[Elecraft] K3 with switching power supply??

[Joe Subich, W4TV]

I have no disagreement with anything you've said, particularly
keeping the pass transistor mounting refreshed and relocating
the diodes to a heat sink if your particular RS-35(M) uses the
stud mounted diodes.

However, the 723 has a relatively high dropout threshold.  Other
regulator topologies, like that in the BWC Electronics article,
provide lower dropout and allow the pass transistors to be mounted
without insulating hardware providing for lower heat dissipation,
more efficient cooling and more reliable operation.  In addition,
the other topologies can also provide improved transient response
(resulting in improved IMD, etc. from the transceiver).

> For a any linear PSU, even a small fan wafting air over the
> heat-sink will make a world of difference too, resulting in a much
> cooler PSU that is much more reliable as a result. If you don’t like
> the continuous sound of that, fit a thermal switch to the heat-sink,
> say 50'C, to turn the fan on when it gets hot...

Yes, a fan is certainly useful - particularly for RTTY or contest
service.  However, I'd suggest a dual speed scheme with a constant
low speed (quiet) that switches to full speed at say 40C.  One does
not really want the heat sink (and by extension the pass transistors)
to exceed 50C.  The Astron supplies have been known to reach 85C or
more with "brick on the key" modes - certainly not good for the long
term health of the supply.

73,

    ... Joe, W4TV


On 2/14/2017 4:25 AM, Dave B via Elecraft wrote:
> On 14/02/17 00:54, elecraft-request at mailman.qth.net wrote:
>> Subject: Re: [Elecraft] K3 with switching power supply??
>> Message-ID: <6796bb5c-c5da-c0d2-13ce-0b5bd4ba16f0 at subich.com>
>> Content-Type: text/plain; charset=utf-8; format=flowed
>>
>>
>>  > I am on the way to change my old Astron RS35M power supply....
>>
>> Why would you change out an Astron RS35M?  Unless you need to
>> make the K3 station portable where the weight and size of the
>> Astron is an issue, the RS35M will be both much more quiet and
>> much more reliable than any switcher.
>>
>> Even if the Astron is getting a little unreliable, an overhaul
>> (replacing the diodes, pass transistors and possibly filter cap)
>> is likely to be more cost effective than purchasing a switcher.
>>
>> For repair/overhaul information see:
>> <http://www.repeater-builder.com/astron/astron-repair/astron-repair.html>
>>
>> or for a more in-depth modification/overhaul that replaces the 723
>> regulator see: <http://www.bwcelectronics.com/articles/WP20A190.pdf>
>>
>>
>> 73,
>>
>>     ... Joe, W4TV
> There is absolutely nothing wrong or bad about the 723 regulator.  But
> there are a lot of badly designed units using that IC out there.  The
> designers did not read (let alone fully understand) the data sheet.  And
> then not helped by the physical layout people, who don't appreciate the
> effect "inappropriate" wire routing can have on a regulator with a high
> loop gain. Don't blame the device for poor product design.
>
> What does go bad over time with most "Ham market" linear PSU's, is not
> so much the main cap (often too high a value) diodes or pass transistors
> (unless this next issue has happened.)
>
> High value reservoir caps can overly stress the rectifier diodes, and
> transformer.   Ripple is to be expected across that, and is what the
> regulator is there to remove...
>
> Anyway.  The thermal interface between the pass transistors and heat
> sink often deteriorates, not helped by the usual sloppy build quality in
> the first place.  (I've even seen pop-rivets used to mount regulator
> pass transistors!  BAD BAD BAD!)
>
> Same for the main bridge rectifier if that is heat-sink mounted as
> well.    If it is not, consider moving it to the heatsink, or fitting
> one to it if there is room.
>
> The cooler you can keep a semiconductor (within reason) the longer it
> will last.   Wide variations of temperature are also bad, often for
> where the leads are connected to a PCB, if not using flexible wire
> resulting in failed solder joints.   Always "kink" the lead with a z
> formation for mechanical stress relief, it'll last much longer as a result.
>
> Power transistors with sockets too, can suffer intermittent lead
> connections over time, due to thermal cycling and the minute repeated
> relative movement between lead and socket, usually damaging the plating
> on the semiconductor lead, resulting in bad connections.   Replacing the
> socket will not cure that.
>
> Dismantle, clean and re-apply fresh compound or thermal interface pad.
> Use fresh insulating (clamp) washers too if needed.   Most of all, do
> not over-tighten those device mounting screws.   No, I can't quote a
> torque, you'll have to research the matter, and consult the device
> makers data sheet, if you can identify the devices that is...
>
> Re solder any joints that appear a little suspicious around the high
> power devices.   Replace any suspect socket connections by removing them
> and soldering the wires to the device leg’s directly.  (If the device
> legs plating is not badly corroded, if so, replace the device too.)
>
> The same holds true for the pass transistor (and their drivers) emitter
> ballast resistors.  Check their condition and connections.
>
> No emitter ballast resistors?   Bad design...
>
> For a any linear PSU, even a small fan wafting air over the heat-sink
> will make a world of difference too, resulting in a much cooler PSU that
> is much more reliable as a result.   If you don’t like the continuous
> sound of that, fit a thermal switch to the heat-sink, say 50'C, to turn
> the fan on when it gets hot...   (Or based on PSU output current...)
>
> 73.
>
> Dave G0WBX.
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