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Found these inside a non-working 71B
03-14-2018, 06:19 PM (This post was last modified: 03-14-2018 06:24 PM by MikeSD.)
Post: #89
RE: Found these inside a non-working 71B
I'm still puzzled about why the RAM didn't work on my unmodified 71B. I did not have HPIL attached when testing.

But there is something different about the original unit it works in, which might explain why in it and not an unmodified one.

I've attached a pic below of the back of the PCB on the one that works. The U9 (I think) bottom chip, pin 9, has a jumper to the Math ROM (top chip, pin 8), which one might expect, if expanding to that top chip spot.

But that U9-9 also goes to pin 10, which picks up the trace to HPIL, when the top chip slot isn't needed. Not sure why it needs to go to pin 10 at all. If they wanted to also go to the HPIL, they could have done it without going to pin 10.

That said, the trace is broken where it connects to the HPIL trace. So, since U9-9 doesn't go directly to HPIL any longer and the trace is cut, how does HPIL get back in the loop. AND, might this extra wiring and cutting, be the cause for the RAM not working in the unmodified unit.

And now that I think about it, the rest of the chips is how it gets back to HPIL. I think pin 10 is just connected but not used, or possible for some other reason. The chain goes from Math, to RAM, to FORTH, THEN to the HPIL.

Dang! Still don't have an explanation for why RAM didn't work in unmodified unit. Smile

[Image: Pin9-10x.jpg]
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RE: Found these inside a non-working 71B - MikeSD - 03-14-2018 06:19 PM
What's U10? - Dave Frederickson - 03-15-2018, 03:43 PM

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