2016-01-22 21:48 GMT+01:00 js at cimmeri.com <js at cimmeri.com>: Well. It depends if you read the schematics before you do so. In this case I powered the startup voltage from a 12 V bench supply. Normally the startup voltage comes from a small mains transformer and a 7812. Then I supplied the primary side voltage for the main switch transistor using a variac. Works perfectly well if you know what you are doing. Have done this practice with many SMPS supplies.

Yes of course. If I have a PSU where I have doubts about the function and there is no schematic I spend a few hours trace out the relevant portions. Normally this is not a big chore. Then there are of course complex PSU and simple ones. I would not like the idea of tracing out the VAX-11/750 PSU, but a VT100 PSU is very small circuit on the primary side. OK. No I am not using onIy a variac. That wouldn't work very well. I usually try to identify the control circuit and power it independently of the switch circuit. That is usually no problem. Some PSU has a simple voltage divider from mains input other like the VT100 has a normal small iron core transformer to supply the control circuit supply. Many of the PSUs I work with have been not been powered up for decades. I think it is a good practice to take it slowly in the beginning. Observing it so that everything behaves well in the circuit using a very light load. Verify that the switch control circuit and regulation circuit behaves well and that the switching transistor waveforms looks good. Nothing is getting abnormally hot or so. Also be a little bit kind to all the electrolytics in the circuit etc. That is why I do this and I think that this method have served me well. Then of course I also use a protection transformer and very often some kind of input current limitation, not just a variac and a bench supply. /Mattis

I for one think it is a good practice to start carefully. I like the idea of bringing up the voltage over capacitors and other electronics in a controlled manner. Electronics that had been sleeping for decades. I use a protection transformer, variac and also often means to limit the current. Then I could easily measure how the switching transistor behaves. The base/gate drive etc. All this depend on what load your are using. When running on the bench with variac I have a very modest load. In this case 100 mA at full 5V. Just 1 % of rated output. In that case there is very unlikely to overload the switch transistor. That was the case for the VAX-11/750 supply which didn't enable switching under low main input conditions. Something I deliberately disabled to be able to test. In the case of the VAX-11/750 PSU I was able observe and detect a number of failures in the supply at low and non-harmful voltages. Regardless of protection transformer or not I don't like the idea of working with a PSU with 300VDC everywhere when probing with the scope probe. But "safe" depend on the load applied to the supply. On the other hand my question to the list was not if the use of variac is a good practice or not. I will continue to use this method since it has served me well and I am not forcing any one else to use it if they feel it is wrong. The question I have is why R27 in the snubber network is getting what I think excessively hot. The schematic for the primary side can be found here: http://i.imgur.com/VlInF90l.png One input that I had that if C19 is marginally bad then that might happen. Like if the dielectric as deteriorated over time and cannot whit-stand the voltage in the circuit. The capacitor is not cracked like the RIFA ones. It looks perfectly fine. It is a SPRAGUE 0.033uF 1600VDC. I don't have an capacitance / ESR meter so I cannot check it. Maybe I should just go ahead replacing it. But I don't like idea of replacing things without really knowing they are bad. Another question is whether the R27 is normally getting hot or not. What is VT100 owners experience here? /Mattis

On Sat, 23 Jan 2016, Mattis Lind wrote: They can. The translucent yellow Rifa (now owned by Kemet) class X/Y safety capacitors in particular have a 100% failure rate and are on my replace-on-sight list. They usually begin to show visible signs of cracking in their outer casing before they finally go out with a bang. It seems to be due to chemical decomposition of the paper insulator which swells over time and causes the case to crack. I usually replace them with a film capacitor from Epcos of the same value and safety class.

On 1/23/2016 5:40 AM, Mattis Lind wrote: Not highly likely, but possible. Just put another one in temporarily and see if it solves your problem. Or try to measure what kind of current is causing the resistor to heat up -- is there AC across it, or is that cap leaking? - J.

On 01/22/2016 06:26 PM, Tothwolf wrote: It's a well-known fault, but has anyone ever known one fail and actually cause any damage (other than to itself)? AIUI, they're there to reduce noise from the device leaking back out onto the AC supply - a system should run quite happily without them. I've had maybe four or five fail on me over the years, out of several hundred systems. They're a definite weak spot, but given that I've never heard of one damaging anything it's the sort of thing I'd consider doing only if I was carrying out some other repair work on the PSU. Is the voltage rating on the US ones different (new vs. old)? I can't remember now. I know all the old UK ones seemed to be 250V, while modern parts were rated at 275V. cheers Jules

On Sat, 23 Jan 2016, tony duell wrote: Actually, yes. I had one fail in one of my scopes. I don't recall the brand/series right now, but it is apparently a known issue with that particular series of filters. I still need to replace the same filters in the other scopes since they will most like do the same thing eventually. Most of the IEC320 input filters I've used have a schematic on the can or in the datasheet. They are usually also careful to denote the medical-rated filters which lack the Y-type safety capacitors between the two supply terminals and ground.

On 01/23/2016 01:26 AM, Tothwolf wrote: I have heard similar from a few other places. And I'm replacing a five of these in a bigger SGI PSU as preventive maintenance. A few of them are showing cracks. All I could find were new similar Kemet capacitors. Do you know if newer Kemet suffer from the same problem and I will regret buying them? Thanks, Pontus.

On Sat, 23 Jan 2016, Chuck Guzis wrote: Kemet bought out Rifa and the parts from Kemet with the same P/N also use a paper insulator. Rifa used to tout the paper insulator as a selling point, but after seeing so many fail, I won't touch them, new manufacture or not. I'm using Epcos brand polyester film safety capacitors as replacements. So far I've been able to match them up 1:1 with the correct physical sizes (lead spacing) and values.