On 14/02/17 00:54, [email protected] wrote: > Subject: Re: [Elecraft] K3 with switching power supply?? > Message-ID: <[email protected]> > 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. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
