[TenTec] Argonaut 505 alignment, filter, mod?

[Rick - DJ0IP / NJ0IP]

Hi Neil,

You are very lucky man to find a 505 in such great condx.

First of all, no special or unique tuning tools are required.  
All you need are the standard things we used for aligning radios back in
those days.
...> A non conductive screw driver for adjusting the trimmer caps
...> A non conductive Allen wrench that fits inside the IF cans

Radio Shack used to sell these.  Are they still in business?
I was very lucky to have a porcelain screw driver, but most were plastic, or
plastic with a tiny metal tip on the end.
There were 2 or 3 sizes of plastic Allen wrenches.  I don't recall the size,
but they were standard.

CAREFUL!!!  When you first try to adjust the IF cans (and mixer can), it is
very easy to break the iron power core inside.  DO NOT TURN IT WITH FORCE.
Instead, rock the tuning too back and forth gently and eventually it should
break loose. Then you can adjust it for maximum signal.


CLEARLY AN AUDIO FILTER *INSIDE* THE AGC LOOP IS THE WAY TO GO!

It is no problem to insert an audio filter in the 505.  I did it and it
worked just fine.
Since I couldn't find an original audio filter, I simply copied the circuit
design of the Model 208.  I built it on a universal printed board which had
just holes with copper rings around them.

Start by studying the circuit diagram of the IF board on the 505 vs. the IF
board on the 509.

...> 505:  90184 IF Board, Page 4-7 of the manual
...> 509:  80208 IF Board, Page 3-16 of the manual

The two boards are almost identical, except for 3 things:

..1) Q3 on the 505 is a bipolar transistor, but an FET on the 509
..2) The specific transistor types of all of the bipolar transistors have
different model numbers (2N... for the 505 transistors and MPS... for the
509 transistors) but my guess is they only changed manufacturers; the
transistors are all the same.
..3) The 509 has provision for the external audio filter, the 505 does not.

You can ignore points 1 and 2.  It doesn't matter.

You can easily modify the jack on the side of the board which has the audio
output side (right side in the schematic) by rewiring the 2 pins.  Let's
count them, starting with pin 1 being on top:

PIN......*505..........*509*......
.1......GROUND.........GROUND.....
.2........BFO...........BFO.......
.3.......+12V.......*FILTER OUT*..
.4.......+12V...........+12V......
.5.....AUDIO OUT......AUDIO OUT...
.6......GROUND......*FILTER IN*...
.7......GROUND.........GROUND......

It is not necessary to have multiple connections for ground and +12v. We
will use one of each.
If you want to work with an external audio filter (Model 208 or home brew),
you can modify the 505 by removing the jumper between pins 3 and 4, AND by
removing the jumper between pins 6 and 7.

THEN on the PC board, check to see if the solder run to these pins is
connected.  If it is, you will have to cut the run with a sharp knife or
scalpel.  I can't recall for sure, I modified my 505 in 1975.  If you cut
the solder runs, be sure the run to the components is still connected where
it should be.  You may have to install a wire jumper.

If you choose to build the audio board into the radio, you don't have to
modify the jack (as described above), just make flying connections to the
board.

Here is the procedure for either case:

...> cut the wire between the collector of Q3 and the base of Q4.  Actually
the cut for Q4 will be before capacitor C15, which is just to the right of
Q4.
...> Add a .1mF capacitor to the collector of Q3.  This is identical to C15
and is called C19 on the 509.
...> Then connect "Audio In" of the filter to the new capacitor whose other
lead you have just connected to collector of Q3. Use a shielded cable for
this, such as RG-174 and ground the shield.  If using external, it will go
to pin 3.
...> And connect "Audio Out" of the filter to the right side of C15, also
using shielded cable. If using external, it goes to pin 6.
...> Voila!

You certainly will want to provision for a switch to switch the filter in
and out.
I used a small slide switch which I mounted on the right side of the radio,
in the plastic panels.

You'll find this mod makes a world of difference.

TO MOD OR NOT TO MID, THAT IS THE QUESTION.

Everyone has their own beliefs on this.
Most people are worried about the resale value of a radio.
I NEVER worry about that.  I buy my radios to use, not as an investment.

If you want to keep and use the radio, do the mod.
If you don't know, well then don't do it.
A good compromise would be to first go the EXTERNAL route.
Make the mod but run it to a jack or jacks on the rear panel and place the
filter external to the box.
 
SENSITIVITY
Funny, I did not find my radio insensitive.  Keep in mind that it is a QRP
radio and if the radio is aligned correctly, you're probably not going to be
able to work anything that is S-1 anyway.  

I built a small 40m preselector for mine, connected it externally, and I
could switch it in and out.  I also added an attenuator inside the
preselector.  It was certainly necessary in Europe.  As I recall, there were
2 spare RCA Phono jacks on the back panel.  I interrupted (cut) the RG-174
coax going from the T/R relay to the RX RF amp and ran each end (where I
cut) to these two jacks.  I used them for Preselector-Out and
Preselector-In.  You can do the same thing for an attenuator, keeping it all
inside the box.  You will need to have a switch.   A slide switch on the
side panel near the RX pre-amp will do the trick.

ENJOY your new radio!

73 - Rick, DJ0IP
(Nr. Frankfurt am Main)


-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Neil
Martinsen-Burrell
Sent: Monday, January 20, 2014 5:55 AM
To: tentec at contesting.com
Subject: [TenTec] Argonaut 505 alignment, filter, mod?

I am the proud new owner of a fine condition Argonaut 505.  I'm very happy
to find a five band QRP rig that is happy running off a 12V battery, even if
it is 40 years old.  I do have a few questions and this seems like the best
place to ask.

1) The receiver seems a bit deaf on 80m ( by A/B comparing with a TS-520)
and I wonder if the problem could be solved by following the alignment
procedure in the manual.  I can get my hands on an oscilloscope, but I can't
tell by looking inside what hand tools I need to do the actual adjustments.
What are the proper tools to perform an alignment for this radio? And where
is a good place to get them?


2) I am primarily interested in CW with this radio and so I am looking for a
narrower audio filter.  I have seen a Model 208 CW audio filter for sale,
but it is set up for use with the accessory jack on the Argonaut 509.  The
TenTec wiki says "a slight modification is necessary when using the filter
with the older Model 505 Argonaut. Service Note SN-6-505 details the
modifications."  I assume that the slight modification is to separate out
the audio in, audio out and power in lines.  Does anyone know how to obtain
a copy of that service note for more details?


3) Some have suggested that an audio filter outside of the AGC loop is not
very useful.  Comparing the schematic of the Argonaut 509 to that of the
505, it appears that the patch point for the audio filter on the 509 which
is inside the AGC loop could be added to the 505 as well.  Does anyone know
if this mod is possible or desirable?  I have seen people talk about these
radios as collector's items and this one appears unmodified, so I would also
welcome thoughts on when it is appropriate to make a modification like this.

Thanks in advance for any help you can offer on any of my questions.

-Neil KD0UKC
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