Older 1040STF Inductor(?) Issue

[Chewbode]

I bought an inexpensive 1040STF on eBay and am doing a bunch of work to get it back to working order. A previous owner (tried to) install a Tech-Specialties 1040A RAM expansion board. When they did this, traces and resistors were cut. I've fixed the traces with bodges and re-soldered the resistors, now I'm trying to get the machine to boot.

When I turn it on, I get a high-pitched whine from the PSU (crappy old Tokin 4501U1 that I want to replace). This whine quickly faces out and I then get some magic smoke from an inductor on the motherboard. It's marked as L1 and is located on the far left of the board near the RAM.

inductor.jpeg

I'm not sure how to troubleshoot this, and since this board is old (C070523-001 REV.C), I've not found a schematic for it, so I'm not sure what it does or what I should replace it with.

Any advice is greatly appreciated. TIA!

[BigSteve]

You would need to meter it out but I suspect this is a filter on the 5V rail and would suggest a short/near short. If it is - then your L1 would be L51 on a C070789-001 rev board. 10uH at 10%. Check filter caps on/near supply rail. You should get ~0 increasing to ~100ohms when measuring an isolated board between 5v and Gnd with a digital multimeter.

[frank.lukas]

The inductance and the 470uF capacitor feed the 5V voltage for the dram banks from the mainboard. So the memory only draws too many amps.

See the 1040ST Schematics ...

[rubber_jonnie]

First thing to do if you seem to have a short or something other wrong, then go back and review any work you've done with a magnifying glass or microscope to look for solder bridges, which could even be something as small as a solder whisker.

Otherwise, I'd agree, you need to meter out the 5v rail in continuity mode and try to find out where the short is. One probe on GND and then work through the other components looking for shorts.

Do you have a bench PSU? You could try injecting 1v and look at the amps drawn, and check to see what is getting warm, other than the inductor.

You may ultimately need to replace the inductor as it may have become hot enough to be damaged, but you might be lucky if it wasn't run for too long.

[Chewbode]

Thanks for the tips on this. I'll start testing the caps and see if I can narrow this down. I'll also review the bodges as @rubber_jonnie suggests. I'm pretty sure they're good, but another review is always a smart tactic.

I do have a bench PSU. I'll try using that which will also help rule out any weirdness with the junk Tokin PSU in the computer.

I'm a bit slammed with projects the next couple of days, but will update you all by the end of the week at the latest. Thanks again!

[BigSteve]

I do have a bench PSU. I'll try using that which will also help rule out any weirdness with the junk Tokin PSU in the computer.

The ST can do most things with just 5 volts. Croc clips on the biggest axial cap works for me.

[Chewbode]

Well, I've tested my bodges I put in place to fix the broken RAM traces, and there's no apparent short there.

Here's some interesting data I've been able to produce:

I started by testing the Atari PSU separately. After plugging it into mains, it's properly generating 5V (5.22V) and 12V (12.63V) at the 6-pin connector that plugs into the MB. That's a good sign the PSU is ok.

When I plug the Atari PSU into the MB, I get a 13.5kHz whine from the PSU (PSU is silent until plugged into MB). If I meter the 5V rail, I see 1.8V 5.05A. If I disconnect the Atari PSU from the MB and power the Atari PSU's 5V pin from my bench PSU, it looks good (5V 0.11A), until I plug the Atari PSU into the MB. It then drops to 1.5V 5.05A.

So, I think the next step is to re-cap the board. I see no evidence of issues, but it just makes sense to get this out of the way.

I'll keep you all in the loop.

[Steve]

So you've got some seriously massive short going on. I think you should identify where the short is rather than just recapping the whole board. When you say you see no evidence of issues, the voltage drop and high amp pull means a short to ground, that is evidence of a rather significant issue!

[rubber_jonnie]

Unless you have proof positive that it is a shorted cap, which it certainly could be, I wouldn't go changing any caps.

You clearly have a huge 5v short, but your time would be better spent working through the 5v rail and using your multimeter to figure out where the short actually is.

You may change all the caps and not get any further ahead, which would be a bit of a downer. Better to test logically and thoroughly to work out where the problem is.

I'd start by removing the inductor at one end to see if it's gone. It's easy to do, just lift one end. It is possible it has an internal short.

Look closely all over the board with a magnifier or microscope and look for shorts where you have worked, then in other areas.

If there are ICs in sockets, remove them one at a time and see if the short is gone, if not then work through the rest.