[cctalk] Re: Xerox 820II U07 Power Supply magic smoke....

Best I can tell the diodes are Passivated Silicon, Bead diodes, of which there are three flavors. https://pdf1.alldatasheet.com/datasheet-pdf/view/1073401/NJSEMI/A15A.html On EEVBlog it was suggested that these were Sintered Glass Bead diodes. I do believe they are still in okay working condition. My thought process is that they are probably quite noisy. Installing a modern type of diode would also I believe dramatically reduce generation of heat. It is nice that a Power Supply board is laid out in a way that you can access all of the components. Nothing like some of the small 5v switching supplies in which most of the components are shoved close together. I downloaded the 1977 GE Semiconductor manual, but it’s not easy to find it without a part number. Back when I was in my early 20’s a neighbor whom was an Engineer at Xerox in the power supply division at 701 South Aviation Blvd. El Segundo, gave me a bundle of those soft bound Motorola reference manuals, some RCA TTL manuals, Zener Diode reference etc. The books sat on my shelf for many years until I ran out of room. I moved them to my shed which at the time was safe from the weather. After years of storage many of them were damaged from dampness etc. In the last 10 years I realized I should have taken better care of them. I realize they are almost worth their weight in gold. I was able to find a few of those books used and purchased them at (luckily) reasonable prices. I regret that and a home brew vacuum tube combo preamplifier/amplifier stereo unit. I was forced to part with. My father referred to it as “junk taking up space”. It contained 4 6V6GTs, 2 12AU7As, and a 5U4 Rectifier. The front end was early GE (germanium I suppose) transistors. They were of the metal oval shaped black painted metal can type with the pinched evacuation nipples. Don Resor From: Joshua Rice &lt;Rice43(a)btinternet.com&gt; Sent: Thursday, October 13, 2022 9:44 AM To: D. Resor &lt;organlists1(a)sonic.net&gt; Subject: Re: [cctalk] Re: Xerox 820II U07 Power Supply magic smoke.... On Oct 13, 2022, at 2:32 PM, D. Resor &lt;organlists1(a)sonic.net <mailto:organlists1@sonic.net> > wrote: A couple reasons I'd like to have a circuit diagram is to know what the RIFA capacitors purpose are. The other is, a couple of the electrolytic capacitor are double covered with what appears to be rubbery heat shrink tubing, therefore I cannot read what their value are. Generally, though not always, these are simply “coupling” capacitors, that are used to limit RF interference from appearing on the mains supply. Most, though not all, PSUs will work fine without them. Some others won’t function right as they’re used for generating clock signals from the mains supply. I believe that some PDP-11 supplies use them for this purpose. One of these two electrolytic capacitor appears to have a dried substance around the top. The over-pressure venting cuts in the top of these two capacitors are not split. At this point I cannot tell if it is dried electrolyte, cement to hold the sleeve on, or possibly paper. Sometimes it’s just glue , but better safe than sorry, Cut off the sleeving (it’s used for electrically isolating the cap from other components), get the rating off of it, and replace with like. Probably worth heatshrinking them again as well. Seems someone was in hurry on the day this PS was tested. I cannot make out date, is it 1983? <https://www.dropbox.com/s/zlt2ocpqulil13j/pssticker.jpg?dl=0> https://www.dropbox.com/s/zlt2ocpqulil13j/pssticker.jpg?dl=0 Looks like ‘85 What type are the diodes, their rating etc.? I seem to remember this type with a black band were rated at 3 amps, but that's all I remember. The TO-3 transistor/regulator has no P/N silkscreened on it. Having a circuit diagram helps to cover many bases. Apparently it's an Astec AA12070. It’s unlikely that these would have failed. If they have, i would (as another person suggested,) just replace the unit with a modern Meanwell supply. At that point, it’s probably not worth the time and effort to replace all the parts. Placed the PS board back in the garage to continue airing out! 😉 Don’t fear the RIFA, but keep the windows open, just in case. Josh.