Re: Challenging Woodstock Repair Message #9 Posted by Katie Wasserman on 7 Aug 2000, 8:53 p.m., in response to message #7 by Katie Wasserman
I've now managed to repair the broken 22 and 25 that I have in addition to the 27. In both the 22 and the 25 I found that the problem was noisy clock/data signals. The noise quiets down after the calculators have been on for a few minutes and they start working fine (it's sort of interesting to watch this pretty abrupt shift on a 'scope).
On the 22 I found that the 175Khz-clock signal was coming through the unused pins. And did the following fix: I simply connected one of the unused pins on the RAM chip to ground. (At least some of the unused pins appear to be connected to the substrate of the chip.) In particular, I connected a jumper between pin 7 (ground) and pin 10.
On the 25 this repair did not work, although it too had clock noise coming through on the same unused pins. However, I found that the data signal line was quite noisy too. The fix here was to install a 20K resistor between the data line and ground. The data line is pin 11 of the CPU chip and there is a convenient ground just across the chip on pin 12.
In the process of all this work, I found that my earlier assumption about clock generation was not completely correct. There is a sort of external RC circuit governing the speed of the on-ship (CPU) oscillator. It's a 10 ohm resistor shunted across a rather strange diode-looking device (a bilateral trigger diode?). The diode-like device has around 300pf capacitance, which makes no sense given the resonant frequency of around 700Khz and the 10 ohm shunt resistance, but that's what it measures! Anyway, you can slow down the clock with a parallel capacitance (300 pf will cut the frequency in half).
The CPU divided the 700Khz oscillator by 4 and generates the 175Khz clock and by something around 100 to generate the lower frequency clock. (BTW, slowing down the clock was not a fix for either the 22 or the 25.)
The lower frequency clock seems to be the main RAM/ROM access clock (very, very low given that these chips are bit serial address and data circuits) the higher frequency clock seems to be needed for the display driver chips. Why it also connects to the RAM/ROM chips is a mystery to me, unless these are dynamic RAM chips and need that clock for internal refresh purposes.
Anyway, I hope that some of this will help in your repairs. One of these days, I suppose that I should get around to drawing a schematic of the Woodstocks now that I've at least got a good idea of many of the pin functions.
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