[synthetix]

I've got two Indigo2 IMPACT power supplies. One is good, one bad. I've replaced the capacitors on the bad one, but that didn't fix it. The IMPACT power supply is split into two halves, low voltage and high voltage. As a troubleshooting exercise, what's the thought on splitting them both in half and recombining them to figure out which half of the bad one is the offending side? If I can confirm which side of the bad supply is the bad side, that would really speed up the repair process. Am I asking for trouble with this approach?

[weblacky]

After having a few tough PSU repairs recently (not Indigo2 however) I've found that once a PSU stops working, it's already blown a semiconductor (DIODE, FET, etc..). However I've also seen a few designs where large resistors used to limit current go out of spec and choke off the process on the HV side. However an OLD PSU that hasn't been started and just recapped after sitting for 7+ years on a shelf, often works right away...the moment someone decides to "try" an old PSU...they then have actually damaged it.

But a simple Multimeter WON'T tell you anything on many of these unless you plan on removing components and testing them outside the circuit. However I'd bet fair money that you'll likely do a bunch of hacking and reattaching to find...the new (combined) PSU doesn't work well and that's because the VAST majority of the time it's the LV side that fails but the HV side is the largest filtering component for preventing the passing of ripple and spikes into the entire PSU. So combining an OLD HV from a non-working psu to the LV of an OLD (not rebuilt) PSU is going to basically beat the sh*t out of it while it's on and may work for awhile but will accelerate the demise of the working LV side you hacked together.

While that's not 100% true, my personal experience has been it's been true 8 out of 10 times (if it's not a resistor spec issue). The LV side has ALL the high speed recovery diodes and the output caps, those take a beating and often are the failure points.

I think you'd be better to do some research/learning and carefully take out the diodes and measure them with a multimeter and carefully desold and pry up on end of all the resistors and check those against marked spec.

The best tool is a current-limiting Incandescent bulb setup (you'll need a 300W bulb or two 150W bulbs to allow a 500W start on a PSU) so you can SEE an indicator of PSU startup current draw. I recently discovered a rebuilt PSU (I rebuilt) appeared dead from just no fan or power but throbbed/blinked the current limited bulb setup I used and that told me the PSU was actually rapidly restarting! This shows the PSU is in fact alive but has a short in the secondary LV side and the brain was protecting it by trapping the PSU in restart as an OCP. That one turned out to be two bad diodes on the LV side!

I'm not telling you what you should do here, only my experience in terms of putting in the effort and what you're likely to find. Also given the IMPACT PSUs have huge soldered cables, any hack you do to reattach them OTHER than a solder bath or 300W iron will permanently alter/damage the large ribbon conductors. You'll never really get them solidly attached without special equipment (unless you have that).

There isn't many components on a I2 PSU, I'd just start desoldering and check things on the LV side and reinstalling them.

But even if you do this...you cannot "buy" an LV side as a replacement board like a modern TV set. It's one product, split across two boards. So given the amount of hackery, the knowledge of which board it is, isn't worth it. There are MUCH easier ways of telling which board it is then the work it takes to reattach those ribbon cables.

Also when you say you replaced the caps, did you replace ALL electrolytics or only the output caps? Many people get bad advice and don't change them all (even the pricey caps), this is big mistake. Also replacing those old caps with modern LOW-ESR caps is another HUGE mistake, the caps you use should be close in ESR to the vintage caps spec. PSUs are basically giant, high speed, variable oscillators. A simple LC circuit in parts. You can cause problems with that oscillation by changing the spec of the caps used in some parts (depending on the design). So check the ESR, Ripple tolerance, lifespan and temp tolerance, and dimensions when finding a suitable replacement caps.

While old filter caps won't prevent startup they will KILL the mainboard in no time and the PSU won't know it's happening until after you smell the smoke!

I bought an Indy someone burned this way, previous owner turned on the PSU and Indy started smoking WHILE BOOTING, but still booted! Because the PSU was so far out of tolerance that what came out of the PSU wasn't electricity the Indy could handle and it's old parts basically starting dying in the tidal wave of power.

I guess what I'm saying is, don't see the PSU as two boards, treat it holistically as one unit.

[synthetix]

There isn't many components on a I2 PSU, I'd just start desoldering and check things on the LV side and reinstalling them.

Thanks for the advice. I think that's what I'll do. I'll just start from the top and check everything. I have a desoldering station, so it should be a pretty straightforward process. A long one, of course, but straightforward nonetheless.

[weblacky]

Check MOSFETs first (desolder, diode check, reinstall) then do resistors and secondary diodes next, they are the things that go first. Go from most to least likely.