At that point I guess I should have listened to the little
voice in my head that was saying, “Warning!, Warning!, Danger Will Robinson,
Danger!”.
Against my better judgment, I bought the radio. I figured if nothing else, it would be a good
learning experience. When I bought it,
the man I bought it from told me the previous owner had replaced the finals but
was experiencing this intermittent transmit.
Step 1: Build a dummy
load. I needed something I could sink
the transmitter with. I’m cheap at
heart, and I couldn’t see spending an arm and a leg on one. So I resorted to my old standby, Google. Many thanks to all the Hams, that whether they
know it or not have been my Elmers in this situation. I found two different ones that caught my
eye. One that used 4 - 100 ohm, 100 watt
thick film resistors mounted on a heat sink,
and one that used 20 – 1k ohm, 3 watt resistors. Both were built into quart paint cans and
filled with mineral oil to help dissipate the heat. I settled on a hybrid design based on the one
by K4EAA. My design was a single 50 ohm,
100 watt thick film resistor mounted on a heat sink. The part I borrowed from K4EAA was the RF
detector that lets you measure output power using a simple volt meter. I bought all the parts from Mouser since I
don’t have much of a junk box yet. I could have gotten the thick film resistor
for a couple bucks less at Digi-Key, but by the time I paid for the additional
shipping, Mouser was the better deal. I
bought the paint can and miscellaneous nuts and bolts locally. A note about the mineral oil. Mineral oil is used both as a sealer for
butcher block countertops, and medicinally as a laxative. Both are the same. The slightly less than a pint bottle at a
home improvement store (I tried all three in my area) was between nine and
twelve dollars. I stopped at a local
hardware store, and they carried a full pint bottle, but only had one, and it
was just under four dollars. Pressing
on, I stopped at Wal Mart, and in the pharmacy section, I found the mineral oil
for a dollar and sixty nine cents per bottle.
The lesson learned, “Don’t dismiss Wal Mart as a source of parts for ham
radio.” And although I have the oil, I
have not put it in the dummy load yet. I
have the resistor mounted on a computer CPU heat sink. And none of my radios even come close to
being 100 or more watts. So for now I’ve
left it dry. When building your dummy
load, make sure you don’t use binding posts that can accidently short out on
the lid of the can. I spent almost two
weeks chasing gremlins because of that.
The magic formula for using a DVM to calculate a radio’s
power output:
((volts/1.414)squared)/50
Using my cheap DVM’s (I have two of them), I got similar readings
and within the correct amount for the radios I measured.
Step 2: Build an RF
probe. Using the same diode/capacitor
technique from the dummy load, you can build a simple probe. It’s helpful to be able to trace RF through
the circuit to see where it’s being lost.
Step 3: Schematics.
You *need* them. If you can find
a service manual for your radio, get it.
It will have much more than just the schematic. It will help you understand the various
components and their functions in your radio.
You may be very knowledgeable in radio repair and may not need this
item. If that’s the case, more power to
you. Me?
I’m a firm believer in reading and understanding manuals. In this case, I read the text description of
the various parts of the radio several times to get a flow for how things
worked so I could eliminate areas that I didn’t need to work on. I was very fortunate to find a copy of the
service manual for my radio on the web. Again,
Google is your friend. Expect the older
manuals to have been scanned rather than electronically produced. They may not be the best scans in the
world. In my case I had to look at three
different copies before I found one legible enough to use. It’s still not great when it comes to
voltages and things, but it’s still very helpful.
Step 4: Soldering
skills. Again, you *need* them. If you are unsure of your skill set, find
something else to practice on. When
working on high wattage RF, you want to make sure that you are leaving nice
rounded solder joints. Don’t believe
me? Have a look inside your radio at the
RF output section. Leaving pointed areas
of solder or parts leads provides the opportunity for arcing. That’s bad.
Step 5: A good work
area. I live in a small apartment with
my wife. And I don’t have the luxury of
my own work bench. So I usually take the
kitchen table hostage for my projects.
Thankfully my wife has come to accept this idiosyncrasy of mine. You will also want plenty of light. My wife scrapbooks, and has something called
an OTT light. It’s supposed to be a full
spectrum fluorescent. I usually borrow
that from her along with a magnifying lens on a flexible arm.
So now we’re at the point where I’m finally ready to get
started. Are you ready? A word of warning here. If your radio is still under warranty, any
work you do on it will undoubtedly void any you may have left. I am not responsible for your voided
warranty. So think about it before you
turn the first screw. Please? Carefully disassemble your radio. I generally have several sandwich bags around
that I can put groups of parts in that belong together. If it’s likely I’ll forget what the parts are
for, I’ll write on the bag with a sharpie so I can remember later where they
go. In my case, I took off the top and
bottom off the radio, and I took the shields on the RF sections off the
unit. It was immediately clear that the
PA (power amplifier) module on the VHF side of the radio had been replaced but
I wasn’t sure about the UHF side. How
could I tell? Well, without bashing the
last person to have worked on the radio, it was fairly obvious that the tech
had some trouble soldering the module into place. So I started on the VHF side, even though
neither side would transmit. I had
expected the issue to be common to both sides, but it turns out it was
not. After some false starts and some
retracing of steps, I came to the conclusion that the problem was the PA
module. I could prove that the required
400mw signal was reaching the PA module, and I could prove that the switching transistor
was applying the correct voltage to the PA module for the requested power
output, and I could prove that the PA module was receiving a full 13.8 volts
from the radio’s power bus. But when
keying the radio, no power was coming out of the module. So I carefully unsoldered the module. And once I did, three of the four leads from
the module fell out. Oops! That’s a problem. Unsoldering the module could have caused
that. I spent part of an evening looking
for a replacement module online. They seem
to be available, but no one posts prices except one place. And I’m not about to pay more for the module
than I did the radio. So with leads
falling out, my curiosity was piqued. I
felt like I really didn’t have anything to lose, so I decided to take the
module apart. The module had something
white oozing out all of the lead holes and around the sides of the case. I thought it might be potting compound. It turned out to be silicone sealer. I used my Xacto knife to break the silicone
seal and then gingerly pried the cover off.
Here’s what I found:
If you look across the bottom, you can see the pads where
the leads that are missing belong. One
lead is still attached. But take a close
look at the pad on the left. See how
black it is? When I looked at the bottom
of the lead that came from that position, it looked the same way. What does that tell you? Arcing!
That particular lead is the output of the module. At full power, the radio puts out 43 watts. That's about 70 volts peak to peak. So there is plenty of potential for sparking
if there is a bad or loose connection.
Of course at this point, I can’t tell if the module was defective, or
the issue was caused by the previous tech’s soldering problems. So it’s no wonder the radio had intermittent
transmit. I made the decision to try and
repair the module. Again, what could it
hurt? I already have a module that’s not
working. What’s the worst that could
happen? The module still wouldn’t
work? It was worth the risk in my
book. So I cleaned up the excess
silicone, and I cleaned the soot off the pad and the lead. Turns out the arcing pretty well destroyed
the pad. So I ended up soldering the
lead to the end of the part next to the pad.
I reattached the other two remaining leads. Then I got out my computer heat sink compound
and spread it on the back of the module.
Placed the module back in the radio, and soldered it into place. Here’s a picture of the module back in place
with the cover still off:
The results? Believe
it or not, it works! Full power into the
dummy load as measured by my DVM and the magic formula. At this point, I hadn’t tried it on the air
yet because I had the mic disassembled at that moment. And yes, I know I harped on soldering skills
early on. The soldering of the leads onto
the module itself was tough. I had to
use a high watt soldering iron because of the heat sink. But the places where the leads are attached
to the main board are nice and round.
Testing on the UHF side indicated the same problem. Good signal into the module, nothing coming
out. And the switching transistor is
turning the voltage to lead 2 on and off like it’s supposed to. This time I took a picture before I removed
the module.
It looks to me like this module has been replaced too. My reasoning behind that is that although
it’s hard to see here, there was some solder flux around the pads, and the
solder joints weren’t what I would call factory smooth. I’m not sure if it was done by the same
tech. The solder joints on this one are
much better than on the VHF side. Again,
this is a nothing to lose situation. It
already doesn’t work, so it’s not going to hurt for me to take it out, open it
up, and have a look. Here is what I
found:
Pin 2 (second pad from the left) was not connected. Pin 2 is what keys the module and causes it
to amplify the input signal. So in this
case the module was never getting turned on.
I didn’t see any evidence of arcing, so I have to wonder if this was caused
during installation. Opening this module
up was much more difficult than its VHF counterpart. There was considerably more silicone
used. I actually broke a tab off the
cover that goes between pins 4 and 5. If
I decide to put the cover back on, I’ll super glue the tab on before I
reassemble things. But at this point I’m
seriously considering leaving the cover off both sides. I don’t see that it serves any major purpose. Here is the module reinstalled after the
repair, again with the cover off:
The first test didn’t go so well. Power in, nothing out. But as I was probing around, something
changed. I took RF measurements again,
and this time I was getting unbelievable values. I suspect it was my homebrew RF detector
circuit. But I was getting output!
Lessons learned:
- Most importantly, listen
to the little voice in your head.
If it tells you to run, then run.
- If you are bound and
determined to take on a radio as a project, be prepared to improvise and
use some unorthodox methods.
Especially if your project radio is 20+ years old. Some of the major parts may not be available.
- If you can prove to yourself that something like a module is the culprit for your problem, don’t be afraid to disassemble and explore. This is ham radio after all. We are *supposed* to be able to homebrew solutions for our problems. Disassembling and exploring may provide you with a ready solution like it did for me. But if not, nothing ventured, nothing gained and you may learn a thing or two in the process.
I reassembled the radio enough to
test it on the air. Got a great signal
report and the unit behaved well. So it
was money and time well invested. I
still have one thing left to do, and that’s to replace the display lamps. They are both burned out.
To replace the display lamps, I decided to use high brightness white LED's rather than putting in OEM lamps. My reasoning was that the LED's should last longer. Probably longer than the radio does, and I only wanted to do this once. The original lamps were what I would call 12 volt grain-of-wheat bulbs. Something I would have used in model railroading in the 1960's.
The hole for the lamps is about 3 mm in diameter. And of course locally, I can only get standard T 1 3/4 size LED's which are 5 mm. I also wated something that was more of a flood light than a spot light. The LED's I had access to were water clear, so they fell into the spot light category. I could have ordered in some 3 mm LED's that were brighter than the ones I used, but being the impatient sort I decided to have a go with the 5 mm ones. If you choose to do this, be sure to buy spare LED's you *will* ruin some in the process.
I had read somewhere that you could diffuse the light from an LED by sanding it with a low grit sandpaper. So i dug out my Dremel tool, stuck one of the large grit sanding drums on it, turned it down to the lowest speed, and tried roughing up an LED. Lightly at first. That worked pretty well. So I sped the Dremel up and tried "resizing" the LED. I needed 2. I ruined 2. You have to be very careful no to open the "envelope" that contains the diode itself. I don't know what happens, but it gets very dim. I used the grind and fit method to try and avoid ruining any more of them. I finally got LED's fitted into both places and turned the correct direction to connect to the positive and ground pads. Of course the LED is a much lower voltage device than the lamps, so after doing some calculations, determined that I needed to include a 400 ohm series resistor. No such animal at my local Radio Shack, so I took the 470. Here's a picture of the result:
The lamp brightness setting in the menu works. But there's no difference between settings 1 and 2. Honestly, I'm thrilled.
I spent $65 for the radio. Probably more than I should have under the circumstances. And I spent almost two months rehabing the unit. Other than my labor, the only real cost I have involved was a plastic lens restoring kit to take a scratch out of the display, and $5 worth of parts to replace the lamps. But I ended up with a nice little unit that functions well as a base, and that I can use to experiment with without worrying about destroying any of my other equipment.
*** UPDATE ***
I recently had the oportunity to do some computer work for a gentleman that owns a business radio repair service. I detailed my process in this repair to him, and he told me that this issue is not all that uncommon. And it's not limited to ICOM. He told me they've seen this a number of times on several radios that used manufactured PA sections. It seems the issue started when the industry started using lead-free solder. Sometimes the joints don't hold. His suggestion was exactly what my solution was. Disassemble the module, resolder the leads, and put the module back in. He said he actually talked to a factory rep who while not directly encouraging the use of lead-based solder in doing the repairs, said that there was certainly nothing wrong with it when used in small amounts and with adequate ventialtion.
*** UPDATE ***
I recently had the oportunity to do some computer work for a gentleman that owns a business radio repair service. I detailed my process in this repair to him, and he told me that this issue is not all that uncommon. And it's not limited to ICOM. He told me they've seen this a number of times on several radios that used manufactured PA sections. It seems the issue started when the industry started using lead-free solder. Sometimes the joints don't hold. His suggestion was exactly what my solution was. Disassemble the module, resolder the leads, and put the module back in. He said he actually talked to a factory rep who while not directly encouraging the use of lead-based solder in doing the repairs, said that there was certainly nothing wrong with it when used in small amounts and with adequate ventialtion.
73
Steve
KDØWSW
Steve
KDØWSW
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