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Looking for Feed Through Capacitor for SNA1

Martin Member Profile All Messages By This Member #59 Hi all I have an SNA1 that puffed smoke, and discoverd it was due to a shorted feed-through capacitor. Now I'm looking for a replacement, but they seem special in that they are very thin on the interiours side, where the bended wire is. I cannot find anything like that on the internt. Below some pics (metric scale!). The parts list says 8c20 / 0855-0020.0008 / C-Keramik / ZZMZ / 4N7 / 20/80 / 63V. So its a 4n7 capacitor with 63V rating. In the SNA this capacitor is handling a 12V supply line. Does anyone have such a capacitor left from a similar donor unit? cheers Martin 20230315_134222 _15.03.2023 - 13h42_.jpg 20230315_134240 _15.03.2023 - 13h42_.jpg
Alan Beard All Messages By This Member #60 开云体育 Hi Martin, BTW: What country are you in? I'm writing from Australia. The pins in the picture look unsoldered. Every other feed through I've seen has pins for soldering. Lots in my clear plastic bottle for them. Also, the top square plate, is usually a hex like the head of a bolt. Are any of these details important? Alan On Wed, 15 Mar 2023 08:10:17 -0700, Martin via groups.io wrote > Hi all > > I have an SNA1 that puffed smoke, and discoverd it was due to a shorted feed-through capacitor. Now I'm looking for a replacement, but they seem special in that they are very thin on the interiours side, where the bended wire is. I cannot find anything like that on the internt. Below some pics (metric scale!). > > The parts list says 8c20 / 0855-0020.0008 / C-Keramik / ZZMZ / 4N7 / 20/80 / 63V. > So its a 4n7 capacitor with 63V rating. In the SNA this capacitor is handling a 12V supply line. > > Does anyone have such a capacitor left from a similar donor unit? > > cheers > Martin > --------------------------------------------------- Alan VK2ZIW Before the Big Bang, God, Sela. OpenWebMail 2.53, nothing in the cloud.
Martin Member Profile All Messages By This Member #69 Hi Alan, have a look at the pics from a board adjacent to the one with the faulty cap. Both wire ends of the cap go into sockets, they are not soldered. On the left its the power distribution board, on the right its bended towards the viewer, entering the contacts in the silver dome. You may also see the flat part of the cap that is very close to the board. Thats why the standard feedthroughs do not fit. When I ran a straight wire to the board the SNA started up and passed self test. After 20-30 minutes, though, the switching supply started cycling off and on, so there is another problem. Fortunately its a W&G design and I have the complete manual of the supply. The biggest problem is how to get it out of the unit. The procedure described in the manual does not seem to work, I can't get the side panel loose. cheers Martin
Alan Beard All Messages By This Member #70 开云体育 Hi Martin, Wavetek had a very bad run with, in particular tantalum capacitors, the moulded ones. So, I'd suspect those in orange blocks, the ones marked 16V +22.? In the 3002 0 - 520 MHz sig-gen, it's the black moulded 5uF and 10uF caps that look like 1A or 3A diodes. They go short. I've had no problems with the epoxy dipped ones. (from 1974 onward) Also, in later models, the surface mounted caps somehow have liquid inside and the leads corrode off internally and leak this goo that then corrodes the tracks. And, there are hundreds in a unit! Lots of computer boards have micro-plate ceramic caps 0.01 to 1 uF and they are fine. Alan VK2ZIW On Thu, 16 Mar 2023 09:44:14 -0700, Martin via groups.io wrote > Hi Alan, > > have a look at the pics from a board adjacent to the one with the faulty cap. > > Both wire ends of the cap go into sockets, they are not soldered. On the left its the power distribution board, on the right its bended towards the viewer, entering the contacts in the silver dome. You may also see the flat part of the cap that is very close to the board. Thats why the standard feedthroughs do not fit. > > When I ran a straight wire to the board the SNA started up and passed self test. After 20-30 minutes, though, the switching supply started cycling off and on, so there is another problem. Fortunately its a W&G design and I have the complete manual of the supply. The biggest problem is how to get it out of the unit. The procedure described in the manual does not seem to work, I can't get the side panel loose. > > cheers > Martin > --------------------------------------------------- Alan VK2ZIW Before the Big Bang, God, Sela. OpenWebMail 2.53, nothing in the cloud.
Martin Member Profile All Messages By This Member #71 Hi Alan, but this shorted through capacitor is a ceramic one, according to the parts list. I find that very strange... As I said, the unit worked fine once I replaced the through cap by some wire, at least until when the power supply failed. Yesterday I managed to get the power supply out and opened it. It looks as if built by NASA for Mars explorations... I noticed one large electrolytic caps had corroded srews. Maybe thats the reason for its malfunctioning. I'll try to make some measurements today. BTW, I'm sitting in Germany (Stuttgart). cheers Martin
Martin Member Profile All Messages By This Member #72 After an hour or so measuring around the power supply, following the diagram in the service manual, I ended up at the circuit where the suspect eletrolytic cap is. And indeed, it has a total short. I've never seen that with such a large cap, though. The screws have some sort of brow-black goo on them, as if something leaked but its not corrosive. I did check the other cap that has the same specs, and will check some others too. Made some pics of the cap and the whole wonderfully complicated power supply. Enjoy! cheers Martin

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