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Re: ft-736

The FT-736 internal supply is a switcher.  The switching pulses are murder 
on the capacitors.  The major failure mode of a swithcher is in the 
capacitors.  They loose capacitance and the ESR (equivalent series 
resistance) increases.  THe switching supply starts by rectifying the AC 
primary power and filtering it.  This is applied to a switching device that 
is switched at a high frequency the get the transformer smaller. The output 
of the transformer is rectified with a high frequency rectifier and 
filtered. There are voltage and current sensing circuits that control the 
switching circuit, either by pulse width or frequency.  If by pulse width, 
the wider the pulse width, the more current the supply will deliver.

Now to the failure.  If you have a capacitance meter, remove each 
electrolytic capacitor and check its capacitance.  Any capacitor the 
measures the rated capacitance or below, change it.  If you have an ESR 
meter also check the ESR of any capacitor that measures ok for 
capacitance.  If the ESR is too high, replace it.  The replacement 
capacitor should be rated for switching service and also for 105 degrees 
C.  If you cannot make the capacitance and/or ESR measurements change the 
primary voltage converter capacitors. These are the two capacitors that are 
rated at about 200 VDC just after the rectifier for the primary AC voltage. 
Any capacitor that shows signs of overheating, leaking electrolyte or 
bulging is to be replaced.  If in doubt, replace it. Capacitors are 
relatively cheap and most of the battle is getting to them. Once you have 
them out for checking, unless you have a very good reason to use the old 
part, replace it.

This goes against what we were taught about locical 
troubleshooting.  However you are trying to fix a supply that was consumer, 
not engineering driven.  There were cost trade offs to get you to buy the 
product.  These cost trade offs included the use of the wrong type of 
capacitor.  The capacitor that was chosen may have been as good as it gets 
when the supply was designed.  Today we have better parts available.  Aso 
use the largest capacitor that will fit into the space.  The smaller a 
capacitor is, the more heat it will generate for a given capacitance and 
voltage.  The larger parts will run cooler and last longer.  And be sure to 
use capacitors rated for switching service.  These will last the longest.

I have fixed a number of switching supplies with these 
techniques.  Switchers are a very high failure item in VCRs, to the poing 
that there are kits of parts available for most VCR power supplies to fix a 
supply that has failed from bad capacitors.

Your symptoms are classic of a failed capacitor.

Hope this helps

    At 10:54 PM 2/4/02 -0500, n8anr@juno.com wrote:

>Hi all
>I know there was a post on this subject before but I cannot find the
>reply.  I believe the power supply in my 736 is acting up.  It does not
>always turn on and the transmitt power will fluctuate during long periods
>of transmission (FM),  I remember some one saying something about
>resistors heating and drying out the filter caps causing the supply not
>to start.  I would be grateful for any help on this subject.  I have the
>service manual and there is no schematic of the power supply at all.  I
>opened the radio up and there are two resistors raised above the board
>and the circuit board is blackened beneath them.  Yhe last resort is to
>unplug the internal supply and hook up an external one.  Thanks in
>Juno offers FREE or PREMIUM Internet access for less!
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Glenn Little                         glittle@awod.com   QCWA  LM 28417
Amateur Callsign:  WB4UIV            wb4uiv@amsat.org   AMSAT LM 2178
QTH:  Goose Creek, SC USA (EM92xx)                      ARRL  TAPR

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