ICOM IC706 classic and FAN noise reducing

I know the IC9700 VHF UHF SHF is the latest development  SDR based sets maybe  will purchase one. From a ham friend i tested  the 144MHz all mode TR751E and its 70cm twin TR851e

Very nice design, small  25-30W rf very good receiving excellent, brilliant audio RX TX with standard microphone , no need to puzzle menu settings just brilliant a pleasure to use. (except repeaters.. fuzzy settings no CTCSS)

If i hear the struggle with modulation settings in the new FT991A some HAM have?

Sorry enough these Kenwood's /E  version miss the essential CAT connection and CTCSS it will not work with repeaters

Same aged?  IC706 classic  is still a good performer at 144MHz. and does have CAT and CTCSS  and tunes  Hz precise

I had to assemble a custom desk microphone with electreet and MAX9814 amplifier to have a good modulation report the original HM100 could not bring reasonable audio

Home made desk microphone electreet max9814 and blue led glow to scare off  Covid...

 

The classic without TCXO option is by default frequency stable enough for the most demanding WSPR mode, frequency drift stays within a few cycles in a 2minute TX period. 

Not bad at all for such an old rig. 

144 148 MHz band pass is automatic active  in this tuning range removing strong out band signals IMD.

The rx preamp on board is a uPC1658G  MMC broadband low noise about 2dB nf  it is used for all bands.

I estimate with the extra circuitry the snr could be 3.

So even with low cable loss an external LNA could improve weak signal receiving.

My cable loss 27m H2000  1dB at least, the VHF Design LNA is placed close to the 12eM2 antenna should bring  nf from  3 or 4  down to  0.5 roughly.

I can select  or by-pass the LNA down at the PA that makes a quick compare possible.

In practice  weak signals are clearly raised  out the noise, dough not as much as i expected i keep hearing most weak signals with the bare IC706 preamp on of course.

Maybe noise figure is for the human ear less important as most hams assume ? 

In digital modes FT8 one can compare the snr figure .

But hearing a 3 dB difference is not that easy.

The drawback of a preamp without input filter is that strong local signals IMD 

will reduce the receiving f.i. 28MHz tx nearby

The VHF-Design AFT53189  does have its 3 stage band filter at the output of the HEMT Fet.

 I use the IC706  at 144 in combination with a 200W SS-PA and an VHF-design mast preamp ATF53189

And the Micro KEYER2 as interface to  sequence  switching the external PA and inline preamp. 

also the MK2 interfaces CAT and audio. the MK2 makes the things  working well!

Had to modify the preamp with a surge arrester to keep it EMD protected and made its milled aluminium case weather resistant The 12V power over the feed line So the combination old IC706 with MK2  with its sequenced timing works like a charm!

The IC706 is also in use as i.f. driver for 1296 MHz  then 146-148 MHz range for 1296- 1298MHz

 

But now to the constant noise of the IC706 internal FAN in receive, weak but annoying hum all the time I notice a relief pfff when the Icom is switched off..

I do not have to point out, low noise audio and RF wise  is a key factor for receiving.

Fortunately this can be solved  . When studying  the diagram in the PA part the fan is running at 13.6V with R65 = 47Ohm in series. Q15 is shorting R65 at HF TX,  then  the FAN runs full speed.

Not at VHF TX time dough but then only the PA driver is active.

I replaced R65 with 100R  1W SMD type that keeps the FAN running at a much lower level at RX but still cooling the inside parts. The noise is dropped to non annoying level. 

  Bottom cover removed shows the PA PCB ,the position of R65 is at the right between PAO and Q15

 R65 47R replaced with 100R  lowering the standby  speed of the fan.