HP27 Restoration (Lots of Corrosion)

[Duane Hess]

Hi:

Interesting. Certainly a lot of tracings look like they may be incomplete. Should the chips be OK and you can re-lay/repair the tracings I wanted to point out one specific trace. As if it is incomplete, an otherwise functional unit behaves in a manner similar to the "fried ACT".

This might not apply to you, but might help others:

I make comments due to my own experience and not because I even begin to be knowledgeable in the matter. After obtaining a super cheap, claimed non-functional 27, I checked the insides. Obviously nearly every component was unsoldered and resoldered. Evidently the previous owner was trying to diagnose the unit.

The behavior of the unit was as follows:
- when powered on, the display was ALL zeros, but shimmied, VERY rapidly
- closer examination showed the zeros blinked numerous times, then changed
- the change was, steadily spaced, all zeros counted then up/down (forget) 0 thru 9
== steadily paced means after 'x' flickers of all zeros, the display blinked once all 1s
== then 'x' blinks all zeros, and one flash of all 2's; 'x' all zeros & one flash all 3's
(0, 1, 2, 3...etc. or 0, 9, 8...etc. recall it counting up)
- it was really hard to see, as it was so fast; needed to get somewhere dark

The previous owner was unsuccessful. By accident I got it to work 100%. Which involved shunting one defective trace. As so much was resoldered, performed many continuity checks. Had screwy resistance values on one trace. Found the problem when a section of the trace simply fell off.

Your 4/6 12:57 post with the before and after pictures shows the trace I dealt with. As does your 4/6 06:44 back-lit pictures.

Looking at the 'after' picture from 12:57:
you see the PCB cut-out for the battery pack. Look immediately left of the cut-out to the 1st trace you see. This trace goes towards the top of the calc. It also goes towards the bottom of the calc. Connecting to the 3rd hole from the left for the top chip (16 pin). Amungst other connections, following the tracings on both sides of the PCB, it connects to a hole on the bottom edge of the PCB. The 1st hole immediately left of the sticking-up connector pins at the bottom of the PCB. The perspective is as you look at the 'after' picture as shown.

The trace goes upwards to a chip under the display panel. Examining the trace, as a whole, shows directly connecting a wire from the top-edge chip pin directly to the bottom edge PCB hole you do not by-pass any functionality of the tracing. But, instead, complete the tracing.

That is how I tested it. Which was a bit tedious, as for testing I didn't want to solder anything. Tightly wrapped a wire around the pin of the top chip and inserted the other end into the bottom hole. Placing the components in the case knocked the wire loose, so held it all in my hand. Attached the keyboard, held the battery in place and after some dexterity was able to hold it all steady. The unit worked flawlessly.

Looks like you have your work cut out for you. May have no pertinance to you. But after you're done, if you have the display behavior, you might want to look towards that trace. The 'after' picture looks like part of the tracing is missing just above the yellow blob (capacitor?).

Overall, that methodology is how I managed to fix the problem. Don't recall every detail, but assuming I examined the pictures correctly, it all should be correct. If not, you'll figure it out. As the 1st trace to the left of the battery cut-out on the surface of the PCB facing the camera of the 'after' picture is definately the tracing I'm referring to. And I remember connection a pin from a top-edge chip to some hole on the bottom of the PCB.

Whatever its worth.