[Collins] Dial slippage, audio distortion, and humming iron in KWM-2A

[Dr. Gerald N. Johnson]

On Mon, 2007-02-26 at 23:10 -0800, Chris Kepus wrote:
> I just woke up my round emblem KWM-2A after it had been sleeping for nearly
> five years.  This unit was worked over and updated by Dennis Brothers about
> 5 years ago but it was not put back into service. I bought it nearly two
> years ago and, I am sad to say, it sat unused. This condition is about to
> change as I am finally getting my shack in order. So tonight, I woke it up.
> The good news is that it plays.  The bad news is:
> 1. The PTO dial drive is slipping in several places during rotations.  The
> amount of slippage seems to be decreasing slightly with use but it still is
> slipping.

Section 4.9 of the 32S-3A manual:
"4.9 DIAL COUPLING ADJUSTMENT
Slippage or rough feel in the dial mechanism can be corrected by one of
two adjustments.
a. Inside the lid of the receiver [transmitter, transceiver] at the edge
of the dial discs is a nylon gear that allows the discs to rotate at
different speeds. Loosening the screw that mounts the gear and slightly
changing the mesh of the gear with the discs will help improve operation
by reducing any binding that causes slippage.

b. Coupling between the main tuning shaft (large knob) and the drive
ring inside the dial discs is adjusted by rotating the main tuning shaft
bushing. The bushing is an eccentric and by loosening the hex nut on the
outside of the front panel and using a sharp instrument like an awl in
the small holes on its perimeter, the shaft will be moved closer or away
from the engagement point. A small amount of rotation should be all that
is required."

> 2. The power transformer (choke?) seems to be humming.  The power supply
> unit was professionally solid stated. The humming (vibration) is not
> terribly noisy.

This could be from loose assembly/mounting screws or excess current from
leaky filter capacitors in the power supply or in the radio.

> 3. The audio sounds distorted to me.  It is not as clean sounding as on my
> 75S3 and no way as clean as on my TS-850S.  (Gotta fix that!!)

Distorted audio can come from low bias to the audio output tube. That
can come from a weak bias rectifier in the power supply, a leaky bias
supply filter capacitor, a burned up cathode resistor for the audio
output tube, a leaky audio coupling capacitor to the grid of the audio
output tube (a likely spot for an infamous leaky black beauty), all of
which cause the output tube to draw excess current and could contribute
to the power supply noise. Distorted audio can come from leaky AVC time
constant (often black beauties) and bypass capacitors and from IF tubes
with grid emission that counteracts the AVC voltage. Conditions that
cause the audio output tube to draw excess current also shorten its life
and at end of life it can add to the distortion. A weak or gassy first
audio tube can add distortion too.
> 
> None of these is a show stopper. But they are annoying and I would
> appreciate any and all suggestions that would allow me to correct these
> problems, especially the dial slippage.
> 
> Thanks es 73,
> Chris
> W7JPG
> 

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