[1000mp] Switching supply slow to start

[Rick Stealey]

Here's a followup to this problem I reported back in November.  Thanks Mike, W2AJI for the tips.  I found a shorted FET in the switching part of the supply, (replacement is STP16NM50N $3.52 at Mouser) which didn't fix the problem.  Then I found an open 10 ohm resistor in the input area and that got it running again.
Apparently it was acting as a fusible resistor. For what it's worth here's what I learned:
the supply is made by Futaba.  I could find no info about it on the net.
I traced out some of the circuitry and found a rather straightforward input, isolation transformer (110v in, 110v out), a bridge rectifier giving around 260 vdc with 2 big electrolytic caps.  Then that 260 volts runs the switching part of the supply where there are 2 big FETs.  
As an aside I was motivated to get back to work on this supply because I was running an external 12 v supply and hearing very loud pops when I keyed the rig.  I also got a report of chirp.  I found 1000 mv of RF on the 12 v lead when I was on an antenna, but nothing when I was on the dummy load.  I installed 0.01 cap on the PS but I guess the long leads were delivering RF into the rig.
You can read below to get some background on this problem.
What a fantastic rig.  I still love it after 12 years.

Rick  K2XT
________________________________
> From: cherokeehillfarm at earthlink.net
> To: 1000mp at mailman.qth.net
> Subject: Re: [1000mp] Switching supply slow to start
> Date: Sat, 22 Nov 2008 09:01:19 -0500
>
>
> Hello Rick:
>
>
>
> It is pretty tough to know whether you have a bad
> component or a bad solder joint with a heat sensitive problem like
> yours.
>
>
>
> There are a few techniques which you could try, to
> locate the problem area.
>
>
>
> You may be able to heat areas of the board with a
> hair dryer to get the power supply playing. This will help to localize the
> problem.
>
>
>
> Then, with the supply running, use a can of cooling
> spray, with the spray wand, to identify the exact location of the component or
> solder joint.
>
>
>
> When you know which component seems to be the
> problem, before you remove and replace it, reflow the connections. Some
> folks just start reflowing many connections to try to locate the faulty joint,
> without trying to pinpoint it first.
>
>
>
> You should be able to locate the culprit quickly
> and simply.
>
>
>
> 73 and Happy DXing,
>
>
>
> Mike
>
> W2AJI
>
> Administrator: 1000MP Reflector
>
>
> ----- Original Message -----
>
> From:
> Rick
> Stealey
>
> To: 1000mp at mailman.qth.net
>
> Sent: Saturday, November 22, 2008 8:35
> AM
>
> Subject: [1000mp] Switching supply slow
> to start
>
>
> The internal supply in my 1000MP doesn't like to start when the
> rig is cold. As the
> ambient temperature rises it takes off and
> operates normally. I see there is no info
> in my technical manual,
> just a schematic which I note has FUTABA on the PCB.
> I've never tried
> troubleshooting a switching supply. Any ideas as to what is most
> likely
> the cause? Some resistor has changed value in 11 years?
> Some capacitor is leaky?
> Transistors and ferrite transformers don't get
> cranky with age do they?
>
> Oh by the way, I am sure it's not something
> external to the supply (it is getting 110 v, and
> there is no 12 volts when
> it is cold).
>
> Thanks,
> Rick K2XT
>
>

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