My IC-735 sojourn: travel or travail?

My IC-735 sojourn: travel or travail?

Sometime in December 2015, I bought a used  Icom IC-705 (white one in the picture) and I proceeded immediately disassemble it so that I can attach a spare front panel that my friend Ricardo gave me.

The front panel came from an Icom IC-731, the Japanese domestic model of the famous IC-735.  

The front panel has a damaged LCD.  

The IC-705 is a rare creature however.  I was told that  the local distributor took some new IC-735 replaced the front panel with a VFO less one so it can be sold to the commercial fixed frequency market.

I first saw the ad for the 705 months earlier.  There were two that were advertised for sale by a ham from Lucena City but too many questions in my head back then so I did not buy it.  Eventually a ham from Marikina, Ed bought the pair and a few days later, I was at his place trying to buy one of them.  The one he sold me looked better than the one he retained.  Normally you would wonder why. But I did not quibble as i got to have since I already have the proper front panel for it with a good VFO encoder. I had thought to use the good LCD of the 705 too. The plastic polarizer film was falling off the 705 but I did not worry about that.

Apart from the LCD, everything seemed to be fine when one day, while I listened to a CW transmission, the clean tones suddenly became raspy sounding almost sounding like a buzz. Now that sound like FM (pahse modulation), I said to myself.   I researched and saw several artciles about the common faults for the Icom IC-735 and found that a few trouble prone "plastic" trimmer capacitors had to replaced. with better ceramic ones.  So I ordered them from RS online.

They came after a week or so and I followed instructions on the net, carefuklly unsolder the large shield can that contained the HPL VCO banks and replaced the trimmer set.  I also replaced the master oscillator trimmer capacitor.

I was almost ready to volunteer the radio as a backup for the upcoming INFEX in February when I decided to ragchew with the radio as  a final toruture test.  Up to then, I only used for short contacts and the very short check-ins to the DU net.

And failed it did. After it warmed up a bit, power became intermittent, sometimes cycling between full power and low power in two second intervals and some parasitic oscillations started to occur the moment one transmits above 21MHz. If I unplugged the cable from the main unit providing the drive for the PA, there is no oscillation,  So for several days, I tried to figure out if the fault was in the main unit or the PA.  Well this early morning at about 5:00AM I discovered the cause:

An hour later,  I found  a used transistor of the same type  (a precious "original" that came from a junked Kenwood) in my junk box and replaced it immediately.  I was pleasantly surprised that the oscillations ceased and it now transmits well.

Due to thermal stress a microscopic crack may have developed at the transistor die a which gives it the intermittent nature of being normal one second and low powered the next,  This part is perhaps the reason the unit looks so fresh inside compared to the other 705 that came with it is because, the radio did not get much use as it always developing a problem and got shelved most of the time.  Well we are grateful for what is provided us by fate in this life.  I also found that the socket at the main board which supplied the drive to the transistor was a bit worn out and loose and had cracked (cold) solder connection for the center conductor.  This could have contributed to the instability of the stage.

 During trouble shooting, I installed a resistor (380 ohms) in parallel to the original 220 ohms to ground at the base circuit to decrease somewhat the bias on the device.  I left that in, in the hope that the pre-driver will run cooler.

I am thinking of adopting the temperature conpensation scheme used in the TS-450S for its pre-driver, also a C1971.

The setting of APC in the service manual is a bit cryptic (Icom IC-735). I suggest: 1. adjust R268 to power output of 110 watts; 2. adjust R260 to bring power to 105 watts; 3. adjust R262 to bring power down to 105W and finally 4. adjust RR268 again to bring power to 100W. Almost all modern transceivers be it HF or VHF have some sort of automatic power control which scales back power output in the event of high SWR or excessive current flowing to the PA. APC designs vary from model to model.

I just put in the special order green LEDs to replace the provisional white ones, I put in earlier. I had to use fine grit sandpaper to give the clear LEDs a frosted finish for better light dispersion. Going green is a big improvement in viewing comfort. Using white LEDs specially bright ones gives the sensation of trying to look into a flashlight. HIHI. Now if it were anegative display, white would be better. The display is a lot closer to the original IC-735 look with the incandescent bulbs and green rubber boots.

I have finally restored the FM and AM transmit functionality of this radio.  It seems the guy who did the amateur to commercial conversion disabled AM and FM transmit by the easiest way possible. He adjusted the series inductor (adjustable core) to the crystal used for FM and AM transmit.  The crystal should be trimmed to operate at 9.0115MHz.  what he did was to adjust the core down and well away from the center of the winding so that the circuit will stop oscillating.

A lot of work still has to be done on the radio.  But we'll leave that for later.