bushings before replacing the front 
panel to the shafts a bit of "play" so they'll be less likely to bind.

The sequence in which the front panel bolts are tightened will make a 
difference in the bushing alignment. Set the receiver frame on blocks so the front 
panel hangs free when reassembling the front panel. Leave the bushing nuts loose 
on the bushing barrels when first mounting the front panel. The IF deck green 
bolts can be loosened to shift the IF deck and improve the alignment of the 
extension shafts. The same can be done to the RF deck. RF deck alignment does 
not offer as much shift as the IF decks appear to have. But it has been found 
to work. A little here and a little there and soon you have a smooth running 
assembly.

The bushing are "standard interchangeable parts" that are not perfectly 
centric. Thus the bushings are eccentric by definition. That hole is not exactly in 
the center of the mass. So some time rotating the bushing to one position 
will allow some more freedom than other positions. Almost always some combination 
of deck shift, front panel bolt insertion sequence and bushing rotation will 
allow all the shafts to operate very smoothly with bushing nuts tightened. 

If you have a bent KC change shaft, you can operate the receiver with the 
front panel bushing nut loose on the barrel until you have time to get into a 
shaft bending maintenance period. The shaft may be straightened with out removing 
it from the RF deck. Do not strike shafts. A proper diameter tube of good 
length should be placed over the shaft and gentle (this is steel to be bent 
gentle) pressure applied to remove the bend as best as can be judged with available 
resources. Feel free to use all the resources available to you when ever 
necessary. 

The most common problem is the MC shaft binds. There are two sources to this 
problem. The detent spring is often seated with two much force against the 
detent stop ring or the MC shaft bushing needs service. Service may be cleaning 
or adjustment. Often removing all the front panel bolts and reinstalling them 
will shift the front panel and provide less binding of the bushings.

These receivers are getting over a half-century old. If the bushing holes in 
the front panel needed filing, it likely has been done. Mostly getting every 
thing aligned to operate smoothly is just a mater of persistence. Knowing that 
the front panel and decks can be shifted gives the maintainer insight into the 
problem. Also knowing the bushing may be eccentric and rotating the bushing 
may provide a better fit can help the maintainer achieve a smoother operating 
receiver from a mechanical point of view.