I’ve never had a need of the term power mod on a Apple II SCSI card but now I have a situation where it’d be handy. The diagram for the Rev C SCSI card says to connect the diode to the front end of C16. I’m thinking that they meant C18 as C16 is on the far end of the card.
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Can you check voltage against ground at both locations?
A HBA that supplies TERMPWR should do so through a fuse to protect the cable against excessive power draw. The typical fuse rating is 2 A.
Just to draw attention to this... the Rev C card is really a crapy way to identify the card because rev C not a card revision it's a firmware revision.The way to identify if a card is FW Rev C is to boot in a GS, anything other than revision C FW will produce an incompatibilty error and requries the card to be removed before system can boot.
I think Rev C FW was also found on a 27128 and not the 2764 as the card's silkscreen states.
The 602-0291-? is the card revision. I belive there are two versions of the card A/B.
IIRC The B card is difference around the area of the circuit called out in the term power mod.
But I've never actually done the mod, that's what I seem to recall when I looked at what would be needed. Then I decided I haven't needed it yet, so it wasn't worth doing. So if something is "off" with the notes on the mod, that may be why.
It can still be done but you may need to just tap in a different way, still doable.
Rev C is a nickname that’s been in common use for about 3 decades. If you say Rev C SCSI card everyone knows what you’re referring to. But if you say 607-0291 or 820-0193-A then most have to look it up. I know it's not correct but it's easier to use terms everyone understands.
I have cards numbered 607-0291 with the letters A, B or C stamped on them. The cards appear to be identical. I believe the letters refer to the ROM version the card originally shipped with.
Rev A & B ROMs were 2764 and Rev C 27128. While I did upgrade all my cards to Rev C, I did keep 1 copy of each Rev in my ROM collection. I could just keep an image but the original ROMs have the correct labels.
Today I’m not tired which helps a lot. Another look at the diagram clearly shows the other end of the diode going to the front end of C18. So I went with that and it worked
I understand, the why but I think everyone would agree it's just about the worst way to identify what is important which is the firmware, not the card revsion. Plus there's an easy way to differentiate based on the firmware.
I suspect this came about because some random user had revision C board with the 16K firmware blasted the "Rev C" found on the board on AOL and the rest is history. Although, at the time if you had a GS and this SCSI card you know the original firmware found on the rev A/B card needed to be upgraded to a 16K ROM for the 16K firmware so the card would work with GS/OS.
As you likely know any board revision (A.B or C) with the 8K firmware is not this mythical "Rev C" SCSI card. Any board revision (A,B, or C) with the 16K firmware is this "Rev C" card. That's confusing! Adding to that is the firmware revisions.... there are two 8K firmware images, 341-0112A and 341-0112B, the 16K firmware aka "Rev C" is actually revision A of the 16K ROM, 341-0437-A. So we're approaching wicked confusing!
So "Rev C" is really firmware ROM 341-0437-A, and becuase this firmware can be found on any board revision the firmware is how this card should be referenced.
As to what the actual differences between Rev A/B/C cards is, that I don't remember. If you say the mod description is correct for all three revisions (it sounds like you have all three) I'll take your word for it. I thought I remember something wasn't right for rev A, but it's been years since I looked and my memory could be failing. =)
FWIW with Apple's part numbering schemes, the board rev changes, A to B was likely the result of circuit changs. Something like a firmware part change would not always mean the board would be reved, but sometimes it was. This was one of the confusing things with internal part numbers. If parts (like firmware) changed but were direct swappable, there wasn't usually a part change. Using a 27128 in place of a 2764 in the factory would not equate to any change either. If the firmware exceeded the size of the 2716 (8K)size and required 16K for firmware (27128) that would require a board revision change even if the only change is the ROM part number because the 27128 part is functionally different than the 2764.
I remember people having a card with the 341-0437-A FW and thinking that they had Rev A FW.
I’m convinced that the A, B & C weren’t meant to be a card revision designation. Usually it’s part of the number when they build the card. It doesn’t make any sense to build a card then have to stamp part of the revision number after. Apple did put white ovals when they wanted to note other stuff
Rev C SCSI cards copy.jpg
Here’s a pic of the 3 “Rev C” cards I have handy.
If you look back at the discussions regarding this product in the 1990s you'll likely find the people talking about hte card addressing the "firmware rev C" with part number 341-0437-A. The card revision wasn't called out, it was in reference to the firmware version. Which is also interesting because the only way this would be rev C is if the firmware part number ended with C and, yeah that A isn't what would be expected. The "revision C" really means firmware ROM release 3, as stated earlier A and B were 8K firmware ROMs, C was the first 16K firmware ROM.
If interseted from the GS/OS source code this is the message when you try to boot to GS/OS with firmware revision A or B installed: The SCSI card in slot *0 has an old version ROM and cannot be used by GS/OS. See your dealer for a free upgrade to the Revision C ROM...
Referencing "frimware revision C" really is the only correct option. There really are several different revisions of parts on the board in addition to the product. For example, there's a product revision, firmware revision, PCB revsion, PAL revision, and others.
The assembled board part number is 607-0291- with the revision in the white box (just to note, the board product number may actually be 607-4291). If you look around the board there will be a 820- number etched in the copper, this is the PCB product number and version. I'm guessing your 607-0291-A card has a PCB product code of 820-0193-A. I am interested if all your A/B/C cards have the same rev A PCB. If so, that means the PCB was not changed, but component did for each assembly revision. As mentioned before, if it's a 1:1 swapable part the product revsion would nto change. For example, the different SCSI controller IC (5380) are different in all cards, but each may be listed alternate parts and if sowould not warrant a BOM change so no revision change. But, if a component value changes (capatiance, or resistance, etc) that's a BOM change so the revision bumps too.
To close this out, if you rreead your OP referencing the "Rev C SCSI card" I'm sure you can understand why I called this out. The use of the "Rev C" label makes the question unclear. Can you even confidently determine what the Rev C in terms of the notes for the modification refers to? If we're talking about a hardware mod firmware revision should be moot but the board revision makes sense when talking about modifying hardware, right?
Side note since you mentioned the HS SCSI card... while there was one revision, there was actually two used in production. But you should never see the first because the fatal bug in the first revision was caught while the card was in production. That meant the line was stopped, all complteted products were reworked but the firmware revision was not changed since nothing had made it out the door. I guess it's possible one of the early boards could be out there with the bad firmware, but finding that would be nearly impossible. But... if you got that first revision, I can corrupt any drive in about 5 minutes.
To go back to the original question, I think the confusion on this is caused by how the PCB has the silk screen printed on some PCBs. While it looks fairly clear it's an 8 in 2 of the 3 the images posted above, I found at least one other image where a via cut into the silkscreen in the upper right of the 8, making it look like a 6 (similar to one of the pictures above):
Apple II SCSI C18.png
Looking at a schematic, I can see C16 connected to ground and an input of U1C, which lines up better with where it is physically located on the board:
Apple II SCSI C16 Schem.png
Oddly, I can't find C18 on the schematic. Unless I'm over looking something. But this schematic seems to omit a lot of things, like VCC and GND on the ICs. So if C18 is acting as a decoupling cap on VCC for one of the ICs, that explains why it's missing.
I beleive you're right about the silk screening
Apple II Rev C SCSI cards copy.jpg
If you look in the pic I posted you’ll see 820-0193-A in the upper right hand corner of all three cards.
I think the only thing missed is that's not an Apple schematic. I agree, whomever did that schematic missed some components. I see 5 ceramic caps on the board and at least 20 of those axial filter ones but only find at total of 4 caps in the schematic. Power filter caps are often not called out in schematics, or if they are they appear as a block of caps wired to power and ground.
I made the schematic a searchable PDF file so it's easier to find things:
https://www.applefritter.com/files/2024/04/29/Rev.%20C%20SCSI%20Schematic%20-%20Updated%202-23-6.pdf