Can someone advise on problems with an Icom ic-718?
Suddenly, I can no long xmit on full power. I can get it put out maybe
40W according to the built-in meter and according to my tuner. Also,
when tuning the tuner in xmit mode, there is a hum that gets loud or
soft depending on the tunning - coming from the xmitter. Is it
possible that one of the two 2SC2904's in the PA unit has blown?
Help / advice is appreciated.

On Sat, 13 Sep 2008 17:16:13 -0400, kadlam262 wrote:

Can someone advise on problems with an Icom ic-718?
Suddenly, I can no long xmit on full power. I can get it put out maybe
40W according to the built-in meter and according to my tuner. Also,
when tuning the tuner in xmit mode, there is a hum that gets loud or
soft depending on the tunning - coming from the xmitter. Is it
possible that one of the two 2SC2904's in the PA unit has blown?
Help / advice is appreciated.

Doesn't sound like a bad PA transistor from here.

Two guesses:

- Something has happened to your antenna system. The ground has opened,
or the shield of the coax has opened, or a connection on the antenna
itself has opened. Causing high RF levels in the shack, which are feedin
g
back into the radio.

- Something is wrong with your power supply. It's no longer able to
deliver rated voltage at the current necessary to power the IC718 at full
output. A failed pass transistor (or driver transistor) can cause this.
Actually, so can a loose connection in the cable connecting the supply to
the radio.

Divide and conquer. Get your hands on a dummy load & a voltmeter.

Can you transmit at full power into the dummy load? If so, there's
something wrong with your antenna. (or external tuner, or coax, or groun
d)

If you *can't* transmit at full power into the dummy load, leave it
connected and measure the power supply voltage as you transmit. It shoul
d
stay above 12.5 volts. If it doesn't, there's something wrong with the
supply.

If you can't transmit full power into the dummy load but the supply
voltage is staying above 12.5 then you probably do have a problem with th
e
IC718 itself.

Good luck.

wrote in message
...
Can someone advise on problems with an Icom ic-718?
Suddenly, I can no long xmit on full power. I can get it put out maybe
40W according to the built-in meter and according to my tuner. Also,
when tuning the tuner in xmit mode, there is a hum that gets loud or
soft depending on the tunning - coming from the xmitter. Is it
possible that one of the two 2SC2904's in the PA unit has blown?
Help / advice is appreciated.

Doug has made some valid suggestions but another is that RF is getting into
your PSU and causing it to drop out of regulation. The IC718 does not like a
low supply voltage- once it drops less than 13V the 718 tends to reduce
power. (This could explain the "hum".)

There are a couple of other ideas: you aren't trying to measure the PO in AM
are you? The 718 runs about 40W on AM. Also, you've not wound the power down
and had it slip your mind? (Although never would explain the "hum".


--
73
Brian, G8OSN
www.g8osn.org.uk