Re: Cambridge Workstation woes - VMS 2691 PSU smoke
On Sun, Jul 30, 2017 at 8:17 PM, Jules Richardson via cctalk wrote: > On 07/30/2017 11:28 AM, Tony Duell wrote: >> >> I certainly traced out the schematics of mine, but I forget which PSU was >> fitted to my machine (something tells me it was made by Farnell). > > > That could well be - I do think that I've seen another type of PSU used in > addition to the VMS one, and Farnell would be an obvious choice. By chance I came across the diagrams today. The PSU in my Cambridge Workstation was made by Zenith. So I guess the diagrams aren;'t going to help for this. -tony
Re: Cambridge Workstation woes - VMS 2691 PSU smoke
On Sun, Jul 30, 2017 at 8:17 PM, Jules Richardson via cctalk wrote: >> The former are the ones that normally fail, but I would change the lot. > > > I'd be inclined to agree in the case of any class X/Y caps as they're a > known failure point in any piece of vintage equipment... but not every > single cap in the PSU. I know that's not what you're suggesting, but there > seems to be a trend amongst collectors to blindly replace every cap in sight > as part of some "refurbishment process" these days, whether they're good or > bad. Personally I like to keep my systems as original as possible, which > generally precludes replacing perfectly-good vintage components with > perfectly-good modern ones :-) Yes, I was not clear. I don't believe in replacing perfectly good capacitors either, and in my experience capacitors (at least not in classic computers) do not fail as often as some people will have you believe. I meant to say to replace all the capacitors in the mains filter circuit. There will probably be a bfilar-wound filter coil (which doesn't need replacing). On one side of it (electrically) will be a class X capacitor between live and neutral and a pair of class Ys from each of those line to earth. On the other side of the filter coil expect at least the class X between live and neutral maybe more class Ys to ground. I would replace all those. And nothing else unless you know there's a fault. Thinking about it, the ACW has a built-in Microvitec monitor with its own SMPSU (on the monitor board). That probably has its own mains filter componets which I'd probably replace while I was in there. -tony
Re: Cambridge Workstation woes - VMS 2691 PSU smoke
On 07/30/2017 11:28 AM, Tony Duell wrote: On Sun, Jul 30, 2017 at 5:18 PM, Jules Richardson via cctalk wrote: On 07/30/2017 10:59 AM, Ian Frost via cctalk wrote: Any body any experience in fixing these old Acorn PSUs - or managed to source alternative modern supplies? In general smoke from a power supply that keeps on working is from the mains filter capacitors. Some makes are well-known for this. I would carefully examine them, they crack when they fail Aha, I missed the "kept working" bit... agreed there, then, it's probably one of the input caps which failed (and that seems consistent with brown goo on the PCB, too). The former are the ones that normally fail, but I would change the lot. I'd be inclined to agree in the case of any class X/Y caps as they're a known failure point in any piece of vintage equipment... but not every single cap in the PSU. I know that's not what you're suggesting, but there seems to be a trend amongst collectors to blindly replace every cap in sight as part of some "refurbishment process" these days, whether they're good or bad. Personally I like to keep my systems as original as possible, which generally precludes replacing perfectly-good vintage components with perfectly-good modern ones :-) I certainly traced out the schematics of mine, but I forget which PSU was fitted to my machine (something tells me it was made by Farnell). That could well be - I do think that I've seen another type of PSU used in addition to the VMS one, and Farnell would be an obvious choice. cheers Jules
ATX PSU breakout/adapters - Re: Cambridge Workstation woes - VMS 2691 PSU smoke
On 2017-07-30 12:18 PM, Jules Richardson via cctalk wrote: On 07/30/2017 10:59 AM, Ian Frost via cctalk wrote: Any body any experience in fixing these old Acorn PSUs - or managed to source alternative modern supplies? There wasn't anything "special" about them, in that the outputs were typical for "small computer with a hard disk" (i.e. +5V and +12V at a few amps, -5V and possibly -12V, main regulation done on the +5V rail, "hard start" via physical switch). I expect it's possible to find a modern PC PSU in some kind of slimline form-factor which will physically fit (and rig the soft-start so that it powers up as soon as AC is applied). FWIW I recently discovered this little PCB which looks darn useful for power control + breakout (ignore the RPi part). https://hackaday.io/project/20963-mini-atx-psu https://www.tindie.com/products/tomtibbetts/piryte-mini-atx-psu-for-raspberry-pi/ > I expect the current limits for the different rails are printed on the side of the VMS PSU's frame, so as-good-or-better should be fine. ... cheers Jules
Re: Cambridge Workstation woes - VMS 2691 PSU smoke
The PSUs in the Acorn BBC Micros are we'll know for blowing caps. Replacing caps is a standard thing when bring any BBC Micro back to life. The StarDot forum is a good place to ask specifics about a Cambridge Workstation - http://www.stardot.org.uk/forums/ On 30 July 2017 at 17:28, Tony Duell via cctalk wrote: > On Sun, Jul 30, 2017 at 5:18 PM, Jules Richardson via cctalk > wrote: > > On 07/30/2017 10:59 AM, Ian Frost via cctalk wrote: > >> > >> Any body any experience in fixing these old Acorn PSUs - or managed to > >> source alternative modern supplies? > > In general smoke from a power supply that keeps on working is from the > mains filter capacitors. Some makes are well-known for this. I would > carefully examine them, they crack when they fail > > There will be class X capacitors between live and neutral and class Y > between > live and earth and neutral and earth, The former are the ones that > normally fail, > but I would change the lot. > > > Unfortunately I did trace out some, possibly all, of that PSU (most > likely, > > it was certainly a VMS-something in my ACW) years ago, but even if I do > > still have the schematics, they're on the other side of the Atlantic. The > > PSU in mine had the rectifier diode on the +12V rail go intermittent, > so it > > would start sometimes and not others. > > > > Tony D. might have schematics, but I have a feeling they weren't part of > the > > ACW service manual as the PSU was bought in from an external supplier and > > not an Acorn part. > > I certainly traced out the schematics of mine, but I forget which PSU was > fitted to my machine (something tells me it was made by Farnell). I can > try to find them. > > -tony > -- 4.4 > 5.4
Re: Cambridge Workstation woes - VMS 2691 PSU smoke
On Sun, Jul 30, 2017 at 5:18 PM, Jules Richardson via cctalk wrote: > On 07/30/2017 10:59 AM, Ian Frost via cctalk wrote: >> >> Any body any experience in fixing these old Acorn PSUs - or managed to >> source alternative modern supplies? In general smoke from a power supply that keeps on working is from the mains filter capacitors. Some makes are well-known for this. I would carefully examine them, they crack when they fail There will be class X capacitors between live and neutral and class Y between live and earth and neutral and earth, The former are the ones that normally fail, but I would change the lot. > Unfortunately I did trace out some, possibly all, of that PSU (most likely, > it was certainly a VMS-something in my ACW) years ago, but even if I do > still have the schematics, they're on the other side of the Atlantic. The > PSU in mine had the rectifier diode on the +12V rail go intermittent, so it > would start sometimes and not others. > > Tony D. might have schematics, but I have a feeling they weren't part of the > ACW service manual as the PSU was bought in from an external supplier and > not an Acorn part. I certainly traced out the schematics of mine, but I forget which PSU was fitted to my machine (something tells me it was made by Farnell). I can try to find them. -tony
Re: Cambridge Workstation woes - VMS 2691 PSU smoke
On 07/30/2017 10:59 AM, Ian Frost via cctalk wrote: Any body any experience in fixing these old Acorn PSUs - or managed to source alternative modern supplies? There wasn't anything "special" about them, in that the outputs were typical for "small computer with a hard disk" (i.e. +5V and +12V at a few amps, -5V and possibly -12V, main regulation done on the +5V rail, "hard start" via physical switch). I expect it's possible to find a modern PC PSU in some kind of slimline form-factor which will physically fit (and rig the soft-start so that it powers up as soon as AC is applied). I expect the current limits for the different rails are printed on the side of the VMS PSU's frame, so as-good-or-better should be fine. Unfortunately I did trace out some, possibly all, of that PSU (most likely, it was certainly a VMS-something in my ACW) years ago, but even if I do still have the schematics, they're on the other side of the Atlantic. The PSU in mine had the rectifier diode on the +12V rail go intermittent, so it would start sometimes and not others. Tony D. might have schematics, but I have a feeling they weren't part of the ACW service manual as the PSU was bought in from an external supplier and not an Acorn part. cheers Jules
Cambridge Workstation woes - VMS 2691 PSU smoke
Hi all I try and boot up most of the working systems I’ve got at least 2-3 times a year, and on a recent boot up of a Cambridge Workstation, I got the dreaded smoke. The system works fine, but the PSU is clearly stressed. The PSU is a ‘VMS 2691A’, made in the UK, and it looks like the problem is around a large / ceramic 22K resistor. There is a fair amount of brown leakage on the PCB too. Any body any experience in fixing these old Acorn PSUs - or managed to source alternative modern supplies? Thanks Ian
