ICOM HM-98 mic mod

I don't know why ICOM didn't design a lot of their radios to mute the mic when it is keyed from the data port in the back.  Its easy enough to overcome this problem if you have a micro-miniature relay.

I bought a couple NEC ud2-5nu 5 volt coil relays.  They are VERY small and are double-pole double-throw.  This way the PTT button can trigger the relay coil, and there are two available contacts on the relay.  The first set replaces the PTT button.  The second completes the circuit on the mic element.

This mod is not for the faint of heart.  I find that using a diamond-tipped rotary engraver tool to cut the board traces works very well.  Once the trace is severed the protective coating can be scratched away leaving enough bare copper to solder some small wires to.

I cut the board traces in three places.  One in the hot side of mic line.  Second on one side of the PTT button.  Third on the other side of the PTT button.

The coil minus side goes right to the common ground plane of the board.  This gives the relay something to hold it in place.  I put it "dead bug style" on the top of the board, since there is plenty of room in the mic case to mount it.  The + side of the coil is fed from the PTT button with the red wire.  The other side of the PTT button goes to the +5v supply, with the yellow wire back to the 100uF capacitor.

Relay contacts fed with the black wires replace the PTT button.  The other relay contacts bridge the place where I cut the mic trace using the yellow wires at the bottom of the photo.

It may not be pretty, but it works pretty well.  The Q1 transistor seems to have enough capacity to supply the hungry relay coil another 20mA of current.

Now I can run my Signal Link for our local NBEMS practice net on FM using my favorite IC-207H radio, and not have to unplug the mic every time the SignalLink keys the radio.

Icom HM-151 microphone data mute mod

I found the design of the Icom HM-151 lacking in that it did not mute the microphone when running data modes through my SignaLink.  The solution is simple.  Install a tiny relay so that the microphone element is only connected to the radio when the PTT button on the handheld mic is pressed.

The tiny relay I found was actually Double Pole Double Throw.
NEC Miniature Non Latching DPDT Relay 5V 1A SMD SMT EB2-5NF

The specifications were quite impressive and the relay coil draw is very low, which is ideal for powering from the little Q1 pass transistor in the mic.

It was easily tacked into the circuit.  First the mic trace on the board had to be cut off.  I did this with an engraving tool.  See the X on the board where the trace was ground away.

The Common (Pin 8) and Normally Open (Pin 7) relay contacts were soldered directly to the board to reduce lead length.  The un-used pins were bent carefully out of the way.

The coil minus lead (Pin 1) was soldered to the PTT button contact.
The coil plus lead (Pin 10) was soldered to J2 pin 2, which is +5 volts.

The results are good.  I can run the local Data Net without fear of background noise, and I don't have to unplug my mic cable.

Amateur Radio - IC-207H 2m power module repair

The Icom IC-207H is a nice radio.  I was fortunate enough to obtain one from a friend.  It was used hard and its beginning to show.  However, as a mobile rig goes, its a small tough package.  The problem is that I've begun receiving reports that my signal is breaking up when the radio gets hot.

A fellow ham also has a IC-207H and recommended to inspect the SC-1091 VHF power module.  Apparently this module has a ceramic substrate and will form cracks due to uneven heating.  He recommended bridging the cracks with solder and a thin wire.

Since the IC-207H dual band mobile is a fairly simple rig, disassembly is easy.  First remove the head unit and the screws that hold the head connector on the main unit.  Remove the four bottom case screws and then remove all the screws from the main board and the two retaining screws from the power modules.  Unsolder the antenna connector and slip the board out.

The 2 meter power module is the one with fewer wires coming out of it.  (The 70cm module is actually sealed and you can't get into it anyway.)  Unsolder the 2 meter power module and GENTLY pry the black cover off.  Its on there pretty good, so just work it off a little at a time.  Don't break the ceramic substrate.  Once the cover is off inspect the ceramic for hairline cracks.  Use a soldering iron with HIGH heat >40w and bridge any gaps in the traces with some light wire and good solder.  The module dissipates heat quickly, don't use too much solder or you'll just have a blob.

You can see the hairline crack on this module and then how it was repaired.  You have to look really hard to see the crack.  Its in the center of the board.

Forgive the globbly solder, its on there good, but its hard to get it hot enough to melt uniformly.  I didn't want to make it worse by overheating the ceramic in one spot with the soldering iron.

When re-assembling grind the little clip areas off the inside of the 2 meter power module cover.  This will reduce the stress on the ceramic during re-assembly.  After this repair procedure the radio works great.

Amateur Radio - Icom IC-746 meters go full scale

There is some documentation floating around concerning the failure with the Icom IC-746 and self oscillation on the 6 meter band.  The suggestion is that the radio begins to receive itself through coupling of the open contacts of the TX/RX relay.  When this happens the radio goes out of control, with all the meters going full scale.  The power output is real, but there is no audio.  It started during the January VHF contest and got consistently worse.

I replaced Q25 and D22, but that didn't solve the problem.  I also saw that my radio had the factory mod for the PIN diodes around IC151.  They were found to be ok, but replaced anyway.

I found a page on the same problem on a IC-756PRO, that looked like it would solve my problem, but it needed adaptation to the 746.
http://www.hampedia.net/icom/ic-756proii-oscillation-problem.php

The concept of the "crowbar" relay to ground the RX line during transmit seemed the right thing to do.  There was also plenty of space to install the relay.  I chose a very small 12v relay that could mount right above the L28 inductor.  I removed some of the green surface of the ground plane of the board with a diamond bit engraving tool to expose pure copper of the board.  This allowed me to solder the relay's contact leads directly to the surface of the board.  The other side of the NO contact went to the trace that joined L28.

The coil wires of the Crowbar relay were run external to the board over the edge and to the same points as RL4.

The installation is clean and the radio works great now!  Full power restored to 6 meter band and the autotuner now works again.

Control Board from the bottom showing the Crowbar relay coil wires.

Postlogue: 

The crowbar modification was working well for almost 10 months.  Then something in the setup changed which stopped it from working.  The Receive Antenna Relay Box modification plugged into the ACC2 port seems to draw enough current from the trigger line that it stops the crowbar relay from working.  When the plug was removed from the ACC2 port, the crowbar relay started working again.  Make sure the main power supply voltage is at specification as not to produce a brown-out condition on these modifications.