Although I have a later digital transceiver (Icom-751), I recently acquired a
Swan-500-C from an estate. It had been in storage for 25 years. I have had fun
working on it's quirks and returning it to very good performance, even by
today's standards:
The first quirk was not staying on frequency (not related to time/heat drift).
Slightly wiggling the bandswith knob caused slight changes in voice pitch. And
what was worse (and intolerable in a group discussion) was that often Transmit
on a slightly different frequency than it received on. A change of more than
100 Hz or so is usually intolerable.
My initial thought was oxidized bandswith contacts. I bought some "Deoxit" at
Radio Shack dowsed all the contacts except the ones in the VFO compartment. This
may have helped slightly but problem was essentially the same. Then I "Deoxed"
the bandswitch contacts in the VFO compartment which I SHOULD NOT HAVE DONE,
because the material is a different plastic and was severely affected. It changed
the frequencies by more than 30 KHz...probably by changing the dielectric constant
of the plastic material. I quickly rinsed this off with FREON-TF and frequencies
returned almost to where they were before.
The original problem still mostly existed after all the contact cleaning. I had
even used "PB-Blaster" rust solvent from Auto-Zone on the main bandswitch (not the
VFO) which made no noticeable difference. I finally flushed all the contact
cleaners back off with FREON-TF which pretty pretty much leaves them clean and dry.
HERE'S WHAT SEEMS TO HAVE BEEN THE MOST SIGNIFICANT "REAL" PROBLEM:
While trouble-shooting the problem, I monitored the actual VFO frequency (which is
always the operating frequency plus or minus 5500 KHz in "Normal" sideband), with
my Icom-751 listening as a frequency-meter:
I found that tightening, or loosening the mechanical linkage between the two
band-switches (VFO and lower gang), the "pitch" of tone I was listening to on
the Icom changed by about the same 100 or 200 Hz as exhibited in the origanal
problem. The long band-switch shaft passes through several wafer sections and
is only grounded near the knob and de-tent assembly which provides a poor and
un-reliable ground. (Slightly wiggling the band-switch knob always caused a
significant change in the tone pitch I was monitoring).
Apparently, when the shaft was not firmly grounded, it provided a capacitive-
coupling path between certain wafers that caused the problem.
AS A CURE, I soldered a flexible jumper (Pig-tail) between the lower shaft and
chassis ground, and also a jumper on the larger section of the mechanical linkage
above the chassis to ground.
On a different aspect of improving the SWAN-500 performance, I used and external
wall-receptacle-mounted transformer to light a 12 volt auto tail/stop light and
placed it against the VFO compartment to simulate heat when the transceiver has
been on for awhile. Although probably not necessary, I took the opportunity to
negative-rectify this 12 volts and send it in one of the accessory socket pins (I
don't have the pin# handy, but it goes the 25W 10-volt zener(-10v) mounted under the
chassis. You may have to limit the current to the zener with a resistor if your
voltage is excessive. This keeps the transistorized VFO section running at all times.
I have found that remarkably, there is virtually no warm-up time required for
SSB standards operation...comparable to the digital frequency-synthesized more
modern transceivers. Of course you disconnect the external heat while operating
the transceiver.
If better voice quality is your preference, the SWAN provides this by using a
wider crystal-filter band-pass of 2.7 KHz which is noticeably better than the
Icom-751 which is 2.3 KHz bandwidth, and some transceivers even use 2.1 KHz.








Perhaps I should add a cautionary risk-warning for the less experienced
who choose to energize the VFO during off-hours as described above:
During normal operation on mine, the current to the -10v zener set the
voltage at -10.1 volts. When I substituted an external source when the
transceiver was turned off, I selected a resistor to supply enough
current to read -9.9 volts at the zener....thus ensuring that the current
is not excessive. Accordingly, you should be sure to disable the external
current supply (and heater light) before you turn on the transceiver to
prevent BOTH currents energizing the zener. It would probably tolerate it
temporarily but if it were destroyed so as to "open up", the soaring
negative voltage might destroy the VFO transistors.
A wide variety of external power sources could be accommodated...AC or
DC, but if AC is used, it will require an external rectifier with the
Cathode toward the transformer, and Anode toward the internal zener. No
filtering required.
In my particular case, I had a unit labeled 13.5 vdc @ 1A.
On the auto brake/park bulb, I chose to solder to the two lead contacts
(ignoring the shell). This runs the Brake and Park filaments in series
and of course most of the illumination is the Park filament only. My
voltage across these measured 11.25 volts dc but of course it could
also be 11.25 ac. Although with my negative dc supply, I didn't need
the rectifier diode, but I used one anyway as a precaution against
accidentally reversing the dc supply. As noted above, this amount of
heat seems to simulate the normal operating heat quite well since I
experience virtually no warm-up time required for freqeuncy stability.
On accessory socket J6 on the Swan 500-C, chassis ground is pin #9
and the external zener current goes to pin #3 or #4 (They are jumpered
together in the octal plug) and of course you leave the plug inserted.
For other Swans that might have different wiring, the zener is a
25 watt, insulated, stud-mounted 1N2974-A.
----------------------------------------------------------
A couple of other tips that might be useful to some:
Although my receiver was functioning satisfactoriy, the S-meter
readings seemed stingy compared to my Icom. Removing a capacitor
is an easy fix for this:
The high-end of the Audio Gain pot (R-1201) has a capacitive-divider
of two .001uf's in series which divide down the audio voltage derived
in the Product Detector. This midpoint goes to AGC amplifier/detector
(pin#8 V-11, 6BN8)
to generate the AGC. Simply removing the lower (grounded) .001uf, C-1104
allows much more audio to generate AGC without affecting anything else.
In my case, this makes the S-meter readings more normal, and better
AGC control of signal levels.

Another change I did was to lengthen the AGC time-constant which seemed
much faster than the Icom (I use it excusively in SSB mode). They used
..01uf, C-901, from pin#6, V-11, 6BN8, to ground. I added an additional
..05uf across it for a longer release-time.

Although the Swan-500 is usually sufficient at its higher power levels,
I prefer to use mine to drive a Linear Power Amplifier. Even so, I prefer
to leave a 50 ohm dummy load paralled at its output to ensure stability
at light loading...the two 6LQ6's are still not working very hard and
should last a long time. The dummy load does not affect the Receive
sensitivity noticeably.





An Electret type element provides a high quality, low cost microphone for many
purposes, including some amateur radio transceivers. They might also be
referred to as a "capacitor mike". The element is small, about the size of a
large pea. Radio Shack doesn't sell them anymore but several types are available
from Mouser Electronics (no minimum order). My version had 5" leads, and required
only +2v to +10v supply ( at 1/3 milliamp). The shield on the output cable is
also grounded to the case. It delivers about 140 mv P-P when capacitively
coupled to a high-impedance load. A SWAN-500C has about a 1Meg input, and its
microphone gain can be set at about 50%. With +6v, the electret's output is about
the same as a typical Ceramic type microphone. An Electret's output is quite high
fidelity, essentially flat from 100Hz to 5000Hz. (Cost $3.40 +shpg).
My contention is (at least in theory),that if you offer the full hi-fi range
of audio input to the transceiver, the resulting fidelity being transmitted is
virtually dictated by the response curve of the steep-skirted SSB Crystal Filter.
For example, the SWAN-500C has an SSB filter 2.7 KHz wide. (i.e. 5500.3 KHz to
5503.0 KHz), and their reference crystal oscillator of 5000.0 KHz provides an
ulimate audio range of 300 Hz to 3000 Hz which is quite good for voice communication.
On the other hand, if an SSB filter is only 2.1 KHz wide, you must be very
selective in choosing the reference crystal oscillator....you can decide whether
to utilize that smaller width to emphasize low-freqency audio response, or high-
frequency response, but you can't have both. You could choose 300 to 2400 Hz, OR
1000 to 3100 Hz, for example.