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[W4AJF]

Hi all, I bought an Icom IC-751A and was told it had low output but I find it actually won't even go in to transmit mode. Whether I use the SEND switch or microphone, when I key up (any band any mode) it does not go in to transmit, transmit light does not come on, and it stays receiving even when attempting to key up. All the settings on the radio are in the correct positions. I am hoping someone is specifically familiar with the IC-751A and can tell me what the issue might be. Thanks for any help.

Adam Farber W4AJF
Cooper City, FL

[ZS5WC]

 :)Hi Adam,
Does it recieve?. Note that the IC-745 / 751 had a volatile ram that holds the PLL info for the various bands.
If the memory battery goes low, the data is lost and no rx /tx.
I know PIEXX made an non volatile replacement board.
Best Regards,
William
ZS4L / ZS5WC

[W4AJF]

Hip Willian, thanks, I should have mentioned that receive is fine. Adam

[ZS5WC]

OK Adam, have you tried vox, might use a parallel keying path.
I suspect however , you probably have an keying transistor fault.
William, ZS4L / ZS5WC

P.S. Adam, If you come across an Parts Radio Yeasu FT-2000 pse let me know, I'm specifically looking for the tuner control board.
That is if you are still dealing with radios..

[VR2AX]

Make sure you have selected 'HAM' and not 'GEN' (general coverage mode which is RX only).

[W4AJF]

Hi guys, Ham or General makes no difference HOWEVER William is on to something, in VOX it does key and in SSB I can get full output but I also here myself through the speaker when I talk but there is some progress here. What I can do to isolate this problem? Adam

[W3PX]

Going from memory here, and it's not what it used to be  , but examine the area on the back of the microphone jack inside the radio, there is a small ribbon cable coming off the mic connector pins, I think this is where the mic PTT line and the line from the Transmit switch are brought together, look for damage in this area and it may be hard to see so look carefully.

[W4AJF]

Yes, will do later, that makes sense. Thanks, Adam

[W3PX]

I took a quick peek into the IC-751A Service Manual. The diagram on page 8-10 shows that the front panel mic jack is connected to the "AF VR" board via a small ribbon connector. The front panel "SEND" (Transmit-Receive) switch is also connected to the "AF VR" board via 2 wires from the switch.

These two PTT connections are paralleled on the "AF VR" board and then exit as a single PTT line via a small 2-pin plug designated as P4. Look for a loose P4 plug coming off the "AF VR" board, that would affect both the mic jack PTT line as well as the manual T/R switch on the front panel.

73 Frank W3PX

[W4AJF]

Thanks Frank, will dig a little later and report back. Thanks, Adam

[W3PX]

Hi Adam,

Here's a little bit more info. The "blended" PTT line (comprised of the front panel PTT switch and mic jack PTT line in parallel) exits the AF-VR Unit in (3) places:

1. Via (2) wires coming off the AF-VR Unit PCB (labeled as P4 in the service manual) which are terminated in a white plug, probably 2 pins, that go to another unit in the IC-751A. I haven't traced it out, but my guess would be the MAIN Unit.

2. Via the 10-pin white plug on the AF-VR Unit labeled in the service manual as P6/J9.

3. Via the 3-pin white plug on the AF-VR Unit labeled in the service manual as P2/J11.

The AF-VR Unit is in a difficult location to access behind the front diecast panel, some disassembly may be required. Good luck, I'm pretty sure the trouble you're having is in this area.

73 es HNY, Frank W3PX

[VR2AX]

Does it TX with a morse key (or a shorted phono plug) in the CW socket?

[W4AJF]

Hi, in CW mode it works fine, just as it should with a key. Adam

[VR2AX]

Good news that the TX chain is fine. I'd look for the simple before the complicated. Some great advice above. Switches and connectors do go dodgy over time, solder joints can go bad due to age and need reflow, or some cleaner, even blow away dust. Audio in the speaker on TX may be a possible separate issue, the mute function does not disable the full RX chain as best I recall, just closes off the RX audio out circuitry (don't trust the separate RX antenna input on the rear it does not protect the RX circuitry when any real TX power is involved).

I have a 751AF (left short eham review) now packed away somewhere ,with every imaginable extra, including a hard copy service manual. HNY.

[W4AOK]

If you hear yourself in the speaker, the Monitor function is enabled.  Just switch it off.