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Re: ft-736

Hank and Glenn,
Had a similar problem with the 736 Power Supply some months back.
Replaced 3 electrolytic's in the vicinity of those resistors and we
had it back on line.
Glenn's advice is spot on.
Best regards,
Colin VK5HI
----- Original Message -----
From: "Glenn Little" <glittle@awod.com>
To: <n8anr@juno.com>
Cc: <amsat-bb@AMSAT.Org>
Sent: Tuesday, February 05, 2002 5:00 AM
Subject: Re: [amsat-bb] 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
> is switched at a high frequency the get the transformer smaller. The
> 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
> rated at about 200 VDC just after the rectifier for the primary AC
> 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
> 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
> 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
> supply that has failed from bad capacitors.
> Your symptoms are classic of a failed capacitor.
> Hope this helps
> 73
> GLenn
>     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
> >advance.
> >
> >..........................................tnx.......................Hank,
> >N8ANR
> >________________________________________________________________
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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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