Kenwood HF-50Mhz TRansceiver TS-680V Review Digest

Icom  "Nana Yonju"  ( IC-740 ) Repair

The radio model is about forty years old. It is a little older and slightly larger than the more popular IC-735. It sits about in between the IC-730 which is compact sized rig and the IC-735, also a compact rig. The 735 sports an LCD display rather than the fluorescent display of the 740 and 730. What drew me to the IC-740 were the reviews on eham.net and other articles on it in the internet. I remember this particular comment by AB4OJ: "IC-740 - $200 to $250 - WARC bands, no general coverage, has IF Shift and PBT,very quiet frequency generation system" in his article "Good Older Icom Radios." (please google the article title.)

I read of a defective radio up for grabs and made my mind up to try and get it. Buying a non-working radio is always a crap shoot but is the sort of thing that interests me most because 1. it usually is cheap and 2. it offers a challenge.  

These are some pictures posted by the previous owner in his ad for the radio:

Despite It's age, it was still a nice looking radio.  and the fact that the VFO knob was responsive tells me the CPU  should be ok.

The radio came in with weak receive and transmit. IT is a consolation that the display did change when the VFO knob was turned. The power switch was bad (one pole of the dpdt sw had bad contacts) and as a quick remedy, I used the pole usually meant for switching AC to the inbuilt switching supply (which I decided not to use anymore. I did an alignment of of the tunable RX, TX portions as well as the PLL multiplier stages and zeroed the PLL VCOs. (the trimmer caps of the VCO were still good.) A problem was an intermittent behavior. All this was possible because these old radios had service manuals that are available. I always said that it used to be that the availability of a schematic and a service manual had always been a condition of sale as far as ham radio equipment (at least for hams in the U.S.A.) I have noticed however that many ham gear nowadays don't come with schematics or service manuals available which almost tells us that these gear are throw away radios.

A stubborn fault of the radio was intermittent behavior of the PLL going out of lock randomly and then going back in lock randomly. IT took me about a week to narrow down the problem to the HPL section  I was touching a scope probe to a voltage point and when I move the probe, it touch an adjoining electronlytic and the radio went out of lock. Soft tapping on the HPL board would sometimes cause the radio to go in and out of lock. Well it became apparent that cold solder joints is the problem.

My rather cramped repair setup.  I always preferred the shortest distance between the actual work and the scope display.  Here you could see the probe on the HPL board, specifically the phase detector output pin of the MB8718 BCD programmable PLL chip. The HPL is responisble for band selection (500kHz step). There is little documentation on this chip on the net, while its sibling the Fujitsu MB8719 is very popular in CB radios however.

 I did a reflow of the entire HPL board.

 Radio fixed! 

After a frustrating week, I was ready to dance my Irish jig of joy!