IIgs Vulcan repair adventure

I posted some time ago about my Vulcan I’ve been fighting with. Posting again in hopes to find some more advice as I have made some progress.

The problem before was a result of not paying attention to cap impedance values. I bought replacements that matched as closely as possible to the originals, and initially had some success. I had it mostly disassembled with the hard drive not connected. When plugged in, system booted up fine.

Powered off, fully assembled, tested again. PSU started whining and popped a fuse.

Disassembled again to check for what that might have been. Didn’t see much so I assembled step by step and tested each time as I went along. The last step until it failed was when I connected the hard drive. I had done some past testing with a friend and had fiddled with the trim pot, so maybe that was out of spec. Same friend recommended to connect it to the system and probe the voltages on the drive power cable. I was reading the values, but it started to sputter, and now has hit a point where it no longer fully works. 5v reads 3.8v, and it won’t boot. Adjusting the trim pot does not have any effect.

It was odd seeing it basically falling apart as I was trying to read the voltages, but otherwise not changing anything. I feel I was taking a pretty safe approach: power off, connect probes, power on, read, power off, move and repeat.

Reaching out in hopes that someone has some experience restoring these PSUs. I’d like to get this working instead of just shoving in a modern replacement.

For testing you need the PSU under load, check the HD is working OK on another system and ensure it's running on the bench. When I say working, if the drive heads are stuck or the drive has "crashed", it might cause undue strain on the PSU so make sure it works OK.

Noting the fuses blowing, check for any RIFA or mains filtering caps on the PSU too. Have you matched the caps with correct voltage and microfarad rating (aka impedance) now or are they still out of spec? You can go the same voltage or higher uF rating (for PSUs I'd try to get close).

JB

For a load, I have the whole IIgs system with multiple cards installed. Tomorrow I’m going to give it a shot with the cards removed in case that produces a different result. And, the PSU did initially work multiple times without the drive in.

I’ll test the drive power tomorrow on an external molex power adapter. Even so I don’t intend to use the original drive, even if it is working.

As for the fuse popping, I included it just to give full detail but it only happened once; it has not happened again. During my chat with my very helpful friend I guessed that maybe the metal part of a large power transistor was touching the metal housing as the thermally coupling / insulating pad wasn’t really in place. He did say in some situations that can cause a short, but wasn’t sure. There’s little info about these online anyway. It’s just a loose pad so if that is actually to electrically insulate it from the housing it’s a real weak solution since there’s nothing other than friction to hold it in place.

I had in the past done an initial recap only matching voltage and microfarad ratings and it did not work. After some digging I found a blog post online about how the impedance (or was it the ESR) NEEDED to match, so I rechecked the parts I bought against the originals, and bought fully compatible replacements. Once that was done, again, it did initially work just fine.

Ok, some progress.

Removing almost all the cards (removing the Transwarp is a bit more involved than I’d like to be doing repeatedly) the system boots fine. 5v line is at about 4.5v. Moving the pot too high in one direction causes instability and I can hear some electric “sputtering” coming from the PSU. I don’t know exactly what that pot is controlling either. Anyone?

The drive will not even spin up when powered externally. It beeps for a short bit, presumably trying to spin up but just stays silent. Now, I didn’t connect it to any controller. I assume it would spin up even if not connected to anything requesting data, but let me know if I should dig up some more parts to do that as well. I got this idea from eBay; there are two bare Vulcan PSU boards on there from the same seller, stating the same behavior: works when the IIgs is empty, does not when there are cards installed. Maybe it’s a somewhat common issue, some component aside from the caps on the board is going bad.

Those two tell me that the PSU does not like being loaded too much. What that means from a troubleshooting perspective, well I’m hoping again for some sage advice here on what next I can look into.

I’m at least satisfied that there’s progress in the right direction.

I know very little specific about IIGSes, but +4.5V is still out-of-spec low for plenty of ordinary +5V logic; take this 74LS374 datasheet for example. +4.75V is the minimum listed. Or, to go with some ICs found in the IIGS:

65C816: +4.75V when running in a +5V system (PDF page 25, very top of Table 4.2)
74HCT251: +4.5V minimum recommended supply voltage (PDF page 5)
74HCT245: same (PDF page 4)
8530 SCC: same (PDF page 47)
74LS02: +4.75V (PDF page 4)
74LS74: same (PDF page 5)

And so on. So, if your +5V rail is really sagging to +4.5V, you're not providing enough voltage to some ICs, and several others are right on the margin. I don't know where you're measuring voltages, but I would double-check on the power-supply pins of various ICs on the mainboard (be extremely careful to avoid shorting power pins to any other pins), as if you're measuring on some more distal component (e.g. the floppy port), perhaps connector pin tarnish is a factor.

I would continue investigating power. Besides a PSU fault, is it possible that another component inside your computer might be shorted or conducting much more current than normal? If you aren't one of those lucky folks with one of those handy IR cameras, then finger check: is anything getting hot?

I’ve been measuring off the molex drive power connector out the Vulcan PSU. I’ll next investigate voltages on the board itself, see if I get any dramatically different results.

I don’t expect much of a discrepancy, or at least don’t feel too bad about measuring off the connector that I’m using; it’s not as if it has its own rail or juts off the board with its own set of components; it’s just a branch off the same outbound power cable from the PSU.

I’ll finger check some chips. Would there be any use in checking side-by-side voltages with a recapped original PSU? I have that off to the side, ready for use as needed.

As for IR camera, no, but I have been looking for an excuse to buy one! Perhaps I’ll check around.

If it's right off the PSU and you have a good connection, then I would not hold out much hope for higher voltages on the board. Still, it's worth checking just in case there's something we've missed.

Since you have an original PSU, is it worth seeing whether the IIGS will boot with that instead of this Vulcan PSU? I gather from searching that this means not providing power to a hard drive somewhere. If you can do an apples-to-apples comparison with both power supplies in terms of the loads they're powering, and if the machine works with the original PSU, then you can have a pretty high confidence that it's the Vulcan PSU that's the culprit.

But if the original PSU won't work either, then it seems more likely that some component is consuming a lot of current and dragging down +5V. This means it should be getting hot. It might not be an IC; one of the passives like a capacitor may have shorted. Then again, a small component will be moving a lot of current across a little space in order to make a rail sag, and it might start to look toasty if it's drawing amps.

Overall, if your logic isn't getting voltages in spec, then abnormal behaviour and brokenness may be expected.

Ah, sorry. Should have started this by saying this is a known good IIgs. Working when I got it but recapped the PSU anyway, and then after bought the Vulcan. A friend also ran an additional cable on the board, apparently this board revision has a bit of an issue with a voltage trace being poorly designed. He’s got a literal pallet or more of IIgs’s so I trust his work unconditionally. I’m happy to check the IIgs itself but I don’t suspect it really at all considering those facts.

OK, seems very likely not the computer then, no need to check for hot components on the logic board. (And finger-checking for hot items inside a PSU is a pretty bad idea, not that you were considering doing that!) Good luck fixing the PSU, the problem seems like it must be there.

Yes, that’s the aim but I’m stuck with what next to check. I haven’t the experience to troubleshoot a PSU beyond the basics that I’ve covered here. Let’s hope someone can come to my rescue!

Did you install the replacement polarized caps with the correct orientation?

A good question, I’ve installed caps backwards before.

The board is pretty well marked, but I’ll double check against a schematic available online tomorrow to be certain they are installed correctly.

I’d generally expect a cap backwards to just pop, though, unless it’s a particularly large cap. Would a backwards cap cause (what appears to be) poor current performance?

The caps might not pop immediately, but an out of spec capacitance caused by reversed installation could definitely cause problems with regulation.

So, caps are all the right way around.
But, in checking, I managed to notice a mistake I made a while ago; Caps C4 and C5, for some silly reason I put in 100uf 250v. They need to be 150uf.

Can't believe I missed something so basic! But at least there's something clear about a next step. Thank you!

Hmmm, scratch that. There's different values for different board revisions.
I can't seem to definitively find my board revision. I have two 4 digit codes on the board, which don't line up to the list on appleiioz.com.
Even so, I did keep the original caps, and C4 and C5 I removed are 100uf, 250v. So, in that regard they're good.

Still, a bit odd. The closest number I have is 4189, on the warranty sticker. The board revision for 4289 states 150uf.

I assume the caps I pulled are the originals, but maybe they're not? From a previous botched repair?

Easy enough to put higher value caps in their place, assuming the desoldering/soldering process doesn't lift some traces. However, since C4 and C5 are on the input side of the switcher their exact values may not be too critical.
Have you checked that the diodes for the 5V supply (MBR2 on that schematic) are both good? If one of the two has failed open there might not be enough current to hold up 5V under load.
Although according to the parts list on appleiioz maybe it is really MBR1 (the dual common cathode diode package)?