Thank you Ian Morrison VE3IEM for typing the Service Bulletin from the jpg's.
During the original design and pilot run of the model 250 six meter transceiver, it was found necessary to stabilise the receiver R. F. amplifier stage by using a small ferrite inductor in the grid lead and a 10ohm series plate resistor. Noise figures and sensitivity suffered very little on the first sets built, and performance appeared to be entirely adequate. However as often happens when mass producing a product, these stabilising measures have not proven satisfactory in all cases. The main purpose of this bulletin is to describe some simple front end modifications which will be relatively minor, while in a few cases it will be substantial. It is our recommendation that since the modifications are quite simple, they should be made to all 250's which were manufactured prior to July 25,1967.
Check your serial number. If the last two digits are higher than 12, the modifications have been included at the factory. If the last two digits are 12 and followed by the letter "B", the modifications have also been included. But, if the last two digits are 12 or less, and there is no letter "B", the modifications should be made. We also would like to encourage each owner to make the modifications himself, in the amateur tradition of being a technician as well as an operator. If any problem develops, the set may be taken to your dealer for servicing, or shipped to the factory at Oceanside, California where we will do the work as re-ship within a few days.
1. Remove bottom cover. Locate V6 and V7, the 6HA5 receiver R.F.
amplifier and mixer stages, respectively. The attached
pictorial drawing shows V6. The tube socket immediately next to
it is V7.
2. Remove the ground wire going to pin 3 of V7. It is simply to
be clipped out.
3. Unsolder the two leads going to pin 4 of V6 and connect them
instead to pin 3 of V7. (If the 500 kc calibrator is
installed in your set, the heater connection to it should
also be moved from pin 4 of V6 to pin 3 of V7.)
4. Remove the wire lead from pin 3 of V6 and connect it instead
to pin 4 of V6. This lead comes from pin 4 of V7 where it
remains.
5. Connect a bare wire jumper from pin 3 of V6 to the nearest
ground lug on the socket. The heater hook-up should now look
like the revised schematic of the accompanying fig. 3.
6. Remove the ferrite bead which is around the grid lead going
to pin 1 of V6. This is done by simply crushing the bead
with a small pair of pliers.
7. Remove the 10 ohm resistor going from the plate coil to pin
5 of V6.
8. Locate the 5 pf disc capacitor which connects from the plate
coil terminal to a 1/8" copper strip which leads over to the
4 gang P.A. grid capacitor. Unsolder this 5 pf capacitor
from the coil terminal and connect it to pin 5 of V6 instead.
Then shorten the copper strip accordingly so that the
connection to the 4 gang variable is as short as possible.
9. Remove the other 100 pf capacitor from the other plate coil
terminal, and replace it with a 50 pf silver mica capacitor
(included with this bulletin). The ground end of the new
capacitor should go directly to the ground lug. Make
connections as short as possible.
10. Check connection of the .001 mfd cathode bypass going from
the ground lug to pin 7 of V6. Its leads should be no longer
than 1/16". Shorten them up if they are longer.
11. Install the included brass shield as illustrated. Solder at
3 places: (a) pin 3 of V6, (b) to the center sleeve of the V6
socket, and (c) to the ground lug of the terminal strip.
- Keep the shield clear of pins 2 and 7 so that it does not short circuit on them. Move the 82 ohm cathode resistor going to pin 7 down toward the chassis so that it clears the shield. Make sure that no other component touches the shield. Move them slightly if necessary.
12. Now connect a short length of 18 gauge wire (included with
this bulletin) from the vacant plate coil terminal to pin 5
of V6, where the 10 ohm resistor was originally connected.
Cut this lead so that it is minimum possible length. Make
sure that it does not short circuit to the shield or any
other components.
1. Turn the receiver neutralising trimmer clockwise until it is
snug, then back it off exactly 3/4 of a turn. This position
will be correct in most cases. ( Note that this is the small
mica trimmer located near the plate coil, L603, and it is not
to be confused with the P.A. neutralising capacitor, which is
a large air trimmer located in the P.A. compartment.)
2. Connect the antenna (50 or 75 ohm coax.) to the transceiver.
Set the tuning dial to 50.2 MC and adjust the P.A. grid for
maximum background noise.
3. Adjust the receiver input and interstage coils L601, 602,
603, and 604 for maximum background noise.
4. If it appears that instability is present, it may be
necessary to turn the neutralising trimmer slightly, and to
re-adjust the coils. Swing the P.A. grid control back and
forth through the noise peak. It should sound smooth. If
"birdies" or "bubbles" are noted, retune the neutralising
trimmer and coils.
- The antenna feed line should be 50 or 75 ohm coax , and
should be quite well matched to the antenna.. Under some
conditions of mismatch, it may be difficult to stabilise the
receiver R.F. amp. If in doubt, check the receiver with 50 or
75 ohm dummy load.
RECEIVER SENSITIVITY
With these modifications your Swan 250 will provide sensitivity
on a par with the finest converters preamplifiers. We have
received a number of inquiries regarding the use of a glass
envelope tube instead of a nuvistor type. The fact is that at 50
MC we find no measurable advantage in the nuvistor type.
The 6HA5 is designed specifically for VHF service, and is
electrically a very close relative of the 6CW4 nuvistor.
Also, a number of owners have tried different tube types in the
I.F. stages. The 6GM6 is a 6EW6 except with semi-remote cut-off
characteristics. It will provide an apparent increase in overall
receiver gain because of different AGC control. There is no harm
in trying the change, but it is our feeling that no advantage is
to be gained. Likewise, a 12AU7 may be substituted for the 12BA6
second I.F. stage. Here the situation is reversed. The 12BA6 is
a remote cut-off tube and the 12AU6 is a sharp cut-off. Again,
the AGC control characteristics will be altered, and the
experimenter may enjoy trying it. Our own laboratory test
indicate the best all round performance with the
6EW6 - 12BA6 combination. This is one of the many decisions which
must be made when moving a piece of electronic gear from the
prototype stage into production. Thus, the occasional need for a
service bulletin.
We hope that these notes will be useful and informative. Six
meter openings are coming back strong.
73,
SWAN ELECTRONICS
Oceanside, California
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