I don't know that I remember ever seeing three of the metal clips on a frame before, usually its just the outer two. But maybe I just never noticed the center clip.
I just checked the frame, its an Electrohome G05-802 frame from Jan '81
I'm pretty sure all 802/805 and v2000 frames have three clips. I've just never run into a problem with them shorting anything out, and as a result I've never looked closely to see what each of the clips comes into contact with.
It's an interesting question though, as you'd think it would cause more of a problem. I've *definitely* had the other issue cause a problem (where the board rubs against the back cross-bracket of the frame, next to the upper screw.) If you don't install that screw, and have it in an AD where gravity pulls the board down and into contact with the frame, the rear cross brace will short one of the traces out. But only if that particular board has its trace right at the edge of the board, and not all do. But I kept wondering why I was sometimes seeing sparks on powerup for some boards, but not others.
But I've never looked at the clips, mainly because it isn't easy to see underneath the board.
I just grabbed a spare 802 board from the shelf, and noticed it's a later rev than yours. As a related side note, people misread these all the time, because there are actually four version (aka 'issue') numbers corresponding to four separate aspects of the board. Each aspect has its own issue number, and they are all independent.
There is:
- A 'Foil Pattern' issue number on the solder side (for the actual copper trace layout)
- A 'Solder Mask' issue number on the solder side (for the pattern/shape of the mask)
- A 'Comp Symbol' issue number on the solder side (for the component number silkscreen)
- A 'Comp Layout' issue number on the parts side (for the part symbol/number silkscreen on that side)
See pic below for the first three, outlined in red. The other one is just printed on the parts side, similar to the Comp Symbol number.
You can get different combinations of all of these issue numbers. I actually record all four numbers on every board I refurb, and have a bunch of data. But I've never analyzed it to see how many unique combinations there are, but it's more than a few. The main point is, if the parts side says 'Issue 5 Comp Layout', some people see that and call it 'Issue 5', but that's not the whole picture. You have to look at the other numbers too.
The reason I mention this is because for the board I grabbed, it's a Foil Pattern Issue 7 (versus your Issue 6), and it looks like they changed the layout around that D600, to make more space (presumably to increase clearance around that clip). See the blue oval in pic below.
That's super interesting, as I often wonder why they did so many versions of these, but have never figured out all of the differences, let alone what motivated them. But this one seems to make sense, given what you've pointed out.
