RE: H7878 Fails Under Even Moderate Load
> -Original Message- > From: Maciej W. Rozycki [mailto:ma...@linux-mips.org] > Sent: 01 October 2017 20:28 > To: r...@jarratt.me.uk; Rob Jarratt <robert.jarr...@ntlworld.com>; General > Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> > Cc: 'shad' <shado...@gmail.com> > Subject: RE: H7878 Fails Under Even Moderate Load > > On Sun, 1 Oct 2017, Rob Jarratt via cctalk wrote: > > > Just to let everyone know, I have now fixed this PSU. The final write > > up is here: > > > > https://robs-old-computers.com/2017/10/01/decstation-5000-model-240-ps > > u-fixed/ > > > > Thanks everyone for all the help. > > Great, congrats! I see many failed Chemi-Con SXE parts on your photos and > these do require replacement, bad or good, as they are among the ones that > suffer from the quaternary ammonium salt issue. > I don't think *all* of them failed. Some definitely, others I wasn't sure but seemed reasonable candidates. I have kept them all. Regards Rob
RE: H7878 Fails Under Even Moderate Load
On Sun, 1 Oct 2017, Rob Jarratt via cctalk wrote: > Just to let everyone know, I have now fixed this PSU. The final write up > is here: > > https://robs-old-computers.com/2017/10/01/decstation-5000-model-240-psu-fixed/ > > Thanks everyone for all the help. Great, congrats! I see many failed Chemi-Con SXE parts on your photos and these do require replacement, bad or good, as they are among the ones that suffer from the quaternary ammonium salt issue. Maciej
RE: H7878 Fails Under Even Moderate Load
Just to let everyone know, I have now fixed this PSU. The final write up is here: https://robs-old-computers.com/2017/10/01/decstation-5000-model-240-psu-fixed/ Thanks everyone for all the help. Now back to the DECstation 220, a ZX Spectrum, and then the daunting task of fixing the TURBOchannel Extender I got for this thing. Regards Rob
RE: H7878 Fails Under Even Moderate Load
Thanks for that. The slow start one sounds like an interesting avenue to explore. I have been told some of the capacitors on these tend to be bad. Finding the capacitor (if it does indeed do a slow start) will be a challenge for me. I have been reverse engineering the schematic a bit, but some of the parts are hard to see as they are hidden by a heatsink which is not easily removed... In doing the reverse engineering I have spotted a 10R resistor near the output section that is relatively high power that seems to have some physical damage (it is one of the hard to see components). It is marked as 10R and measures 10R (in circuit), but measures 0V across it when the PSU is powered up. I may remove it to check it over. Regards Rob > -Original Message- > From: cctech [mailto:cctech-boun...@classiccmp.org] On Behalf Of shad > via cctech > Sent: 18 September 2017 18:42 > To: cct...@classiccmp.org > Subject: Re: H7878 Fails Under Even Moderate Load > > hello, > well it's difficult without the piece in hands to understand the circuit, and > without the schematic, but I could give a try. > > 1) > On a switching PSU, only one output is really stabilized, here it seems 5v. > The other outputs are "unregulated", but in some way related to the stabilized > one, for example via transformer windings ratio. > However, to protect circuits, some voltage range or current range detectors > are added, to avoid these outputs to go outside acceptable values. Maybe > something in these circuits on secondary section detects a false condition of > error, cutting off supply current on primary section. > > 2) > voltage seems to go high too slowly, as if a soft start is going too slow. > Normally SS is done charging via a resistor a capacitor on the feedback, > affecting maximum current on the primary. > If the capacitor is leaking current, it will charge too slowly, and will > never reach > maximum voltage required, so basically you will never have maximum current > in the output. > This could be a small capacitor charged by a done kohm resistor. > > Andrea
Re: H7878 Fails Under Even Moderate Load
hello, well it's difficult without the piece in hands to understand the circuit, and without the schematic, but I could give a try. 1) On a switching PSU, only one output is really stabilized, here it seems 5v. The other outputs are "unregulated", but in some way related to the stabilized one, for example via transformer windings ratio. However, to protect circuits, some voltage range or current range detectors are added, to avoid these outputs to go outside acceptable values. Maybe something in these circuits on secondary section detects a false condition of error, cutting off supply current on primary section. 2) voltage seems to go high too slowly, as if a soft start is going too slow. Normally SS is done charging via a resistor a capacitor on the feedback, affecting maximum current on the primary. If the capacitor is leaking current, it will charge too slowly, and will never reach maximum voltage required, so basically you will never have maximum current in the output. This could be a small capacitor charged by a done kohm resistor. Andrea
RE: H7878 Fails Under Even Moderate Load
On Sun, 17 Sep 2017, Rob Jarratt via cctalk wrote: > Many thanks for all the suggestions. I will go through them all, although it > may take me a while > > On your last point, at first I thought that there isn't any feedback from > the machine. However, apart from a DC OK (which will be an output), there is > a wire labelled "Warning", which could possibly be an input, so I will look > at that too. That's pretty surely the PSU overheating status sent to the computer -- readable as the CSR register's PSWARN bit (bit #27) in the 5000/200 or signalled as the IOASIC interrupt (SIR register's bit) #4 in the 5000/240 and /260. Maciej
RE: H7878 Fails Under Even Moderate Load
Many thanks for all the suggestions. I will go through them all, although it may take me a while On your last point, at first I thought that there isn't any feedback from the machine. However, apart from a DC OK (which will be an output), there is a wire labelled "Warning", which could possibly be an input, so I will look at that too. Regards Rob > -Original Message- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Peter > Coghlan via cctalk > Sent: 17 September 2017 21:26 > To: General Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> > Subject: Re: H7878 Fails Under Even Moderate Load > > > > > Can anyone explain the behaviour? > > > > It's hard to know what to do from a distance but here's what I think I'd look at if > I was faced with this problem. > > I think failing under moderate load could be explained by one or more of the > following possibilities and probably others I haven't thought of: > > - The power supply is not capable of producing sufficient current. Check how > the voltage across the main input smoothing capacitors which have been > replaced varies while the load is applied. If it dips severely, check input > components such as filters, surge limiting devices, connectors and so on for > breaking down under load. If you can measure the ripple here while changing > the load, an increase may indicate that one side of a fullwave rectifier is > going high impedance or open circuit under load. Also check for damage that > might have occurred in the struggle to remove the capacitors. > > - Overcurrent sensing is kicking in too soon. Look for low value, moderate to > high power resistors in the output current paths and check their values and > how the voltage across them varies with applied load. If they seem good, > check associated small components. > > - Regulation is not working correctly. Try to figure out how the regulation > is supposed to work and take measurements to see how it is behaving in > reality. Easy to say but may be difficult to do in practice. If the PSU > uses a chip to provide regulation and drive to a chopper device, the > data sheet for the chip may provide some guidance on how it is supposed > to work. Be careful taking measurements as accidentally shorting something > out could lead to big bangs. > > - The PSU may be looking for feedback from other parts of the machine in the > form of remote voltage sensing or remote current sensing or inputs which > cause particular supply lines to be switched on or off or come up in a > particular order. If this is the case, the fault may be elsewhere in the > machine or may be as result of operating the power supply without it being > connected to the rest of the machine. > > Hope this helps. > > Regards, > Peter Coghlan.
Re: H7878 Fails Under Even Moderate Load
> > Can anyone explain the behaviour? > It's hard to know what to do from a distance but here's what I think I'd look at if I was faced with this problem. I think failing under moderate load could be explained by one or more of the following possibilities and probably others I haven't thought of: - The power supply is not capable of producing sufficient current. Check how the voltage across the main input smoothing capacitors which have been replaced varies while the load is applied. If it dips severely, check input components such as filters, surge limiting devices, connectors and so on for breaking down under load. If you can measure the ripple here while changing the load, an increase may indicate that one side of a fullwave rectifier is going high impedance or open circuit under load. Also check for damage that might have occurred in the struggle to remove the capacitors. - Overcurrent sensing is kicking in too soon. Look for low value, moderate to high power resistors in the output current paths and check their values and how the voltage across them varies with applied load. If they seem good, check associated small components. - Regulation is not working correctly. Try to figure out how the regulation is supposed to work and take measurements to see how it is behaving in reality. Easy to say but may be difficult to do in practice. If the PSU uses a chip to provide regulation and drive to a chopper device, the data sheet for the chip may provide some guidance on how it is supposed to work. Be careful taking measurements as accidentally shorting something out could lead to big bangs. - The PSU may be looking for feedback from other parts of the machine in the form of remote voltage sensing or remote current sensing or inputs which cause particular supply lines to be switched on or off or come up in a particular order. If this is the case, the fault may be elsewhere in the machine or may be as result of operating the power supply without it being connected to the rest of the machine. Hope this helps. Regards, Peter Coghlan.
