RE: PSU Input Rectifier Question
> That is absolutely the classic circuit. I assume the AC terminals of the > bridge > rectifier come from the mains input, possibly via fuse,switch,filter. > > To understand it, draw the bridge as 4 separate diodes. Now assume the triac > is > turned off (open-crcuit) and it's just a standard bridge rectifier circuit. > Then > consider the triac turned on (effectively shorted). You have the classic half- > wave voltage doubler using the 2 capacitors and 2 of the diodes. The other > 2 diodes end > up in parallel with the capacitors (remember the triac acts a short circuit > now) > and are always reverse-biased so don't do anything. > > For 230V mains input, I would expect the triac to be turned off. So it's a > full > wave rectifier with 230V AC in. Giving around 325V out. > > I suspect those capacitors! > I have now managed, after a bit of a struggle, to replace to two big rectifier output smoothing capacitors. The DMM shows about 325V output now, so I think this side of the problem is fixed. However, the PSU still does not work. The first time I tried the new caps the PSU fans twitched for a fraction of second, but I did not have any other load attached. Now they don't move at all. Could the earlier defect with the rectifier output have caused any other damage? Regards Rob
RE: PSU Input Rectifier Question
> -Original Message- > From: Maciej W. Rozycki [mailto:ma...@linux-mips.org] > Sent: 30 August 2017 19:30 > To: r...@jarratt.me.uk > Cc: 'General Discussion: On-Topic and Off-Topic Posts' > ; 'Tapley, Mark' > Subject: RE: PSU Input Rectifier Question > > On Wed, 30 Aug 2017, Rob Jarratt wrote: > > > > The purpose of the sleeve is mainly electrical insulation -- so > > > that you > > don't > > > have a battery of large live components mounted next to each other > > > posing > > a > > > risk of a short circuit if their surfaces accidentally touch. > > > In the past some electrolytic caps were manufactured without a > > > sleeve (especially axials), however I haven't come across such one for > years. > > > > > > There is a big red sticker saying that the heatsink is live, so I > > guess this one is not insulated and may be straight to the > > electrolyte. I will drill and drain I think. > > Well, that's a warning for the service engineer about the large heatsink in the > middle of the PSU, not the capacitors which are standard parts and have > nothing to do with any heatsink. Obviously (if anywhere) they had to stick the > label somewhere where it would be clearly visible, would not obstruct heat > dissipation and would not risk the label itself crumbling with time under heat > (so not on the heatsink itself). > Of course! Not thinking straight again. > Obviously you'll do whatever you wish, it's your PSU after all. :) > > Maciej
RE: PSU Input Rectifier Question
On Wed, 30 Aug 2017, Rob Jarratt wrote: > > The purpose of the sleeve is mainly electrical insulation -- so that you > don't > > have a battery of large live components mounted next to each other posing > a > > risk of a short circuit if their surfaces accidentally touch. > > In the past some electrolytic caps were manufactured without a sleeve > > (especially axials), however I haven't come across such one for years. > > > There is a big red sticker saying that the heatsink is live, so I guess this > one is not insulated and may be straight to the electrolyte. I will drill > and drain I think. Well, that's a warning for the service engineer about the large heatsink in the middle of the PSU, not the capacitors which are standard parts and have nothing to do with any heatsink. Obviously (if anywhere) they had to stick the label somewhere where it would be clearly visible, would not obstruct heat dissipation and would not risk the label itself crumbling with time under heat (so not on the heatsink itself). Obviously you'll do whatever you wish, it's your PSU after all. :) Maciej
RE: PSU Input Rectifier Question
> -Original Message- > From: Maciej W. Rozycki [mailto:ma...@linux-mips.org] > Sent: 30 August 2017 09:21 > To: r...@jarratt.me.uk > Cc: 'General Discussion: On-Topic and Off-Topic Posts' > ; 'Tapley, Mark' > Subject: RE: PSU Input Rectifier Question > > On Tue, 29 Aug 2017, Rob Jarratt wrote: > > > I would just like to make doubly sure, before cutting into the outer > > body of the caps, am I likely to hit any gotchas? Just how wet are > > they inside, do I need to take any precautions with the electrolyte? > > I suggest that you cut the sleeve only first (with a utility knife, scissors or > whatever tool you find most convenient or at hand), in which case you won't > get to the electrolyte (except for any preexisting leaks of course), which is only > inside the aluminium can underneath. > > The purpose of the sleeve is mainly electrical insulation -- so that you don't > have a battery of large live components mounted next to each other posing a > risk of a short circuit if their surfaces accidentally touch. > In the past some electrolytic caps were manufactured without a sleeve > (especially axials), however I haven't come across such one for years. There is a big red sticker saying that the heatsink is live, so I guess this one is not insulated and may be straight to the electrolyte. I will drill and drain I think. Regards Rob > > > I found some LED holders/clips which do seem to fit the bill, although > > I am not sure I see how you get the old LED out, unless you have to > > break the thing. > > From experience these seem pretty much single use only and either of the > 2 parts of the holder gets damaged while disassembling. > OK, good to know that I haven't missed a trick. Thanks Rob
RE: PSU Input Rectifier Question
In my case the cap is epoxied to the board so I can’t rock it :( From: drlegendre . [mailto:drlegen...@gmail.com] Sent: 30 August 2017 07:24 To: r...@jarratt.me.uk; Rob Jarratt ; General Discussion: On-Topic and Off-Topic Posts Cc: dwight Subject: Re: PSU Input Rectifier Question snap-cap removal goes like this: heat up the joints and solder-suck them clean as possible. Then hold on to the cap body, and attempt to rock it back & forth has you re-heat one pin and then the other(s). Once all the pins are broke free, you can extract it. On Tue, Aug 29, 2017 at 5:41 PM, Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> > wrote: Good idea to drain them first, I will do that. Having never encountered snap-ins before I don't know, but from the way you describe removal I suspect they are snap-in, as the leads are quite tightly against the sides of the holes and won't move even if I have heated them up. I hope that cutting the tops off will eventually expose the individual pins so they will be more easily removed. Regards Rob From: dwight [mailto:dkel...@hotmail.com <mailto:dkel...@hotmail.com> ] Sent: 29 August 2017 22:25 To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> ; Rob Jarratt mailto:robert.jarr...@ntlworld.com> >; General Discussion: On-Topic and Off-Topic Posts mailto:cctalk@classiccmp.org> > Subject: Re: PSU Input Rectifier Question They would have electrolyte in them. It is mildly corrosive as it is mostly a borate solution. You can drill a couple holes in the tops of the case and drain them. Since you say you've unsoldered the bases, are they wire or snap in. If snap in, you must get the lead clear of any solder touching the edge of the hole before you can think of any prying. Dwight _ From: cctalk mailto:cctalk-boun...@classiccmp.org> <mailto:cctalk-boun...@classiccmp.org <mailto:cctalk-boun...@classiccmp.org> > > on behalf of Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> <mailto:cctalk@classiccmp.org <mailto:cctalk@classiccmp.org> > > Sent: Tuesday, August 29, 2017 1:01:14 PM To: 'Maciej W. Rozycki'; 'General Discussion: On-Topic and Off-Topic Posts' Subject: RE: PSU Input Rectifier Question > -Original Message- > From: Maciej W. Rozycki [mailto:ma...@linux-mips.org > <mailto:ma...@linux-mips.org> ] > Sent: 29 August 2017 01:06 > To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> > <mailto:r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> > ; Rob Jarratt mailto:robert.jarr...@ntlworld.com> <mailto:robert.jarr...@ntlworld.com <mailto:robert.jarr...@ntlworld.com> > >; General > Discussion: On-Topic and Off-Topic Posts <mailto:cctalk@classiccmp.org> <mailto:cctalk@classiccmp.org <mailto:cctalk@classiccmp.org> > > > Cc: 'Tapley, Mark' mailto:mtap...@swri.edu> > <mailto:mtap...@swri.edu <mailto:mtap...@swri.edu> > > > Subject: RE: PSU Input Rectifier Question > > On Tue, 29 Aug 2017, Rob Jarratt via cctalk wrote: > > > Not really no. The base of the capacitors is actually touching the > > board, so I can't get under it to cut anything anyway, which is why I > > am thinking I would have to cut the tops off them. Even if I could cut > > one of the leads, I can't solder a capacitor underneath as that is > > where the board is mounted into the enclosure, there is only room for > > the mounting standoffs on the underside. > > If you need to replace the caps anyway, then I think just cutting through the > outer PET sleeve should do as chances are it's only the sleeve that's got glued > and not the leads or the closing cap. Then you might be able to just pull the > inner aluminium can containing the capacitor itself from the sleeve and with > the component proper out of the way get rid of any remains from the PCB. > Thanks Maciej, I would just like to make doubly sure, before cutting into the outer body of the caps, am I likely to hit any gotchas? Just how wet are they inside, do I need to take any precautions with the electrolyte? > NB the originals are just snap-in capacitors AFAIK, e.g. Chemi-Con KMH series > (I can't figure that out from your photos and luckily none of my H7878s has > failed so far, so I haven't made this a part list for this PSU for myself), still > manufactured, so modern replacements should do. With the PCB hole > diameter supposed to be 2mm by the manufacturer's spec I wouldn't be > bothered with a small lead pitch variation, you can always bend the leads > slightly as there's some space between the closing cap and the rim that rests > against the PCB. > I did wonder if there would be enough tolerance. I have never encountered these s
RE: PSU Input Rectifier Question
On Tue, 29 Aug 2017, Rob Jarratt wrote: > I would just like to make doubly sure, before cutting into the outer body of > the caps, am I likely to hit any gotchas? Just how wet are they inside, do I > need to take any precautions with the electrolyte? I suggest that you cut the sleeve only first (with a utility knife, scissors or whatever tool you find most convenient or at hand), in which case you won't get to the electrolyte (except for any preexisting leaks of course), which is only inside the aluminium can underneath. The purpose of the sleeve is mainly electrical insulation -- so that you don't have a battery of large live components mounted next to each other posing a risk of a short circuit if their surfaces accidentally touch. In the past some electrolytic caps were manufactured without a sleeve (especially axials), however I haven't come across such one for years. > I found some LED holders/clips which do seem to fit the bill, although I am > not sure I see how you get the old LED out, unless you have to break the > thing. From experience these seem pretty much single use only and either of the 2 parts of the holder gets damaged while disassembling. Maciej
Re: PSU Input Rectifier Question
On 08/29/2017 11:24 PM, drlegendre . via cctalk wrote: > snap-cap removal goes like this: heat up the joints and solder-suck > them clean as possible. Then hold on to the cap body, and attempt to > rock it back & forth has you re-heat one pin and then the other(s). > Once all the pins are broke free, you can extract it. That's pretty much the way I do it--but I first remove as much solder as possible (sometimes wick works better in these cases than suction), then I clip the protruding leads as close to the board as I can. Big snap-ins soldered to heavy power traces or planes can be a challenge to get heated sufficiently. Use two irons if you have to. --Chuck
Re: PSU Input Rectifier Question
snap-cap removal goes like this: heat up the joints and solder-suck them clean as possible. Then hold on to the cap body, and attempt to rock it back & forth has you re-heat one pin and then the other(s). Once all the pins are broke free, you can extract it. On Tue, Aug 29, 2017 at 5:41 PM, Rob Jarratt via cctalk < cctalk@classiccmp.org> wrote: > Good idea to drain them first, I will do that. Having never encountered > snap-ins before I don't know, but from the way you describe removal I > suspect they are snap-in, as the leads are quite tightly against the sides > of the holes and won't move even if I have heated them up. I hope that > cutting the tops off will eventually expose the individual pins so they > will > be more easily removed. > > > > Regards > > > > Rob > > > > From: dwight [mailto:dkel...@hotmail.com] > Sent: 29 August 2017 22:25 > To: r...@jarratt.me.uk; Rob Jarratt ; General > Discussion: On-Topic and Off-Topic Posts > Subject: Re: PSU Input Rectifier Question > > > > They would have electrolyte in them. It is mildly corrosive as it is mostly > a borate solution. You can drill a couple holes in the tops of the case and > drain them. > > Since you say you've unsoldered the bases, are they wire or snap in. If > snap > in, you must get the lead clear of any solder touching the edge of the hole > before you can think of any prying. > > Dwight > > > > _ > > From: cctalk <mailto:cctalk-boun...@classiccmp.org> > on behalf of Rob Jarratt via > cctalk > mailto:cctalk@classiccmp.org> > > Sent: Tuesday, August 29, 2017 1:01:14 PM > To: 'Maciej W. Rozycki'; 'General Discussion: On-Topic and Off-Topic Posts' > Subject: RE: PSU Input Rectifier Question > > > > > > > -Original Message- > > From: Maciej W. Rozycki [mailto:ma...@linux-mips.org] > > Sent: 29 August 2017 01:06 > > To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> ; Rob Jarratt > mailto:robert.jarr...@ntlworld.com> >; > General > > Discussion: On-Topic and Off-Topic Posts <mailto:cctalk@classiccmp.org> > > > Cc: 'Tapley, Mark' mailto:mtap...@swri.edu> > > > Subject: RE: PSU Input Rectifier Question > > > > On Tue, 29 Aug 2017, Rob Jarratt via cctalk wrote: > > > > > Not really no. The base of the capacitors is actually touching the > > > board, so I can't get under it to cut anything anyway, which is why I > > > am thinking I would have to cut the tops off them. Even if I could cut > > > one of the leads, I can't solder a capacitor underneath as that is > > > where the board is mounted into the enclosure, there is only room for > > > the mounting standoffs on the underside. > > > > If you need to replace the caps anyway, then I think just cutting > through > the > > outer PET sleeve should do as chances are it's only the sleeve that's got > glued > > and not the leads or the closing cap. Then you might be able to just > pull > the > > inner aluminium can containing the capacitor itself from the sleeve and > with > > the component proper out of the way get rid of any remains from the PCB. > > > > Thanks Maciej, > > I would just like to make doubly sure, before cutting into the outer body > of > the caps, am I likely to hit any gotchas? Just how wet are they inside, do > I > need to take any precautions with the electrolyte? > > > > NB the originals are just snap-in capacitors AFAIK, e.g. Chemi-Con KMH > series > > (I can't figure that out from your photos and luckily none of my H7878s > has > > failed so far, so I haven't made this a part list for this PSU for > myself), still > > manufactured, so modern replacements should do. With the PCB hole > > diameter supposed to be 2mm by the manufacturer's spec I wouldn't be > > bothered with a small lead pitch variation, you can always bend the leads > > slightly as there's some space between the closing cap and the rim that > rests > > against the PCB. > > > > > I did wonder if there would be enough tolerance. I have never encountered > these snap-in ones before, I don't know what they are like. Sounds like I > can try some snap-ins, which widens the choice immeasurably. > > > > Also I would't be bothered about the LED holder getting damaged; it's a > > standard piece, available online. I can dig out the part numbers, > separate for > > individual pieces of the holder (you can match different kinds against > each > > other, depending on your needs), if that would help; I ordered a bunch a > while > > ago for my own use. > > > > I found some LED holders/clips which do seem to fit the bill, although I am > not sure I see how you get the old LED out, unless you have to break the > thing. > > Regards > > Rob > > > >
RE: PSU Input Rectifier Question
Good idea to drain them first, I will do that. Having never encountered snap-ins before I don't know, but from the way you describe removal I suspect they are snap-in, as the leads are quite tightly against the sides of the holes and won't move even if I have heated them up. I hope that cutting the tops off will eventually expose the individual pins so they will be more easily removed. Regards Rob From: dwight [mailto:dkel...@hotmail.com] Sent: 29 August 2017 22:25 To: r...@jarratt.me.uk; Rob Jarratt ; General Discussion: On-Topic and Off-Topic Posts Subject: Re: PSU Input Rectifier Question They would have electrolyte in them. It is mildly corrosive as it is mostly a borate solution. You can drill a couple holes in the tops of the case and drain them. Since you say you've unsoldered the bases, are they wire or snap in. If snap in, you must get the lead clear of any solder touching the edge of the hole before you can think of any prying. Dwight _ From: cctalk mailto:cctalk-boun...@classiccmp.org> > on behalf of Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> > Sent: Tuesday, August 29, 2017 1:01:14 PM To: 'Maciej W. Rozycki'; 'General Discussion: On-Topic and Off-Topic Posts' Subject: RE: PSU Input Rectifier Question > -Original Message- > From: Maciej W. Rozycki [mailto:ma...@linux-mips.org] > Sent: 29 August 2017 01:06 > To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> ; Rob Jarratt mailto:robert.jarr...@ntlworld.com> >; General > Discussion: On-Topic and Off-Topic Posts mailto:cctalk@classiccmp.org> > > Cc: 'Tapley, Mark' mailto:mtap...@swri.edu> > > Subject: RE: PSU Input Rectifier Question > > On Tue, 29 Aug 2017, Rob Jarratt via cctalk wrote: > > > Not really no. The base of the capacitors is actually touching the > > board, so I can't get under it to cut anything anyway, which is why I > > am thinking I would have to cut the tops off them. Even if I could cut > > one of the leads, I can't solder a capacitor underneath as that is > > where the board is mounted into the enclosure, there is only room for > > the mounting standoffs on the underside. > > If you need to replace the caps anyway, then I think just cutting through the > outer PET sleeve should do as chances are it's only the sleeve that's got glued > and not the leads or the closing cap. Then you might be able to just pull the > inner aluminium can containing the capacitor itself from the sleeve and with > the component proper out of the way get rid of any remains from the PCB. > Thanks Maciej, I would just like to make doubly sure, before cutting into the outer body of the caps, am I likely to hit any gotchas? Just how wet are they inside, do I need to take any precautions with the electrolyte? > NB the originals are just snap-in capacitors AFAIK, e.g. Chemi-Con KMH series > (I can't figure that out from your photos and luckily none of my H7878s has > failed so far, so I haven't made this a part list for this PSU for myself), still > manufactured, so modern replacements should do. With the PCB hole > diameter supposed to be 2mm by the manufacturer's spec I wouldn't be > bothered with a small lead pitch variation, you can always bend the leads > slightly as there's some space between the closing cap and the rim that rests > against the PCB. > I did wonder if there would be enough tolerance. I have never encountered these snap-in ones before, I don't know what they are like. Sounds like I can try some snap-ins, which widens the choice immeasurably. > Also I would't be bothered about the LED holder getting damaged; it's a > standard piece, available online. I can dig out the part numbers, separate for > individual pieces of the holder (you can match different kinds against each > other, depending on your needs), if that would help; I ordered a bunch a while > ago for my own use. > I found some LED holders/clips which do seem to fit the bill, although I am not sure I see how you get the old LED out, unless you have to break the thing. Regards Rob
Re: PSU Input Rectifier Question
They would have electrolyte in them. It is mildly corrosive as it is mostly a borate solution. You can drill a couple holes in the tops of the case and drain them. Since you say you've unsoldered the bases, are they wire or snap in. If snap in, you must get the lead clear of any solder touching the edge of the hole before you can think of any prying. Dwight From: cctalk on behalf of Rob Jarratt via cctalk Sent: Tuesday, August 29, 2017 1:01:14 PM To: 'Maciej W. Rozycki'; 'General Discussion: On-Topic and Off-Topic Posts' Subject: RE: PSU Input Rectifier Question > -Original Message- > From: Maciej W. Rozycki [mailto:ma...@linux-mips.org] > Sent: 29 August 2017 01:06 > To: r...@jarratt.me.uk; Rob Jarratt ; General > Discussion: On-Topic and Off-Topic Posts > Cc: 'Tapley, Mark' > Subject: RE: PSU Input Rectifier Question > > On Tue, 29 Aug 2017, Rob Jarratt via cctalk wrote: > > > Not really no. The base of the capacitors is actually touching the > > board, so I can't get under it to cut anything anyway, which is why I > > am thinking I would have to cut the tops off them. Even if I could cut > > one of the leads, I can't solder a capacitor underneath as that is > > where the board is mounted into the enclosure, there is only room for > > the mounting standoffs on the underside. > > If you need to replace the caps anyway, then I think just cutting through the > outer PET sleeve should do as chances are it's only the sleeve that's got glued > and not the leads or the closing cap. Then you might be able to just pull the > inner aluminium can containing the capacitor itself from the sleeve and with > the component proper out of the way get rid of any remains from the PCB. > Thanks Maciej, I would just like to make doubly sure, before cutting into the outer body of the caps, am I likely to hit any gotchas? Just how wet are they inside, do I need to take any precautions with the electrolyte? > NB the originals are just snap-in capacitors AFAIK, e.g. Chemi-Con KMH series > (I can't figure that out from your photos and luckily none of my H7878s has > failed so far, so I haven't made this a part list for this PSU for myself), still > manufactured, so modern replacements should do. With the PCB hole > diameter supposed to be 2mm by the manufacturer's spec I wouldn't be > bothered with a small lead pitch variation, you can always bend the leads > slightly as there's some space between the closing cap and the rim that rests > against the PCB. > I did wonder if there would be enough tolerance. I have never encountered these snap-in ones before, I don't know what they are like. Sounds like I can try some snap-ins, which widens the choice immeasurably. > Also I would't be bothered about the LED holder getting damaged; it's a > standard piece, available online. I can dig out the part numbers, separate for > individual pieces of the holder (you can match different kinds against each > other, depending on your needs), if that would help; I ordered a bunch a while > ago for my own use. > I found some LED holders/clips which do seem to fit the bill, although I am not sure I see how you get the old LED out, unless you have to break the thing. Regards Rob
RE: PSU Input Rectifier Question
> -Original Message- > From: Maciej W. Rozycki [mailto:ma...@linux-mips.org] > Sent: 29 August 2017 01:06 > To: r...@jarratt.me.uk; Rob Jarratt ; General > Discussion: On-Topic and Off-Topic Posts > Cc: 'Tapley, Mark' > Subject: RE: PSU Input Rectifier Question > > On Tue, 29 Aug 2017, Rob Jarratt via cctalk wrote: > > > Not really no. The base of the capacitors is actually touching the > > board, so I can't get under it to cut anything anyway, which is why I > > am thinking I would have to cut the tops off them. Even if I could cut > > one of the leads, I can't solder a capacitor underneath as that is > > where the board is mounted into the enclosure, there is only room for > > the mounting standoffs on the underside. > > If you need to replace the caps anyway, then I think just cutting through the > outer PET sleeve should do as chances are it's only the sleeve that's got glued > and not the leads or the closing cap. Then you might be able to just pull the > inner aluminium can containing the capacitor itself from the sleeve and with > the component proper out of the way get rid of any remains from the PCB. > Thanks Maciej, I would just like to make doubly sure, before cutting into the outer body of the caps, am I likely to hit any gotchas? Just how wet are they inside, do I need to take any precautions with the electrolyte? > NB the originals are just snap-in capacitors AFAIK, e.g. Chemi-Con KMH series > (I can't figure that out from your photos and luckily none of my H7878s has > failed so far, so I haven't made this a part list for this PSU for myself), still > manufactured, so modern replacements should do. With the PCB hole > diameter supposed to be 2mm by the manufacturer's spec I wouldn't be > bothered with a small lead pitch variation, you can always bend the leads > slightly as there's some space between the closing cap and the rim that rests > against the PCB. > I did wonder if there would be enough tolerance. I have never encountered these snap-in ones before, I don't know what they are like. Sounds like I can try some snap-ins, which widens the choice immeasurably. > Also I would't be bothered about the LED holder getting damaged; it's a > standard piece, available online. I can dig out the part numbers, separate for > individual pieces of the holder (you can match different kinds against each > other, depending on your needs), if that would help; I ordered a bunch a while > ago for my own use. > I found some LED holders/clips which do seem to fit the bill, although I am not sure I see how you get the old LED out, unless you have to break the thing. Regards Rob
RE: PSU Input Rectifier Question
On Tue, 29 Aug 2017, Rob Jarratt via cctalk wrote: > Not really no. The base of the capacitors is actually touching the board, so > I can't get under it to cut anything anyway, which is why I am thinking I > would have to cut the tops off them. Even if I could cut one of the leads, I > can't solder a capacitor underneath as that is where the board is mounted > into the enclosure, there is only room for the mounting standoffs on the > underside. If you need to replace the caps anyway, then I think just cutting through the outer PET sleeve should do as chances are it's only the sleeve that's got glued and not the leads or the closing cap. Then you might be able to just pull the inner aluminium can containing the capacitor itself from the sleeve and with the component proper out of the way get rid of any remains from the PCB. NB the originals are just snap-in capacitors AFAIK, e.g. Chemi-Con KMH series (I can't figure that out from your photos and luckily none of my H7878s has failed so far, so I haven't made this a part list for this PSU for myself), still manufactured, so modern replacements should do. With the PCB hole diameter supposed to be 2mm by the manufacturer's spec I wouldn't be bothered with a small lead pitch variation, you can always bend the leads slightly as there's some space between the closing cap and the rim that rests against the PCB. Also I would't be bothered about the LED holder getting damaged; it's a standard piece, available online. I can dig out the part numbers, separate for individual pieces of the holder (you can match different kinds against each other, depending on your needs), if that would help; I ordered a bunch a while ago for my own use. Maciej
RE: PSU Input Rectifier Question
> Rob, I assume there isn't room (front or back of the circuit board) to just cut > one of the leads, and wire in a replacement in parallel to the old one, leaving > the old one in place? That's not a good answer if the old one is leaking, but if > it's still intact, cutting one lead takes it out of circuit so it might stop > deteriorating. It would save having to cut through the epoxy. > - Not really no. The base of the capacitors is actually touching the board, so I can't get under it to cut anything anyway, which is why I am thinking I would have to cut the tops off them. Even if I could cut one of the leads, I can't solder a capacitor underneath as that is where the board is mounted into the enclosure, there is only room for the mounting standoffs on the underside. Regards Rob
Re: PSU Input Rectifier Question
On Aug 28, 2017, at 5:12 PM, Rob Jarratt via cctalk wrote: >> >>> If anyone has any good ideas for a "clean" way to remove them I am all ears. >> >> Fully unsoldering the leads and rocking back and forth usually does the job. >> > > Indeed. That is my normal method. But in this case the epoxy and the fact > that the base of the caps are in contact with the board make it practically > impossible to get any movement. I have tried cutting away the epoxy I can > reach, but the damn thing is still firmly stuck. Rob, I assume there isn’t room (front or back of the circuit board) to just cut one of the leads, and wire in a replacement in parallel to the old one, leaving the old one in place? That’s not a good answer if the old one is leaking, but if it’s still intact, cutting one lead takes it out of circuit so it might stop deteriorating. It would save having to cut through the epoxy. - Mark
RE: PSU Input Rectifier Question
> > > If anyone has any good ideas for a "clean" way to remove them I am all ears. > > Fully unsoldering the leads and rocking back and forth usually does the job. > Indeed. That is my normal method. But in this case the epoxy and the fact that the base of the caps are in contact with the board make it practically impossible to get any movement. I have tried cutting away the epoxy I can reach, but the damn thing is still firmly stuck. Regards Rob
Re: PSU Input Rectifier Question
> If anyone has any good ideas for a "clean" way to remove them I am all ears. Fully unsoldering the leads and rocking back and forth usually does the job. - Original Message - From: "Rob Jarratt via cctalk" To: "'Tony Duell'" ; "'General Discussion: On-Topic and Off-Topic Posts'" Sent: Monday, August 28, 2017 2:30 PM Subject: RE: PSU Input Rectifier Question > > I suspect those capacitors! > So do I, but I fear they are going to be really difficult to remove. They are both epoxied in two places, in both cases one of the glue dabs is pretty inaccessible. I am minded to just cut the bodies of the caps off (ie above their bases, cutting directly into the body close to the base) and then see how to get the rest of the damn things off the board. I am not sure what the electrolyte is like inside these things, do I need to take any precautions? If anyone has any good ideas for a "clean" way to remove them I am all ears. Regards Rob
RE: PSU Input Rectifier Question
> -Original Message- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Tony Duell > via cctalk > Sent: 28 August 2017 19:35 > To: General Discussion: On-Topic and Off-Topic Posts > Subject: Re: PSU Input Rectifier Question > > On Mon, Aug 28, 2017 at 7:30 PM, Rob Jarratt > wrote: > > > So do I, but I fear they are going to be really difficult to remove. > > They are both epoxied in two places, in both cases one of the glue > > dabs is pretty inaccessible. I am minded > > Are you sure it's epoxy? Every time I've had components glued to a PCB it's > been with a fairly 'rubbery' hot-melt type of adhesive. > Yes, it isn't rubbery at all, definitely not that hot-melt stuff as far as I can tell. I have had to take a Stanley knife to it to cut into the stuff, but I still can't free even the accessible bits. Regards Rob
Re: PSU Input Rectifier Question
On Mon, Aug 28, 2017 at 7:30 PM, Rob Jarratt wrote: > So do I, but I fear they are going to be really difficult to remove. They are > both epoxied > in two places, in both cases one of the glue dabs is pretty inaccessible. I > am minded Are you sure it's epoxy? Every time I've had components glued to a PCB it's been with a fairly 'rubbery' hot-melt type of adhesive. -tony
RE: PSU Input Rectifier Question
> > I suspect those capacitors! > So do I, but I fear they are going to be really difficult to remove. They are both epoxied in two places, in both cases one of the glue dabs is pretty inaccessible. I am minded to just cut the bodies of the caps off (ie above their bases, cutting directly into the body close to the base) and then see how to get the rest of the damn things off the board. I am not sure what the electrolyte is like inside these things, do I need to take any precautions? If anyone has any good ideas for a "clean" way to remove them I am all ears. Regards Rob
Re: PSU Input Rectifier Question
On Mon, Aug 28, 2017 at 5:52 PM, Rob Jarratt wrote: > I have posted some information along with a rough schematic here: > https://rjarratt.wordpress.com/2017/08/28/decstation-5000-model-240-h7878-possible-fault-found/. > Other information is in the previous post: > https://robs-old-computers.com/2017/08/27/decstation-5000-model-240-h7878-power-supply-failure/. > > Interesting that Tony mentions a Triac. There is indeed a triac. I am using > 220/240V input > (being in the UK), would it be expected to double the voltage in this case to > 325V? That is absolutely the classic circuit. I assume the AC terminals of the bridge rectifier come from the mains input, possibly via fuse,switch,filter. To understand it, draw the bridge as 4 separate diodes. Now assume the triac is turned off (open-crcuit) and it's just a standard bridge rectifier circuit. Then consider the triac turned on (effectively shorted). You have the classic half-wave voltage doubler using the 2 capacitors and 2 of the diodes. The other 2 diodes end up in parallel with the capacitors (remember the triac acts a short circuit now) and are always reverse-biased so don't do anything. For 230V mains input, I would expect the triac to be turned off. So it's a full wave rectifier with 230V AC in. Giving around 325V out. I suspect those capacitors! -tony
Re: PSU Input Rectifier Question
> On Aug 28, 2017, at 12:52 PM, Rob Jarratt via cctalk > wrote: > > ... > Interesting that Tony mentions a Triac. There is indeed a triac. I am using > 220/240V input (being in the UK), would it be expected to double the voltage > in this case to 325V? No, 325V DC is what you'd get out of a full wave rectifier (not doubler) rectifying the 240V UK mains. That's because capacitor input filters -- what most supplies in the past 60 or so years used -- produce a DC output that matches the mains input peak voltage, at least under low load. (Inductor input filters are different, they produce roughly the RMS voltage, but inductur input filters stopped being common around the time that solid state rectifiers appeared in regular use. They are a good choice if you use 866 tubes for rectification...) A voltage doubler used in the UK would give you over 600 volts, which you're not likely to see outside of a high power radio transmitter or similar device using large vacuum tubes. paul
RE: PSU Input Rectifier Question
> -Original Message- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Tony Duell > via cctalk > Sent: 28 August 2017 15:52 > To: General Discussion: On-Topic and Off-Topic Posts > Subject: Re: PSU Input Rectifier Question > > On Mon, Aug 28, 2017 at 12:49 PM, Rob Jarratt via cctalk > wrote: > > My recently acquired DECstation 5000/240 has had a PSU failure. After > > replacing a couple of small capacitors with high ESR, I have been > > doing some basic checks to see which stages are working. My first step > > has been to check the output of the input rectifier, which has two > > large capacitors to smooth the output, they are each rated at 200V. > > When I check the rectifier output with a basic DMM I measure about > > 136V on the AC range, and a voltage ranging around 160-170 on the DC > range. > > > > > > > > I know that some PSUs double the voltage here, and the presence of two > > large capacitors would suggest that happens here too. However, from > > what I have read, the doubling is done on PSUs with a voltage selector > > switch. This PSU does not have an input voltage selection switch. I > > don't know if on auto-ranging PSUs the rectifier output should be 325V > > or 163V. However, given that each capacitor is rated at 200V then it > > seems to me that the rectifier output is expected to be 325V. Is that right? > > If you havr 230V mains input and you full-wave rectify it and smooth it (say > using a bridge rectifier) you will get 230*sqrt(2) or about 325V. There is no > easy way to get 163V DC from a 230V AC input (you could do it with a step- > down transformer or a voltage regulator circuit, of course, but not with just > diodes and capacitors). > > The common input circuit for PSUs with a voltage selector switch is quite > ingenious. With the switch open, it's a bridge rectifier and smoothing > capacitor > (the latter is actually 2 capacitors in series, with bleeder resistors, each > charged to around 163V). With switch closed, it becomes a voltage doubler. > Two of the diodes do nothing, but again each capacitor charges to around 163V > (with a 115V AC input). > > A lot of older auto-sensing power supplies used that circuit, with an > automatic > switching component (triac or similar) in place of the switch. > > The first thing to check is if the 2 capacitors are in series. Does the +ve > terminal of one go to the -ve terminal of the other? What voltage do you > measure across each capacitor separately? > > What is your mains voltage? 115V or 230V? If the former then it's possible the > automatic switching has failed and that it's acting as if it is connected to > 230V > mains. > I have posted some information along with a rough schematic here: https://rjarratt.wordpress.com/2017/08/28/decstation-5000-model-240-h7878-possible-fault-found/. Other information is in the previous post: https://robs-old-computers.com/2017/08/27/decstation-5000-model-240-h7878-power-supply-failure/. Interesting that Tony mentions a Triac. There is indeed a triac. I am using 220/240V input (being in the UK), would it be expected to double the voltage in this case to 325V? Regards Rob
Re: PSU Input Rectifier Question
On Mon, Aug 28, 2017 at 12:49 PM, Rob Jarratt via cctalk wrote: > My recently acquired DECstation 5000/240 has had a PSU failure. After > replacing a couple of small capacitors with high ESR, I have been doing some > basic checks to see which stages are working. My first step has been to > check the output of the input rectifier, which has two large capacitors to > smooth the output, they are each rated at 200V. When I check the rectifier > output with a basic DMM I measure about 136V on the AC range, and a voltage > ranging around 160-170 on the DC range. > > > > I know that some PSUs double the voltage here, and the presence of two large > capacitors would suggest that happens here too. However, from what I have > read, the doubling is done on PSUs with a voltage selector switch. This PSU > does not have an input voltage selection switch. I don't know if on > auto-ranging PSUs the rectifier output should be 325V or 163V. However, > given that each capacitor is rated at 200V then it seems to me that the > rectifier output is expected to be 325V. Is that right? If you havr 230V mains input and you full-wave rectify it and smooth it (say using a bridge rectifier) you will get 230*sqrt(2) or about 325V. There is no easy way to get 163V DC from a 230V AC input (you could do it with a step-down transformer or a voltage regulator circuit, of course, but not with just diodes and capacitors). The common input circuit for PSUs with a voltage selector switch is quite ingenious. With the switch open, it's a bridge rectifier and smoothing capacitor (the latter is actually 2 capacitors in series, with bleeder resistors, each charged to around 163V). With switch closed, it becomes a voltage doubler. Two of the diodes do nothing, but again each capacitor charges to around 163V (with a 115V AC input). A lot of older auto-sensing power supplies used that circuit, with an automatic switching component (triac or similar) in place of the switch. The first thing to check is if the 2 capacitors are in series. Does the +ve terminal of one go to the -ve terminal of the other? What voltage do you measure across each capacitor separately? What is your mains voltage? 115V or 230V? If the former then it's possible the automatic switching has failed and that it's acting as if it is connected to 230V mains. -tony
Re: PSU Input Rectifier Question
> On Aug 28, 2017, at 7:49 AM, Rob Jarratt via cctalk > wrote: > > My recently acquired DECstation 5000/240 has had a PSU failure. After > replacing a couple of small capacitors with high ESR, I have been doing some > basic checks to see which stages are working. My first step has been to > check the output of the input rectifier, which has two large capacitors to > smooth the output, they are each rated at 200V. When I check the rectifier > output with a basic DMM I measure about 136V on the AC range, and a voltage > ranging around 160-170 on the DC range. > > > > I know that some PSUs double the voltage here, and the presence of two large > capacitors would suggest that happens here too. ... Can you work out the schematic, at least the critical part: transformer, rectifiers, capacitors? There are lots of ways to arrange the components. A couple of arrangements with two capacitors are doublers, but it's certainly possible to have something else. It all depends on the details. Ham radio references (ARRL Handbook, equivalent publication from RSGB) will show power supply structures and show all these schemes clearly. paul
PSU Input Rectifier Question
My recently acquired DECstation 5000/240 has had a PSU failure. After replacing a couple of small capacitors with high ESR, I have been doing some basic checks to see which stages are working. My first step has been to check the output of the input rectifier, which has two large capacitors to smooth the output, they are each rated at 200V. When I check the rectifier output with a basic DMM I measure about 136V on the AC range, and a voltage ranging around 160-170 on the DC range. I know that some PSUs double the voltage here, and the presence of two large capacitors would suggest that happens here too. However, from what I have read, the doubling is done on PSUs with a voltage selector switch. This PSU does not have an input voltage selection switch. I don't know if on auto-ranging PSUs the rectifier output should be 325V or 163V. However, given that each capacitor is rated at 200V then it seems to me that the rectifier output is expected to be 325V. Is that right? For the record, both of the capacitors do feel like they might have a slight bulge at the top, and on the underside of the board of one of them there is some mottling, which suggests there may have been a leak at some point. The bases of the capacitors are in full contact with the board, so I can't really see if there is any leakage. They seem to have been glued in with epoxy too. Testing them in-circuit with the multimeter, one appears to charge slowly, the other is immediately open-circuit. So clearly there is a problem here and they need to be replaced anyway. The supplementary question is about finding replacements. It seems that almost all capacitors of the exact spec (820uF, 200V) are of the snap-in variety. I don't have experience of these compared to normal leaded ones. The leads seem to be 11mm apart, but the snap-ins I find have them 10mm apart, if the leads are effectively very short then I am not sure if snap-ins are a wise choice. Can anyone advise? Also my usual supplier (Farnell) has a limited choice, if I went for a slightly higher valued capacitance (1000uF instead of 820uF), is that likely to cause a problem? Regards Rob
