YAESU FL-DX-500

A bad problem with this RIG is spurious, although some other in this series
are worse.
When you spot tune to another receiver, I used DRAKE 2-B and later R-4C, you
can hear some spurrs on several points on the scale.
Problem is 455KHz generator and mixer up to 9MHz.

FIRST>
I made 9MHz xtal osc for CW, teltening to the output, it was clean!!
You can try for 9MHz xtal a 27.005 or 26.995MHz xtal for CW.

SECOND>
After some time of not using the RIG, I discovered that there was no SSB
signal output. After several tests I found out that the nechanical filter was
defective!! Well, that is a good opportunity to get rid of the rubbish!!!!
So I made a new balanced modulator at 9MHz with MC1496P, with new xtal
oscillators, but it may be possible to use the USB/LSB oscillators fitted
with new xtals, possibly 27005/26995KHz HC-25/U soldered to the sockets under
chassis. The 12AT7 mixer is now buffer, one half is for SSB, the other half
is for CW. Particularly for SSB you need somewhat more gain to drive the TX,
since output from this new balanced modulator is lower than the old one.
I used an XF-9A filter which I bought for my first SSB transmitter in 1967
(KRISTALLVERARBEITUNG NECKARBISHOFSHEIM, I believe they are TELE-QUARZ now),
some very cheap xtal filters has also been available in United Kingdom, and a
simple filter as the XF-9A is quite sufficient in an SSB TX.
-----
So now the TX is free from all the spurious rubbish, and you do not disturb
the other local amateurs.
Another very FB modification is to connect a 6K8 resistor in series with the
'large' capacitor in ALC circuit. I believe it is 0.1 or 0.22uF, this will
improve the AGC action, and thus improve the intelligibility of your
signal!!! I made this modification when I lived in England in 1974, and my
local friends told me that they could understand me much better, in spite of
my bad English pronounciation, than they could before!!!!!
Later I made similar modification to other RIGS with success.
Now the AGC works inmediately, instead of 90 degrees out of phase, with a
possible delay, starting the reduction in gain of the TX after the pulse has
deceases.
With this modification mike gain level is not too critical, and it is much
more difficult to overload the transmitter.



 EXPERIENCE WITH FL-DX-500

Several modification were made to this transmitter.
Severe Miller effect was experienced in the oscillator/buffer circuit,
oscillator did not tune on L/C frequency but 20-50KHz different.
Oscillator valve was 12AT7/ECC81.
A better valve is ECF82/ECF80/6BL8/6U8 etc, with oscillator as triode and
pentode as buffer, with a just under critical coupled two stage band pass
filter on the output.
73 de Jan-Martin



 FL-DX-500: 145V REGULATOR

One problem observed is that the 145V from V12 ECL82/6BM8 is not stable, the
neon lamp in cathode flashes during speech.
The problem is avoided as follows:
+300V...350V..-----2M2 ---2M2---I---I<--------V12 pin 8 (triode cathode)
                                I  1N4007
                                I----------To neon-lamp
When the neonlamp is not on the voltage rise immediately to a higher voltage
and stabilize again.
The output voltage from the regulator is now very stable, and the lamp glows
during all sorts of operation.
LA8AK 1974
73 DE JAN-MARTIN