Thanks. I had no idea that tantalum capacitors had polarity though, I thought that was only electrolytics. The board has "+" markings for electrolytics, but none for the tantalum ones. Are you sure they have polarity? Regards Rob voltage. <robert.jarratt at ntlworld.com> here:

On Mon, Sep 22, 2014 at 5:50 AM, Chris Pye <pye at mactec.com.au> wrote: Hey, I just had a bad idea. I suppose a non-polar tantalum could be built the same way as a non-polar aluminium electrolytic.. two caps in series, in one package, with the negative leads tied together. ;-) But what a horrible thought - just having one of those obnoxious little bombs in one location is bad enough. If your circuit won't perform without one, maybe you should just re-design the circuit or something...

So sorry, I was convinced it was tantalum because of the shape of the body, but when I checked for images of tantalum and ceramics I realised it is ceramic, although I have to say the shapes are fairly similar and I thought you could use the shape to identify them. So, yes, it probably is ceramic then. Regards Rob On 22 September 2014 17:13, ben <bfranchuk at jetnet.ab.ca> wrote:

At 06:25 PM 9/22/2014, Tothwolf wrote: I have seen tantalum electrolytics used on densely packed boards where the aluminium electrolytics of the time would have taken up much more space (the best quality aluminum electrolytics today are more compact than they were a few decades ago). I have an example in the shop at the moment - a Harris RT-1446 HF military transceiver ca. 1980 (the civilian version was the RF-350K). Unfortunately there seems to have been a problem with the 10 uF 25 V tantalum electrolytics used to bypass the +15/-15 rails where the enter each board. Many of them have failed in many transceivers, so many in fact that when I get one of those transceivers into the shop for repair one of the first things that I do is to shotgun all of those rail bypass caps. Dale H. Cook, Roanoke/Lynchburg, VA Osborne 1 / Kaypro 4-84 / Kaypro 1 / Amstrad PPC-640 http://plymouthcolony.net/starcity/radios/index.html

At 12:10 AM 9/22/2014, Robert Jarratt wrote: You are thinking of aluminum electrolytics, which are today the most common and inexpensive of the many types of electrolytic capacitors. Tantalums are electrolytics, they just have different construction than aluminum electrolytics. Tantalum and aluminum electrolytics have displaced the older forms of dry and wet electrolytic capacitors. Dale H. Cook, Roanoke/Lynchburg, VA Osborne 1 / Kaypro 4-84 / Kaypro 1 / Amstrad PPC-640 http://plymouthcolony.net/starcity/radios/index.html

Little know fact, a small tantalum capacitor can be used as a model rocket igniter in a pinch :-) In early 2000 when I was at NetApp, NetApp was selling filers to IBM to re-badge as their own. IBM got on a 'no tantalum' war path (apparently they had been the source of a number of bad events in the field) and so we worked with them to remove every single tantalum capacitor from the equipment. It was quite the project. On Mon, Sep 22, 2014 at 2:06 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:

On 22 September 2014 21:43, Tony Duell <ard at p850ug1.demon.co.uk> wrote: I think I have been convinced it is ceramic. I was fooled by the packaging, which looks a bit like what I think tantalum capacitors look like. One day I will learn to tell the difference more easily. Annoyingly, after desoldering three capacitors, and breaking the one mentioned above, I still see an apparent short across the outputs of the PSU. Thanks Rob

Aren't those PSUs rated for 600W or so? It seems a stretch to believe that anything as wimpy as a shorted low-value cap could ever last long enough to force a supply shutdown due to over-current. I mean +maybe+ but.. I'd be looking to the heavier, more robust components if you suspect something's dropped a crowbar across the output. On Mon, Sep 22, 2014 at 6:00 PM, Ian McLaughlin <ian at platinum.net> wrote:

On Fri, 26 Sep 2014, Robert Jarratt wrote: Which outputs are measuring a short? I just pulled one of these PSUs from one of my systems so let's see if I can follow along. I doubt that large square Sprague capacitor would fail short. It looks to me like a custom designed flat-style snap-in electrolytic capacitor. The failure mode for electrolytics is usually high esr and open. Ceramic and especially tantalum capacitors are much more likely to fail short.

On Fri, 26 Sep 2014, Robert Jarratt wrote: I had a couple of these PSUs within easy reach. The machine this one came out of hasn't been powered in a number of years, but was working fine last time it was powered up. I have another identical PSU in a companion expansion chassis which I can also pull if needed. The board in question can be removed from the main PSU chassis without desoldering any parts. There are 2 screws in opposite outer corners plus 5 more accessible through holes in the board (only the 5 with black plastic bushings, the others don't attach to the main heatsink but just hold the semiconductors to the two aluminum plates), along with the 4 shoulder screws holding the self-aligning connector board to the rear of the PSU's chassis. Once those are removed / loosened, the board can be lifted out, but be careful not to tear the huge silpad (use a nylon spudger to carefully separate it from the two aluminum plates). You don't want to know what a sheet of silpad material that large would cost to replace... Looking at the connector head-on from the mating side, the two left-most pins are connected to '+', and the next pin to the right, along with the next two pins towards the right on the other side of the connector are connected to '-'. The pin furthest towards the right seems to be a different supply or sense line of some sort. (Anyone have a pinout for a H7874?) Compensating for lead resistance, with a Fluke 177, I'm measuring 12.1 ohms between the '+' and '-' contacts on my PSU's board. Does your board also use a bunch of Nippon Chemi-Con / United Chemi-Con LXF capacitors? I've had so much trouble with various series of those brown Chemi-Con capacitors leaking or popping that I tend to replace them wholesale on sight. Which of the small capacitors was it that you broke on your board? The extra aluminum plates on the bottom of the board wouldn't make repair of this supply easy unless you have a vacuum desoldering tool and a long reach nozzle since you would have to desolder some of the semiconductors to fully access the solder side of the board.

out time it chassis screws chassis. Once not to the two that I have already removed the board in question. Although I did not use a nylon spludger as I don't know what such a thing is, let alone actually have one. However, I managed to avoid tearing the pad you mention. pins two pins The line of You can see the pinout on the backplane itself, at least you can in the machine that mine came out of. The two leftmost ones are +12V, the next three are GND, and the last one is +12V. ohms I have been using the diode testing function, which shows a short, but when I measure the actual resistance it shows about 12ohms too. So perhaps those outputs are not bad after all. My current plan is to put it all back together and attach a dummy load to see what the outputs really are. Trouble is, having broken the capacitor I still can't get the solder out of the holes (see last answer below). brown wholesale Don't know, I see three brown ones, marked SXF. The one marked 224, behind a brown 100uF electrolytic, and connected across +12V and GND, right behind the left hand set of outputs. of reach fully Indeed, I don't think I have the equipment to be able to remove the plates without damaging something. Do you know if there is any tool a hobbyist could afford to do this? Thanks Rob

parts. Yes, I was referring to the taller 330uF ones. They are marked SXF on mine, and they are brown, but the make does not appear to be marked. > >> Which of the small capacitors was it that you broke on your board? > > > > The one marked 224, behind a brown 100uF electrolytic, and connected > > across +12V and GND, right behind the left hand set of outputs. > That one is definitely a ceramic. I couldn't read its value though and it has and > desolder the capacitor to measure it with a capacitance meter. There is no need for you to do that on my behalf. I am confident I have the right replacement for it. My only problem is getting the solder out of the holes to fit the replacement. I have a hand held solder sucker and I have done the thing of adding fresh solder, but still struggling to do it. I am sure I just need a bit of time and I will manage it though.

On Fri, Sep 26, 2014 at 10:12 PM, Tothwolf <tothwolf at concentric.net> wrote: I use a Hakko 472D-01, which I like much better than the 808, because it has a separate base and pencil-style tool, rather than being all-in-one. When I'm desoldering more than a few joints, the light weight of the pencil-style tool makes a big difference. However, it costs around $500. I think it's been obsoleted by the FM-204 or 474 or some such, but for now it seems like the 472D-01 is still available, and I'm not sure that there's any pencil-style handpiece available for those newer models. If you want a vacuum desoldering station, get it soon. They're not used for SMD, so they're quickly going the way of the dodo. I still like my Hakko 936 soldering station. They claim the FX888 is technically better, but it's ugly as sin (even after they started offering a less visually offensive color scheme than the original blue/yellow), so I'm in no hurry to retire the 936. If I could afford it, I'd replace it with a Metcal.

out time it chassis screws chassis. Once not to the two that pins two pins The line of ohms I have just replaced the broken capacitor and reassembled the PSU. I attached a couple of bulbs to the outputs of the PSU. One on the 5V side, and one on the 12V side. The 5V one lit up, but the 12V one did not, and I tested the bulb to make sure it is working. So it definitely looks like the 12V board is the problem. Any chance you could check whether the two PSUs you have are still working? Thanks Rob

Now this varies based upon construction of the resistors, but it's almost impossible for any typical 2W resistor to fail dead-short - they can drift up, they can drift down - but a hard-short is just not what we see. And wirewound parts tend to act like fuses - they simply go red-hot and open up if they are forced to carry high fault currents. If the PSU isn't built for rapid over-current shutdown, and the only other alternative is the 6800uF cap, then I'd put the money on the cap. Have you checked for build errors, like missing insulators between boards & standoffs / mounts? The kind where all is well (often for many years) until the board coating wears-though and creates a short to the standoff / mount point? On Thu, Sep 25, 2014 at 6:18 PM, Robert Jarratt <robert.jarratt at ntlworld.com > I am back home and have had another look at the problem board. The > construction is such that it may prove really hard to get at all the > components, because some transistors are screwed to a large heatsink plate > that is on the non-component side of the board. I would have to > successfully desolder 4 of these to get the plate off to be able to access > the tracks so I can desolder other components. I suspect I would actually > have to cut the pins on the transistors to remove this plate. > > > > From what I can see the possible remaining shortable components appear to > be 3 parallel 150R 2W resistors and a large square Sprague 88D capacitor > (value hard to read, perhaps 6800uF, looks a bit like this one: > http://www.tedss.com/88D682M040BB). It should be possible for me to lift > one end of the 3 parallel resistors to test them for shorts, but the big > capacitor is going to be really hard to remove, for the reasons mentioned > above (ie I can?t reach the tracks on the back of the board to desolder it). > > > > How likely is it that this capacitor, which I believe is an aluminium > electrolytic, could have failed short? > > > > Regards > > > > Rob > > > > > > From: Jarratt RMA [mailto:robert.jarratt at ntlworld.com] > Sent: 22 September 2014 23:35 > To: General Discussion: On-Topic and Off-Topic Posts > Subject: Re: Value of a Tantalum Capacitor > > > > > > > > On 22 September 2014 21:43, Tony Duell <ard at p850ug1.demon.co.uk> > > > > In investigating the fault on my H7874 PSU I desoldered a tantalum > capacitor > > which I have broken in the desoldering process. I can't tell its value, > the > > markings are 224 E5Z on one side and 038 ASF on the other side. > > The value is given by '224' You read that like the resisotr code, that is > 22*10^4, or 2200000. The 'base unit' for cpaacitors is the picofarad > (pF), so that's 220000 pF = 220 nF = 0.22 uF. > > That sounds low for a tantalum capacitor. I'#ve actually seen tantalum > capacitors down to 0.1uF, but most are >1uF Are you sure it's tantalum? > > > > I think I have been convinced it is ceramic. I was fooled by the > packaging, which looks a bit like what I think tantalum capacitors look > like. One day I will learn to tell the difference more easily. > > > > Annoyingly, after desoldering three capacitors, and breaking the one > mentioned above, I still see an apparent short across the outputs of the > PSU. > > > > Thanks > > > > Rob > >

On Sep 26, 2014, at 3:03 PM, Tothwolf <tothwolf at concentric.net> wrote: Just wondering: what tool do you use to show that short? A typical ohmmeter is a low current source plus an ammeter. If you connect such a small power supply to a large discharged capacitor, it will look like a short for a while. For example, if you have a supply of a couple of volts current limited to 1 mA, it will take quite a number of seconds for a 6800 uF capacitor to charge substantially. So in a test like that, hook up the instrument and leave it connected for a minute. Does it still read 0 ohms? If yes, then I would indeed suspect a short. paul

I've had aluminium electrolytic capacitors go dead short very suddenly. One time I was working on a Canon CX printer, tracing through the PSU regualtor module. One day it was fine. Left it overnight, when i picked up my meter probes the next morning all sorts of thigns eemed to be groudned that shouldn't be. Turned out one fo the capacitors had just sshorted overnight. Not powered up, no real reason for it, I guess the oxide film just disolved. -tony