Remco.org

Seductive serendipity / Verleidende serendipiteit

April 18th, 2017

IC-720A refurbishment (2)

Introduction
My intention is to interface my IC-720A to common QSO-logging / contest programs like
N1MM or WinTest. I am not familiar with Ham Radio Deluxe (HRD) and honestly I didn’t miss it over
the last years. Goal is to enrich the IC-720A with an USB interface to allow remote control.
Recently I received the IC-720A bus protocol description (tnx KA6BFB!) so there is hope : -)

Sequencing / timing
When contesting with amplifiers or using seperate RX-antennas, timing is of major
importance to prevent e.g. ‘hot keying’ or detrimental damage due to RF-spikes etc.

Besides the PTT switch on a hand mike, many rigs lack a ‘/PTT input’.
That is, a low current input to switch the rig into TX-mode by making the PTT line low (<– /PTT).

/PTT in
Below the rear side of the IC-720A is depicted. The ‘MEMORY’ cinch connector under the power
supply molex connector looked very tempting to serve as /PTT in.

In my previous posting an internal /PTT point was already addressed. Seemingly the most
easiest way to implement /PTT on the ex-MEMORY cinch connector is to wire an internal /PTT point
to this connector? No . . . !

Nuisance with /PTT’s  (i.e. active low) is that this ‘low’ state is relative towards a ‘high’ state.
Depending on equipment used these ‘high’ states may vary significantly. Therefore ‘paralleled’
/PTT lines may result in detrimental damage! Perhaps the Heathkit SB-200 may serve as an example.
In ‘high’ state I measured around 120V on the /PTT input connector (!) Imagine what would happen
if you simply join the PTT of your rig with the PTT input of the SB-200 . . .

Mostly this nuisance is overcome using relays as isolation instrument. However, I am allergic
towards relays (perhaps besides the RX/TX (coax) relay) and prefer solid state solutions.

Grounding the PTT line I measured around 80 mA current, which is too much to be called ‘low current’.
My initial idea was to use an optocoupler where the switching transistor side had to serve as
/PTT switch. Well, that didn’t work. Despite having enough current through the
LED side of the optocoupler I heard the RX/TX-relay switching, the ‘TRANSMIT’ light weakly
burned but no RF output power was present in RTTY mode or ‘key down’ in CW.

Perhaps my /PTT wasn’t ‘low’ (or ‘strong’) enough?

I took another approach which works flawlessly, see picture below.

‘/PTT in’ is wired inside to the ex-’MEMORY’ cinch connector and ‘/PTT out’ wired to the
grey wire on the MAIN UNIT connector used for the relay accelerator (cf. previous posting).

April 9th, 2017

IC-720A refurbishment (1)

Introduction
Around three weeks ago I won an IC-720A for almost nothing during a live sale
at our radioclub
(PI4RCG). It was presented defect, so I thought: “What the heck?”

Afbeeldingsresultaat voor ic-720a

The next day the rig was repaired within an hour. Main error was, guess what . . .
yes! . . . the rotary relay. After some fiddling, cleaning and lubrification of the
rotary switch everything worked as expected.

It appeared my IC-720A was enriched with a  FL-32 500Hz 9 MHz CW filter, so this good old
buddy may be a likely candidate for ‘permanent’ portable usage in our camper/motorhome.

(Note: I prefer 250 – 300 Hz bandwidth for CW with ~500 Hz pitch . . . )

Some googling revealed that this rig was a quantum jump in its time and (one of) the first rig(s)
able to be remote controlled by a predecessor of the current CI-V Icom (‘CAT’) bus/interface.

The IC-720A features a general coverage receiver in 1 MHz portions. Transmitting was limited to
HAM bands only. With nippers this limitation was ‘fixed’ within 20 msec ; -) (read on).

In order to ‘get the rig going’, N1MM+ and my Winkey compatible Arduino keyer forced the
IC-720A to give CQ in CW for around three hours with full power (100W).

After this endurance test everything worked well, but I noticed I had to enter around 50 msec
‘PTT preamble’ in N1MM+ . . . which is too slow for contest and/or pile up usage.

So, I decided to enrich the TX/RX relay in the FILTER unit with a relay accelerator.

After studying the IC-720A service manual (google on it) it appeared that the /PTT (‘SEND’)
signal towards the filter unit (containing the TX/RX relay) could be easily located and isolated.

The wire was cut and the relay accelerator was piggy bagged somewhere on the MAIN unit.

Below a picture ‘before’ is depicted, mods are marked with A, B and C.
(click on picture to enlarge in a new tab)

A. General coverage TX mod. Cut this wire. (period ; -)

B. /PTT (‘SEND’) wire to filter unit. Cut this wire.

C. μPC2002 (TDA2002) noise reduction mod: place 470n in series with 100 Ohm between legs 2 and 4

Ad. B
Cut the concerning grey wire around the arrow marked position (see picture above).
The piece connected to the connector on the MAIN board goes to the input of the relay accelerator.
The other side is connected to the relay accelerator output by lengthening the wire a little.

The results of B and C are marked B’ and C’ in the picture below (click on image to enlarge in a new tab)

To be continued . . .

April 3rd, 2017

FT-857 audio noise reduction mod

Introduction
Being into weak signal work, either in contests or DXing, I fight against noise.

Europe is noisier than other parts of the world. In The Netherlands man-made noise
is becoming a significant issue, even a problem. However, with the right attention it is possible
to reduce noise/interference and become competitive on shortwave, even from The Netherlands.

Many people know me advocating the (vertically polarized) deltaloop on 40m. When properly fed
this antenna behaves like two stacked full size verticals but .  .  .  moreover, it reduces noise
during reception because it’s a closed loop/system. “If you can’t hear them, you can’t work them!”

In my quest reducing noise I was confronted with a kind of ‘nuisance after discovery’: audio noise.

My good old buddy, a FT-857 (yes . . . not a 857D), always assists me during holiday trips to
make contacts from exotic places. A few weeks ago I was in such an exotic place and was confronted
with a HUGE pile-up on 40m and tried to pull out weak JA’s and W’s out of the noise with my
simple setup: a (heavily modded) barefoot FT-857 (60 – 65W o/p) and 40m (vert. pol.) deltaloop in CW.

Blame my 70, 23, 13, 9, 6, 3cm, 2, 6, 40, 80 and 160m experience, but even on short wave you’ve weak signals!
Because I listened so much into the noise, my brains developed a kind of ‘weak signal filter’ in pile-ups.
During the last operation from this exotic place something became apparent to me which I (apparently)
didn’t notice before: unwanted audio noise in my (LoFi, NOT HiFi!) headphones.

Back in The Netherlands I decided to dig further into this issue.

Observation
Back home I inserted a weak signal (-140 dBm @7025.0 kHz) into my FT-857 and listened through my
headphones. While listening in CW-N (300 Hz BW) I was fighting against the perception ‘do I hear a tone or not?’.

In order to rule out some bias in myself I asked someone else to push the RF-on/off button of my
signal generator. My score was around 60% correctly detecting the tone or not.

With this applied weak signal I discovered I had problems in determining whether I could hear a tone
(or not) was related to the volume but increasing the volume also increased high(er) frequency noise
(more?) in the audio! Certainly a psycho acoustic issue is involved.

In other words, the high(er) frequency noise was interfering with my perception (whether there was
a tone . . . or not). In other other words, this high frequency noise was an extra load in filtering
the tone (in my brains?). The high frequency noise can be described as a ‘high pitched hiss’.

This led me to investigate the audio receive chain of the FT-857 and soon I focussed on the TDA2003
audio amplifier. Simply because this stage has an almost equivalent gain contribution in the whole
chain than it’s predecessing (RF and IF) stages. Assume that the -140 dBm signal has to be converted in
1W audio output (+30 dBm), the whole chain must have an amplication of 140 + 30 = 170 dB.

Knowing that generic audio amplifiers have (let’s say) 80 – 90 (if not 100) dB gain,
it’s relative contribution has might be significant (not for the overall noise figure (NF)
but more the timbre of the sound).

So . . . I decided to dig into the datasheet of the TDA2003 audio amplifier.

TDA2003
The TDA2003 is a standard audio amplifier. On the first page of the datasheet I found the standard application:

Immediately ‘Rx’ and ‘Cx’ drew my attention. Further in the datasheet it is explained that Rx and Cx
introduce a low pass behaviour in the above circuit.

In the FT-857 circuit diagram I noticed Cx and Rx were lacking (see below):

From the formulae given in the original datasheet, and assuming ‘B’ in the formula for Cx is
the cut off frequency, I calculated Rx = 100 Ohm, and Cx around 220 nF for B = 3000 Hz.
Because I hadn’t 220 nF and fiddling with  2x 100 nF parallel wasn’t my cup of tea, I chose 470 nF,
also to be on ‘the safe side’. Thus, 470 nF in series with 100 Ohm were connected between
legs 2 and 4 of the TDA2003.

Measurements
My first impression by ear was that the mod works, in a sense that I was more able to detect the tone.
Again someone else was asked to randomly press the RF-on/off button of my signal generator and
my score was 100% correct in determining whether there was a tone, or not.

We did a second test where I had to leave my shack. After I was called I had to take place at the table,
put on my headphones and the above procedure was repeated. At first glance I didn’t hear anything
but noise, but suddenly I thought I heard something. However, this seemed weaker than the previous tests.

I turned up the volume somewhat and after my brains ‘locked’ into the situation I was able to score
around 70%. It seemed my assistant reduced the signal generator level to -146 dBm by inserting a calibrated
20 dB attenuator and adjusting the output of the signal generator to -126 dBm !

Of course I was curious if I could determine the yield of the mod in a more objective manner.
A few years ago I used the ‘Analyzer2000′ program to determine and optimise signal to noise ratios (SNR).
However, it seems that this program doesn’t run anymore on 64 bit Windows systems. As alternative I installed
Spectrum Lab from DL4YHF. Despite Spectrum Lab is advertised being able to determine SNR’s, I couldn’t
find a separate SNR option or button at first glance. This program is significantly more versatile and
complicated than Analyzer2000, so I reckon I have to dig further in the manual ; -).

Anyway, as alternative screenshots of the waterfalls were saved in order to see the difference in noise
performance. ‘Before and after mod’ screenshots are saved into the animated gif depicted below.
In this animated gif three filter responses are displayed from top to bottom:
CW (CFIL, i.e. original filter), CW (2300 Hz BW, ‘SSB-filter’), and CW-N (300 Hz BW).

The crispy yellow points are the FT-857 audio beeps when selecting another filter.

Conditions: signal of -140 dBm @7025.0 kHz inserted into the antenna connector of the FT-857.
Audio extracted from the front headphone connector with headphones as parallel/speaker load.

It can be seen that the screenshot taken at around 19.48 hrs (‘after mod’) is more dark/black in the
higher frequency region than the 19.36 one. Of course this method is very ‘wicky whacky’ but it gives
some visual information about the audio (power) distribution. There may be some slight volume differences
between the two screenshots, due to implementation of the mod . . .

Anyway, the mod works for my brains, may increase (perceived) SNR with around +6dB as determined with a
‘wicky whacky’ method, and . . . is easy to realize!  See picture below (click to enlarge in new tab).

 

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