Icom IC-751 (IC 751 IC751) transceiver mods reviews software and diagrams

   
 
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24 October 2019, 2:24 UTC 


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Icom IC-751 (IC 751 IC751)
IC-751
Programming interface schematics for Icom:
Programming software for Icom IC-751 (IC 751 IC751) :
PDF User Manual for Icom IC-751 (IC 751 IC751) :
PDF Adjusments Procedures Manual with schematics for Icom IC-751 (IC 751 IC751) :
Schematics for Icom IC-751 (IC 751 IC751) :

Mods for Icom IC-751 (IC 751 IC751) :
    Icom IC-751 (IC 751 IC751) improvement for better CW full break-in
    Icom IC-751 (IC 751 IC751) Cooling, remote jacks
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    Icom IC-751 (IC 751 IC751) RAM Card Backup Battery Replacement Instructions
    Icom IC-751 (IC 751 IC751) Allow CW Xmit & USB recieve split mode
    Icom IC-751 (IC 751 IC751) to 37 MHz
    Icom IC-751 (IC 751 IC751) Bad Cap Problems
    Icom IC-751 (IC 751 IC751) General coverage mod
    Icom IC-751 (IC 751 IC751) Switch Matrix
    Icom IC-751 (IC 751 IC751) How to replace the lithium battery in your ICOM radio

Icom IC-751 (IC 751 IC751) technical specifications :  Icom IC-751 (IC 751 IC751) technical specifications



Icom IC-751 (IC 751 IC751) improvement for better CW full break-in

Modification of an Icom 751 for better CW full break-in operation

I recently worked on my Icom 761 and got out the 751 (not an A model). I have become involved in CW traffic handling and currently am the Kansas NTS CW Traffic Manager. The 751 really leaves a lot to be desired in full break-in mode. It has a terrible "thump" when switching in and out of transmit mode. In fact, it is so bad I couldn't use it this way. This article does not apply to the Icom 751A.

I dug out the schematic and began looking at the differences between the 751 and the 745 thinking it might be the same. I had fixed my 745 from "thumping" by replacing most of the CW audio amp electrolytics and putting additional large electrolytics on the CW lead at the switching point for activating the CW lead. Anyway, no such luck on the 751. It used an entirely different scheme.

I didn't see anything right away so I ordered a batch of electrolytics and a new audio amp thinking this might help. The 751 required turning the volume control almost to 12 o'clock before getting any hearable volume so I thought maybe something was wrong here.
I replaced all the electrolytics associated with the audio path, i.e. after the detector stage and the audo amp. I couldn't really hear any difference but at least I felt better.

At this point, I dug out the scope and started looking at where the thumping was coming from. The 751 uses two electronic switches (IC7 C and D) to mute the audio, one for the actual audio and one for the monitor/sidetone circuit. The scope indicated that maybe these were causing the pulses generated when they were switched. (I should have realized at this point that this wasn't the case because when switching to transmit a large positive pulse occured and when switching back to recieve a large negative pulse was generated. I figured out later what was happening.) Anyway, I started cutting traces and adding jumpers and effectively bypassed the electronic switches. What I ended up doing was connecting the monitor and audio stages to the audio amp with no switching. This basically fixed the problem but I lost the use of squelch and the volume was reduced quite a bit. So I started looking elsewhere.

A couple of things I learned was that the electronic switches were not needed for cw operation. The receiver mutes very well without them and the monitor/sidetone only works when the key is down. This is exactly what was needed. The electronic switches are really only needed for squelch operation but the circuit designers built the 751 so that the audio path is switched off when going to transmit and the monitor/sidetone audio path is switched on when in transmit.

The 761 uses a couple of transistors to "short" the audio path to ground when in transmit. I built up a little circuit to do the same thing using the squelch lead to control it. Lo and behold, I got the same thumps and when looking at the audio input the same pulses were there. I said to myself, self, what is going on.

The audio amp in the 751 runs at full gain all the time. The volume is varied by changing the amount of attenuation in IC 6A that is in series with the audio path. Since the amp runs at full gain, the audio signal is very, very low in the preceding stages. My scope just wouldn't show the levels since they were so low. So I built up a little amplifier for the scope to look at the audio at each point. Guess what I found?

Both the attenuator IC and the buffer amp for the monitor/sidetone have series capacitors feeding the audio amp. I just assumed these were coupling capacitors and wouldn't have any effect. WRONG! They are really DC blocking capacitors more than coupling capacitors. In fact, the audio path has a series electrolytic capacitor. What was happening is that these capacitors were being charged and discharged when the squelch lead (audio) or transmit 8 volt (monitor/sidetone) line activated. This is what was causing the positive and negative pulses on the audio line. With the high gain of the audio amp, these pulses were coming through as major thumps.

So, how to eliminate the thumping? I needed to keep the monitor/sidetone electronic switch from operating when going into transmit. I also needed to keep the audio electronic switch from operating when going into transmit.

Monitor/Sidetone Circuit Modifications

I started first on the monitor modifications. I found the monitor switch on the NB SW board. This board feeds T8 (8 volts on transmit) to the monitor switch and when the monitor switch is depressed on to the electronic switch on the main board via J1 - Pin 3. Luckily, this board also has plain 8 volts fed to it. So I cut the trace coming from the T8 (J1 - Pin 2) connector lead and jumpered the hot side of the monitor switch to the connector lead with 8 volts (J2 - Pin 3). This operates the electronic switch whenever the monitor switch is depressed. This worked great except that the Transmit LED was on all the time. After reexaming the schematic, T8 is fed to the Transmit LED from the hot side of the monitor switch. So I cut the trace between the monitor switch and J3 - Pin1 and tied this connector lead (SNDL i.e., send led) to J1 - Pin 2 (T8). Now everything worked great. This modification reduces the volume very, very, very slightly when the monitor switch is depressed. You just need to touch the volume control to make up the difference.

The following are the circuit schematic changes made.

main board changes. Click to view in full size.

The board layout below shows the changes to make on the NB SW board. The circuit board is shown looking at the bottom.

This board can be removed by just taking the top, bottom, and the front panel off. It is the top board on the left as the front faces you. Remove the connectors, depress all the switches, and remove the three teeny tiny screws going into the top of each switch. The bottom of the three switches have little flanges that hook over the front of the chassis. You can tilt the board back and slide it out toward the center of the rig. This keeps you from undoing the front chassis and tilting it forward, although this makes it a lot easier to remove the board.

NB_SW Circuit Board. Click to view in full size.

Following are the steps I took to make the modification.

  1. Cut the circuit trace from the T8 lead J1 - Pin 2 going to the monitor switch. I cut it between the "T" and the switch.
  2. Cut the circuit trace from the monitor switch to R3. I cut it about 1/3 of the way between the switch and the pad where R3 connects.
  3. Put a jumper from the T8 trace to the R3 trace. You can scrape the circuit board trace at the "T" and have a good place to solder.
  4. Put a jumper from the monitor switch to J2 - Pin 3 (8V).

Audio Mute Modifications

I then started looking to see how to change the main board to keep the audio line from switching when in cw mode. J12 - Pin 2 has a 7.5 V CW signal from the logic board. It is high only when in cw mode, in all other modes it is low. I decided to use this to control a relay that would hold Pin 6 of IC7 high when in cw mode and reconnect it back to the squelch line when not in cw mode. An OMRON G6K-2F-5 relay is very small and I could glue it to the top of IC6 to hold it. This is a 5 volt DPDT relay. 7.5 volts doesn't seem to hurt it and only one pole is needed. It is an SMT mount relay and needs small wire. I used 28 ga wire wrap wire for all of the connections.

The following are the circuit schematic changes made.

main board circuit changes.

Following are the steps I took to make the modification. (Steps 1 thru 5 are on the bottom of the main circuit board.)

  1. I cut the trace from Pin 6 of IC7 going to the collector of Q42.
  2. Connect a 6 inch piece of wire to the collector of Q42. (Squelch line)
  3. Connect a 6 inch piece of wire to Pin 6 of IC7. (Electronic switch control line)
  4. Connect a 12 inch piece of wire to Pin 2 of J12. I did this at the W16 trace connection. (7.5 volts only in cw mode)
  5. Connect a 6 inch piece of wire to Pin 6 or 7 of J8. (Ground)
  6. Twist the wires together and bring them around the side of the board near J8 and over to IC6.
  7. Mount the relay on top of IC6. (I used a small dab of hot glue. RTV or something similar would work also.)
  8. Cut the the wire from the collector of Q42 to length, strip, and solder to the Normally Closed (NC) contact on the relay.
  9. Cut the wire from Pin 6 of IC7 to length, strip, and solder to the Moveable Contact of the relay.
  10. Cut the wire from Pin 2 of J12 to length, strip, and solder to both the Normally Open (NO) relay contact and the plus side of the relay coil. NOTE: You can strip the wire long enough to go around both contacts or use a separate jumper wire.
  11. Cut the wire from Pin 6 or 7 of J8 to length, strip, and solder to the ground side of the relay.
  12. Firmly attach the relay to the rig. I used a small dab of hot glue on the top of IC6. Hot glue can be removed pretty easily if you need to do so. I would NOT use super glue!

When soldering to the traces, make sure there are no solder bridges.

The following are pictures of where the modifications go. The first one is from the top side showing the general location of the changes to be made. The remaining ones are looking at the bottom of the circuit board, i.e. at the circuit traces.

General location information. Click to view in fill size.

The board layout above shows the general location of the changes on the main circuit board.

Changes at IC7

The board layout above shows the changes to be made around IC7.

J12 Connection

The board layout above shows the connection to make at J12.

You are now done! You will find the 751 almost noiseless in full break-in mode. I know I enjoy using it on cw now almost as much as the 761.

You will lose squelch in cw mode. I doubt this will bother anyone. I have also noticed when the noise level is very high on 80 meters, like S9 or better, there are some spikes in the audio when going back into receive mode. I don't know if these were originally covered up by the thumping or what. You can reduce these by reducing the RF gain to where the S meter is above the noise level. I suspect the AGC is losing some voltage during transmit causing the noise to appear loud at the time of switching.

This is not a simple modification. You need to be comfortable working on circuit boards and confident in disassembling your 751. A service manual is recommended.

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