de NØBEL, Kevin

Sometime in the 90's I bought an ICOM 706.  Its a great radio and i used it alot HF mobile, with a rack of 5 hamsticks on the top of my van, plus an LDG Tuner kit I put together.  After getting the FLEX-5000 recently, and putting my IC-736 and IC-820 up for sale, I began to miss having a radio that didn't need a computer behind it, mainly for a portable HF digital briefcase setup I wanted to put together (radio, tuner, interface, 10" netbook). I hadn't used, or even seen my 706 in literally years.   I began to wonder if I sold it or, lost it.  It finally dawned on me that it was installed under the seat in my current van, with the face/head in the glove box under the other seat.

So, as I said, I haven't used it in literally years, so I pulled it out got it on the bench and power it on... "click, click"... wouldn't power on.

 

A little messing around and a couple of internet searches, dis-assembly of the face/head unit, restored power.   In my case the culprit was dirty contacts on the head end part.

"click... hisssssssssssssssssssssss"... ok we have white noise good.   It took a few minutes to re-familiarize myself with the rig controls, menus and such.  I got it plugged into an antenna, and began to tune around on 20m, and heard nothing.   Middle of the day and 14.233 (Digital SSTV freq) was quiet, which made no sense.   I began to wonder whether we had a geomagnetic event closing down the band, so I flipped on the flex, and didn't hear anything either.  I moved down to the 14.070 checking for PSK, which is like "always" there and found some sigs, but the 706 heard nothing. So.. what the?... I turned it off put it on the shelf, and forgot about it for a while.

A day or two later, I decided to try again, and also reconnect the LDG tuner, to see if that still worked.  The tuner was on the shelf in the basement for those several years.   The tuner wouldn't tune.. kept flashing at me... again put it all on the shelf.

Ok another day... Lets see if it transmits... connect up wattmeter, mic.. load... whistle.......... nada... what the?... The radio was still on 20m, so lets see about 40,80... whistle... YES.. power (along with rumbling RF into my pc speakers)... 80m.. good

This is when I began to tune using the MHz step size of the tuner, and connected it to an antenna.   As soon as I got above 8Mhz, the noise floor would drop to S0... as soon as I got about 15mhz the noise floor came back... Ah ha... certain transitions across frequencies also cause an audible click of relay(s), as the radio clicks in different filters.

A few minutes later and a download of the IC-706 service manual, revealed one filter section that was labeled 10Mhz/14Mhz and on the other side 8.0-14.999999Mhz.   More searching revealed that this is a common problem for 706's, and even a youtube video showing repairs (altho with blown diodes and shorted transistors).    Not ready to engage ripping apart the radio, I decided to put it back on the shelf for a while.

Finally getting back to this, I disassembled the radio enough to get a good look at the filter board, and it seemed that I could trace the problem.   It did take a while and some logic and detective work to find the bad relay.   The circuit was not strait forward, with power from one relay being supplied by the next lower filter, in what I think is some sort of fail safe method of never allowing two filters to be in line at once.  Apparently the designer preferred high impedance (no connection) as better than the lower impedance presented by two filters in parallel.  I went ahead and removed the bad relay, and checked coil resistance, and sure enough it was open.   I found the relay at digi-key,and while I was at it ordered some other parts that I had been meaning to.

Between all this I had rebuilt my USB/Soundcard/Comport/Rig Interface (home brew) that I originally made for my IC-736.   With some advice from a friend as to how to make a better enclosure for it, I rebuilt it, painted it, and re-wired it to match the 706 ACC jack.

The next day, impatient that I wanted 20m and DSSTV monitoring, I looked over the schematic, and decided to swap one of the other relays on the filter board with another band.  I chose the .3 to 2Mhz filter figuring that I don't have 160m and probably not going to liisten to broadcast AM any time soon.   So I remove the input relay from that circuit.   I check it, and its coil is open!.. .. what the?   So I either damaged it removing the relay, or it was also failed.  I didn't check those lower frequencies, so I can't be sure which.   

Well each filter section has two relays... Thought I might as well pull the other one.   After doing so, I found that one worked fine, so i popped it on the 14mhz filter area, connected antenna, powered on and it worked!    I reconnected the LDG, and set up a tune and it worked!  (the LDG just flashes when you try to tune but no tx power is detected).   Wow strange, everything works!  (well not .3 to 2.0mhz of course)

I went on to test the interface box, install the software on the netbook (HRD and EZPal), and it all seems to work (tx level is not right, but at this point I just left it receiving DSSTV while I write this little blog entry) 

This whole excersice just goes to show that many times fixing a radio turns out to be something relatively simple.  Luckily the relay is a popular part, and next week I should be able to replace failed ones.