IC290A- A practical repair and fix - other examples follow.
At ham radio flea markets and swap lists you can often pick up radios with problems at a real good price. I paid $25 each for these two ICOM IC290A 2 meter all
mode transceivers. The fellow who sold me the first one was honest and said that the radio worked fine but the encoder for frequency adjustment did not
function well at all. He was true to his word. The second IC290A had not too much wrong with it. The power plug had been changed and the speaker leads and
connections were broken but otherwise worked ok when I connected these things up.
This radio has no interna l battery for memory back up. You need to keep the power on or connect up a wall wart power supply or perhaps a battery to the
accessory jack. Every time you lose or shut the power off..you lose the memories (there are only 5 memory positions) and you lose your dial settings as well. The
rig reverts to 146.000 and you have to tune.. 1Khz at a time to the low end of the band..if you are on the SSB/CW portion. So for me..backup power to save the
settings will be a good idea. I am actually thinking of putting one of these rigs in the truck for mobile use. There are certain high hills around town here and I could
even try running it in a vhf contest with a yagi parked mobile shooting across Lake Huron into the USA and southern ontario.
It wasn't all that difficult to take the front panel off the IC290A and
remove the probable cause of the problem. ..that being the Rotary
encoder. I use the words..probable cause..because it could also be that
the logic circuitry was causing the fault.
I removed the 3 wires from the encoder. I looked for a part number on
the item but I saw only 155X which did not seem to be a complete part
I went to a site on the internet called DK MODs where I downloaded a
free copy of the service manual for this radio. DK mods allows a free
download every 4 days.
I had an old DDS vfo that I bought about 7 years ago. The dds vfo was
not working but I still kept it and the old rotary encoder was still with it.
The rotary encoder was square and smaller than this unit. It had 4
terminals but two connections were bridged together so..it might work. I
connected it in place and it did not work properly. The radio tuned only
in one direction. I then switched the two outer connections and the
radio now tuned fine. It works! -- So sometimes you luck out. I dont know
all that much about encoders but I intend to learn more.
Symptom: VFO tuning is erratic, sometimes skips frequencies in a certain area if VFO knob travel.
Least significant digit in frequency display sometimes "flutters" back and forth instead of changing
incrementally when the VFO knob is turned slowly.
Probable Cause: Bad rotary encoder.
Cure: Substitute rotary encoder with a known good one. It is recommended that the rotary encoder
be replaced as a complete assembly if it is bad.
Remarks: If the unit tunes in only one direction, i.e. either up or down, there may be a problem on
the logic unit. Mine tuned only one way but changing the rotary encoder cleared it up.
My NOTE: the IC290A only did tune in one direction..down in frequency. I substituted in the rotary
encoder from my junkbox..as per photos and the IC290A does tune properly now..although ..it does
have a few quirks. Sometimes it tends to stall out or start turning the opposite direction. I can get it
going again correctly and it functions but not without these backlash type issues.
Perhaps if/when I get the real McCoy rotary encoder as per ICOM original ..the tuning will smooth
out. For now..however..I am happy that I can use the IC290A to some degree. It is an older radio and
I'm not going to spend too much money on it.
Searching the web I found this note about the IC-761 (Icom transceiver with similar symptoms to my
MORE research on ICOM encoders for ICOM transceivers led me to an interesting article (the url
is indicated below). This is for an OPTICAL encoder. What the IC290A uses is a more simple
The optical encoder has electronic parts such as IR LEDs, phototransistors, switching transistors,
resistors all mounted on a PCB. There is a careful alignment to these devices and theer is a rotary
mask assembly to generate the pulses for tuning. I know very little about these devices..but I
presume..they might be expensive to replace and/or the parts may be in short supply.
IN SUMMARY: We as amateur radio people are experimenters by nature and inquisitive as well. I didn't do much research on this part substitution...I
just did it. I had no data sheets to look at or spec sheets. The parts had no part numbers on them. So..sometimes..you just go for it. What is the
worst that could happen? Smoke??? Sometimes with certain circuits and components.
I don't know much at all really about digital encoders. I did not study them in College (1980). I can only assume that the mechanical encoders
generate a pulse or perhaps a glitch waveform which is amplified and squared up by logic circuitry and then that causes a counter to increment or
decrement and the radio changes frequency either up or down.
Waveforms are waveforms are waveforms. If we cannot buy a replacement encoder or repair the existing one..perhaps we as amateurs can build
one or modify an existing commercial encoder in production.
I took a look on ebay, and there are all kinds of listings for mechanical encoders like this one I used in this circuit. Im sure digikey and mouser and
other suppliers sell them.
I intend to learn more and I welcome emails from the readers out there to correct me or set me straight. I may publish a part 2 to this article in the
near future with more SPECIFIC DETAILS and technical details.
73 Earl VE3AB (hamelectronicsmagazine.com)
"I could not order the part because it has been discontinued" as per the above with the IC706. The IC706 is a more recent
production than the IC290a and I have not approached ICOM (yet) about a replacement encoder, but I have doubts whether they
would support this product anymore.
Rotary Encoder Idea from the AM Qrp Club
This is very similar to the one I used in my IC290A. This one was used by the AM qrp club
in one of their dds projects/kits and is supposed to be a fairly good one.
Here are some of the specs
Smooth, detentless, inexpensive encoders for use with microcontroller-based homebrew
2-phase output signals (A and B) as contact closures to a common pin
SPST momentary pushbutton on shaft
48 pulses-per-revolution (each phase has 24 pulses)
These are the same rotary encoders as used in the Micro908 Kit
Noble (a supplier of these types of devices) P/N 100-1166-101
Dimensions: 0.6" x 0.7" base, 1.1" from pcb to top of shaft
I cannot provide details like the ones above concerning the encoder I used because mine was a junkbox item. Mine had 4
terminals on the back. Two were bridged together (COM) and there was an A and B line as well. Mine had a plastic shaft vs
the metal one here but looked very similar. Before I put the encoder in the radio I wrapped it up a bit in electrical tape in
order to keep dust particles from getting inside the unit. I forgot to take pictures of it before I mounted it in the radio.
I received excellent audio report from the IC290a when I had a couple of qsos on a local FM repeater. I checked eham reviews and the
radio was a 3.9 out of 5. The memory backup problem is a problem. NO INTERNAL battery to provide memory backup. One fellow uses/used
a 9 volt battery to move the radio from the shack to the auto. Necessary because the memory is total volatile with this radio. When the
power is removed you lose any saved memory locations and your dial reads 146.000 mid band. YOU then have to dial the frequency 1kc at
|BASIC KINDS OF REPAIRS to Ham Radio Equipment..examples with an
emphasis on component interchangeability and discussion of generic
parts such as TTL chips.
by Earl Andrews VE3AB
FREE ON LINE Hands On kind of HAM
(This article under construction..(as is
this entire ""NEW"" web site 73 Earl
MECHANICAL vs OPTICAL encoders
I have not yet gotten around to reading up on how
encoders work, the codes, ie grayhilll codes and stuff
Most of my older gear like FT301s, FT221, HW7, HR10B,
Galaxy R530, Ten Tec Argonauts 505, 509, 515 -- they are
all manufactured with very little if no digital circuitry in
them. They use either variable capacitors and a coil or
fixed capacitors and a variable coil (slug tuned coil) PMO
(Permeablility Tuned Oscillators, as with the Argonauts.
By the way, I read on my QRP-L list the other day where a
fellow picked up a Drake PMO 5 to 5.5 mhz and he said it
was dead stable and nice to use in a vfo circuit!
Anyways..I did read about IC 735 transceivers and
problems with the Optical encoder. The fellow has a web
site and he said he spent quite a few hours trying to
repair and align his. He said in his article..that IF the
radio tunes OK but sometimes acts up with a backlash
kind of situation..he would just leave it be rather than
struggle trying to repair it. (Apparently ..with optical
encoders..it is not as simple to find a fit form function
This example I give with the IC290A..I just found a
junkbox part ecoder..no part number or markings that
could trace it to a specification sheet. I did have a
schematic of the next higher assembly..the (old) DDS
vfo..which gave a basic pin out of the encoder. I wired it
in and it worked. Maybe I just was lucky..or with mechical
encoders ..maybe they all pretty well work and are
interchangeable electrically. who knows??
I will close this topic off for now..Wed April 27 2011. I will be writing a bit more about some practical examples of parts being substituted in
ham radio equipment. QST magazine..often encouraged builders to try different parts. Sometimes there is no problem. You dont have to
be right on with resistors and capacitors. With resistors and capacitors...there is the value that they say they are: ie a one kilohm resistor
with a tolerance of 5 percent. That means..off the shelf ..the value of this resistor will be between 950 ohms and 1050 ohms for a 5 percent
tolerance. I think this is called the selection tolerance. When the item is in a circuit and is subjected to heat it will change value with its
temperature coefficient. With resisistors if you go to substitute ..say you ran out of 1 kilohm resistors..I would maybe try a 1,100 ohm
resistor as a substitute if I had some of those on hand. With resistors, you always substitute up in value. You add (maybe) a bit more
resistance into the circuit because the most harm that you will likely do is to cause less current to flow in the circuit (slightly). Less
current is better than causing more current to flow (which would perhaps cause more heat-- in term more temperature drift due to
heating (temperature coefficient).
There are some more basic kinds of tips and rules of thumb that are good to know. I will be writing more on this shortly.
The entire hamelectronicsmagazine.com is in its infancy and I am working to develop its structure and as well write up some articles.
If you have an article to contribute ..I would be pleased to look it over and publish it for you. Another thing I do is create temporary spots
on the web for your ham event or ham advertisements or flea market , basement sale...whatever. I do it very economically. A big page for
a month with lots of photos and descriptions I can do for $15 a month which includes periodic ammendments of description what -ever.
And I keep all the information and your private email ect..just that..PRIVATE....73 earl ve3ab
IC 706 example from the web I found. I (think) the IC706 encoder would likely be more
sophisticated than the one I replaced in my IC290A.
NAVIGATION LINK TO MY START PAGE:
A free on line publication by Earl Andrews VE3AB
It is a commercial magazine but with limited advertising and very