HP Deskjet SMPS fails under load?

[xyberlink]

I have a faulty SMPS from a HP Deskjet 960c (I know it's old!). Everything was fine, then after printing a few pages one morning, the printer just cut out, and now only turns on for a couple of seconds, if at all.

ASTEC AA21160, HP Part C6455-60009.
Input: 100-240v ~ 50/60Hz 0.9A
Output: 32v DC 30w

The main power transistor is a K2545
The controller (AS998N) looks to be obsolete.

I've set it up in a controlled environment, series light bulb (40w) on the input and electronic load on the output.

If I attempt to draw 200mA, all appears well.
If I attempt to draw 300mA, all appears well.
Somewhere around 320 - 330mA, it cuts out, i.e. at roughly 1/3 it's rated output.

I've been through all the components, i.e. diodes, capacitors, resistors, transistors etc. as best I can and everything seems ok. There are various SMD resistors and other bits which are tricky to get accurate readings from in circuit, but I've not spotted any shorts, opens or unrealistic values. Fuses remain good.

When it's in the printer and the PSU shuts down, the load obviously disconnects.
When using the electronic load, the load remains in place. In this scenario I'm not 100% sure if it shut down or went into a shorted condition on the input at the series light bulb appeared to stay on (I turned it off fairly sharpish, worried something may blow, causing more problems).

So it seems that something is failing under load. I have fixed various SMPS's in the past, but the issue is normally more obvious.

Any ideas on what may cause this scenario greatly appreciated.

Thanks.

[coromonadalix]

I would change the 3 smallest capacitors, one of them maybe the supply for the smps controller and other for the feedback dc lines ...
with time and aging the load line and regulation may be affected or diminished ?  The printer load is not an constant one, it has current peaks while operating

[drussell]

I have a faulty SMPS from a HP Deskjet 960c (I know it's old!).

That series are great printers.  Who cares how old it is?    Plus, they're long before today's "intelligent ink cartridges" that make refilling cartridges a problematic PITA.

I have three of the large-format 1120C version that I still use for printing on 11x17 (the paper tray takes up to 13x19) which is very handy.  I also have at least one or two 960C units around here somewhere that I kept around, mainly in case I need parts for the 1120Cs. 

They probably all use that same power supply so if you need me to probe anything for a comparison reference to help troubleshoot, let me know.

As to exactly what might be wrong, that is a bit of a strange failure mode to be working fine and then just quit to be a capacitor failure.  Your load test stopping at about 1/3 power with a 40 watt bulb in series makes sense, though.  Did you try your load test without the dim-bulb in series?  Your problem might not be the PSU after all but rather something in the printer electronics themselves.

[xyberlink]

Forgot to mention in the first post that the printer does run. I've had it hooked up to a bench power supply and also to a brick type SMPS that I happen to have and it prints fine.

When using the bench power supply, I noted that the current use was typically <1A, though it did flick to ~1.1A on occasion, which is technically over spec for the official PSU, though I'd assumed within tolerance, especially for momentary current draw. Either that or it's just limited by the official PSU.

I've re-tested the original PSU, this time with a 100w bulb in series and managed to draw 900mA from it without shutdown. Following that I've had it back in the printer and it's working there two, hence it's possibly in a good mood today rather than bad mood it was in previously.

I'm now assuming that some component is flaky, sometimes working, sometimes not, though obviously the plot has thickened somewhat with it choosing to work today.

As a side note, I also have a HP DeskJet 970Cxi and a 940c, which despite looking identical, have a PSU rated at 18v output, hence not swappable.