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Author Topic: IC-733A low VHF output  (Read 3205 times)
K6UJ
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Posts: 315




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« on: April 02, 2013, 03:46:33 PM »

My IC-733A has full power out on 440 but only 7 watts out on 2 meters (with the high power setting)
Would it be foolish to try adjusting the output trimmers on the VHF board ?
I am thinking it probably needs a new output transistor/module.  Any advice
on how to diagnose ?

Bob
K6UJ

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KA5IPF
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« Reply #1 on: April 02, 2013, 05:09:09 PM »

What test equipment is available?

Clif
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K6UJ
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Posts: 315




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« Reply #2 on: April 02, 2013, 07:35:08 PM »

Bird wattmeter with VHF and UHF elements.  50 ohm dummy load.  Digital VOM.
Other UHF radios to confirm if getting out and checking audio.

thats about it

Bob
K6UJ
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KA5IPF
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« Reply #3 on: April 03, 2013, 07:05:33 AM »

1. Check the supply voltage INSIDE the radio in Tx mode. It should be the same as measured at the power supply.
2. Check the current draw of the radio in TX mode. If 8-9A the module is fine. Probably bad solder on the switching diode.
3. On the module numbering left to right, 1 is input, 2 should be ~12v, 3 ~13v, 4 is output.

If the current draw is low 3-5A and the module voltages are correct it's probably the module but you need more test equipment to verify.

Clif
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K6UJ
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Posts: 315




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« Reply #4 on: April 03, 2013, 04:52:25 PM »

I did some testing and this is what I found:

1.  The voltage inside the radio in TX mode is the asme as at the power supply.
2.  The current draw on TX mode is 4.5 amps.
3.  I havent checked the voltages on the module yet, will do tomorrow.

I have another 2 meter radio with the same power rating as the Kenwood and just for comparison
I checked the current draw on TX mode.  It draws 8.7 amps which makes sense, it has full output.

Looks like it is pointing to a bad power module ?

Bob
K6UJ

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KA5IPF
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« Reply #5 on: April 03, 2013, 05:17:28 PM »

You need to measure the voltage on Pin 2 of the module., the current is correct for the power.

Clif
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K6UJ
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Posts: 315




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« Reply #6 on: April 04, 2013, 09:35:12 AM »

Here are the voltage readings on the power module,  transmit, hi power setting

Pin 2     8.57 volts
Pin 3    13.3  volts

the radio is drawing 4.5 amps and the output is showing 7 watts on the bird
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KA5IPF
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« Reply #7 on: April 04, 2013, 10:04:45 AM »

Pin 2 voltage controls the power out. With the power that low(7W) it should be at 12-13v if the power control circuit is working correctly. Normally Pin 2 will be abt 11v. Time to go to the service manual and start troubleshooting.

Clif
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K6UJ
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Posts: 315




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« Reply #8 on: April 04, 2013, 12:08:30 PM »

OK,  Service manual is open.
I will focus on the control circuit feeding pin 2 and see if I discover anything.
Thanks for elmering me on trouble shooting this Phil, much appreciated !

73,
Bpb
K6UJ
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KA5IPF
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« Reply #9 on: April 04, 2013, 01:08:59 PM »

If you have a variable voltage bench supply you can lift Pin 2 and connect to it. Start at abt 6v. Key the radio and slowly increase the voltage up to 11-12v. The power out will go up to over 50w if the module is good.

Clif
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