HP27 Restoration (Lots of Corrosion)

[calc-calcs]

Checked continuity between all inter-chip connection and only one was partially failing, all other where solid OK. All power, data, and ground connections where checked between all ICs.

So found another somewhat intermittent trace that carries the Ø1 clock signal from U3 ACT pin 17 to U2 ROM chip pin 15. The connection would measure zero ohms most of the time, but when moving the board a bit it went anywhere between 135 and 350 ohms. The jumper pictured bellow took care of that.

Next I pulled both chips out of their sockets, cleaned all pins again, and reinserted them. Just in case. Also resoldering both the ACT clock inductor and capacitor which appeared to help a bit as well.

Calculator now seems a bit more stable and almost half the time powers up to 0.00, but the other half it still misbehaves, flashes digits, or just a blank display, all zeros, etc.

Also as mentioned before, when the calculator comes up normally to 0.00, all the basic algebraic functions work, but some higher functions don't seem to work, neither the storage registers or financial functions which use storage registers. The non working functions are: 10[1/X] returns "Error" instead of showing expected 0.10 result. PHI key returns zero. The Trig SIN, COS and TAN also always return zero, and the inverse ARC functions blank the display. I did not test the statistical functions yet, but assume they will not work.

The working functions: LN/Log, x^2, 10^x, Y^x, SQRT, %, H.MS/H, X!, E^x, and of course + - x / are fine.

Also cleaned again the receptacles and the keyboard connection pins, and found that a few keys needed the Deoxit treatment. Among them where the 1, 2 and 6. The 2 and 6 started working well immediately after a drop of Deoxit was put into the back opening of the key, but the 1 key stopped working altogether. Had to snake a bit of braided wire inside the switch and a bit more Deoxit and now it is finally starting to work again but still not entirely reliable so will need a bit more work.

I tested all DC supply busses and found quite a bit of ripple, but I assume this would be normal figures for this simple switcher with such small filter capacitors. I was going to set the scope for 20MHz BW limit (100MHz scope) as a lot of the ripple seems to be high frequency content, and re-read the ripple figures but forgot. Will redo it if these ripple readings appear way too high to anyone that might know the usual expected levels.

Rail Volt Ripple P-P
-----------------------------
Vdd 6.45 290mV
Vgg 12.66 205mV
V- -3,72 200mV
Vdp 4.76 185mV

Even the Vcc (2.5V battery rail) had about 440mV of ripple on it.

I am going to hook up my scope to all the data lines and take a picture and post it next.