ICOM 756 Pro III transmit issue on 6 meters

[WX3K]

I own a ICOM 756 PRO III which I bought brand new back in 2001.

I noted an odd transmit issue that only seems to occur on 6 meters. This only occurs on 6 meters.

Upon keying the rig in CW, the power output will throttle back fully momentarily and then key up. This occurs intermittently. Sometimes the rig works fine. The SWR appears prefect according to the SWR meter when the rig is transmitting full power or any lower level of power. I use a M2 6M5X up 70 feet, fed by 1/2 heliax. I also have a MOSLEY PRO 57B at 65 feet which I use with no issue at all at full power.The shack is in the basement.

If I watch the meter, the ALC appears to slam at full reading when the rig is first keyed, the power output throttles back for about 500ms and then powers up fully again. SWR reads perfect. This behaviour does not occur on the lower HF bands. I note that the ALC reacts odd only on 6 meters. I have 13.8 volts supplied to the rig from a solid ASTRON 35 amp supply.

I did place the PRO III on a dummy load and the issue does not occur. You would think there is something going on in the antenna system but the match is good. I recently used the rig on 6 meter SSB with no issues.

Has anyone else experienced this issue ?

[WA2ONH]

Have you voiced this issue with the Yahoo 756pro3 Group at:

LINK: http://groups.yahoo.com/group/ic756pro3/

Know Adam VA7OJ / AB4OJ can assist you.

LINK: http://www.ab4oj.com/icom/ic756pro3/main.html
LINK: http://www.ab4oj.com/icom/ic756pro3/pro3notes.html

Good Luck & Happy New Year!

[N1EQ]

This sounds like the common oscillation issue, usually due to a faulty RFRX-muting circuit (Q25, D22, D23 on the CTRL Unit), and possibly a bad diode or two on the RF Unit (D104, D801).

The oscillation is usually most noticeable on 6 meters, but it can also affect 10 and 12 meters.

The component damage is usually due to an ESD event, or RF-overload from a nearby transmitter.

More info:
http://www.n1eq.com/tech/page_02.html

[WX3K]

How do you characterize "oscillation" ? What is oscillating and what are the necessary elements(certain power levels,etc) that need to be present for your "oscillation" condition to be present ?

The issue I have is when the rig is keyed and held keyed, the transmitter will react and not output power until about 500ms later. Every instance of keying will sometimes exhibit this. I also read that the PRO II models had some sort of "dit clipping issue" which was resolved fully in the PRO III. What caused that issue ? Are you familiar with this ?

This sounds like the common oscillation issue, usually due to a faulty RFRX-muting circuit (Q25, D22, D23 on the CTRL Unit), and possibly a bad diode or two on the RF Unit (D104, D801).

The oscillation is usually most noticeable on 6 meters, but it can also affect 10 and 12 meters.

The component damage is usually due to an ESD event, or RF-overload from a nearby transmitter.

More info:
http://www.n1eq.com/tech/page_02.html

[WX3K]

Thank you 

Have you voiced this issue with the Yahoo 756pro3 Group at:

LINK: http://groups.yahoo.com/group/ic756pro3/

Know Adam VA7OJ / AB4OJ can assist you.

LINK: http://www.ab4oj.com/icom/ic756pro3/main.html
LINK: http://www.ab4oj.com/icom/ic756pro3/pro3notes.html

Good Luck & Happy New Year!

[K6AER]

I know thisis a bit basic but have you selected the right antenna port? On six meters the radio defaults to port 2.

[W0XX]

I had the same problem on 2, not 1, but 2 PRO2s with the exact component failures.  I sent it to HAMREPAIR.COM and Cal repaired it and put in some protection.  Cost about $200 to repair.  A very common problem in the PRO Series.  Unfortunately, most guys that sell their PRO series on the web don't bother to check the bands out, else you would see the pegging of the meter, etc and no audio or real power out on 6.  You can kinda get by it but selecting the tuner which doesn't fix it but will allow you to use the radio on 6.  Send it to Cal at HAM REPAIR.COM.  He is very very familiar with this design problem. GL.

I am looking at PRO2 Number 2 with the same issue and previous owner never used 6 on it....