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Author Topic: Low RX When Amp Is Online  (Read 510 times)
K0XXX
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« on: January 25, 2006, 07:43:27 AM »

I am new to tube amplifier operating so please forgive me if this is normal but... When I have my LK-450 in the Standby mode I have good rx but, when I switch it to the Operating position, my rx drops way down. The amp has good output and otherwise tunes and operates flawlessly.

Is this normal or, is it some type of mechanical or Operator function?

Thanks,
Mark K0XXX
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K0CWO
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Posts: 419




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« Reply #1 on: January 25, 2006, 09:28:49 AM »

I'm not familiar with your amp, but I suspect that switching from standby to operate powers the RX/TX relay in line.  Sometimes these relays get dirty and will cut the receive sensitivity.  Try running a dollar bill in between the contacts of the suspect relay to clean it.  As always, be very CAREFUL when poking around inside an amplifier.  There are DEADLY voltages to be dealt with!  If you are not sure what you are doing, enlist the help of someone experienced in dealing with amplifiers.

73, and good luck.
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WB6BYU
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« Reply #2 on: January 25, 2006, 03:54:30 PM »

Sounds the amp is going into transmit mode as soon as it
is switched to Operate - that would disconnect the antenna
from the receiver, causing received signals to be weak.

How do you put the amp into transmit mode?  Is it a cable
from the rig?  Check that cable for a short circuit, and
make sure it is properly being switched from the rig,
has the right polarity, etc.
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K0XXX
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« Reply #3 on: January 26, 2006, 08:44:50 PM »

Once I switch the amp from Standby to Operate, the amp is keyed via a cable between the rig (IC-746)and the amp. I have checked and re-checked the cable. I've even tried a couple of others just to be sure. As soon as the switch to Operate is made, the rx drops.

I do have some additional info, though. It is, I believe, something to do with the rig. I have since tried an AL-80 (converted to 6 meters) and the same thing happens. I am re-reading the 746 manual, hoping to find something that I missed about setting the rig up.

With this new info, I may be in the wrong forum since it may not be anything dpecific to the amp after all. I do really appreciate your (as well as everyone else) input and help.

Mark
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WB6BYU
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Posts: 13334




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« Reply #4 on: January 27, 2006, 11:58:30 AM »

So if you disconnect the cable when the amp is in Operate,
the amp switches back into receive mode and the antenna
relay passes signals properly?

Check this while disconnecting the cable at the rig, and
again at the amp.  If it works properly in each case,
then try shorting the connector at the rig end of the
cable - that should put the amp into transmit mode.

If you've gotten this far, the cable and the amp should
be good, and the question is why the rig isn't switching
as expected.  Some options to consider:

1) the plug on the end of the cable is wired wrong, and
really isn't connected to the switched pins.  (With some
connectors the numbering can be confusing.)

2) the rig was used to switch too much voltage/current,
and the switching transistor in the rig has shorted.
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K0XXX
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« Reply #5 on: January 27, 2006, 04:14:41 PM »

Yep, the problem lies in the rig. The keying relay in the rig was stuck closed. Even though the 746 manual says that the rig should be able to handle the voltage and amperage from the amp, aparently it doesn't. The relay is now unstuck and everything is working correctly.

I've had this amp for 4 years and had never got around to using it. It may be four more years before I use it again but for now when I do, I will be using a footswitch to key the amp until I get a buffer built.

I really appreciate everyone's help and patience. Thanks Dale!

73 all,
Mark
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VE7ALQ
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Posts: 349




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« Reply #6 on: January 29, 2006, 03:34:54 AM »

Glad to see you have isolated your problem.  Allow me to  recommend thefollowing company:

http://www.ameritron.com

and order the ARB-704 buffer.  Order the correct cables to go with the ARB-704 to interface with your Icom ic-746 I am using an ARB-704 with my Icom ic-706mkiiG and it works flawlessly.
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