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A Common Woodstock Problem and Repair Suggestions

Posted by katie on 16 Apr 2002, 10:14 p.m.

(The following was extracted and edited from threads in the discussion forum here during August 2000 and some later discussions)

A seemingly common problem on some Woodstock machines is that they don't work until they are left on to "warm up" for some period of time. After leaving the on for several minutes (maybe up to 1/2 hour) then turning them off and back on again they start working normally. More specifically the following problems were found:

In the case of the 22, 25 and 25C, the CPU chip works fine (i.e., you can do any command that just uses the stack) but the if you do any command that accesses the external memory chip you either read a zero or a crazy number, a write will do nothing. On the 27 the whole machine is locked up until it's warm, because the RAM and ROM are both on the same external memory chip. The lock up may manifest itself in the form of a totally blank display, one with all zero's or one with a fixed "0.00".

Internally, the problem seems to stem from the clock signals being generated at double their normal frequency until the warm up and "reboot" of the calculator is done. The problem chip seems to be the ACT chip and the signals it generates but it might be with loading from the RAM/ROM chip. The ACT chip is the same in the 22, 25 and 25C, but is different in the 27, which is based on NMOS as opposed to PMOS. The 21 is also an PMOS design but I have seen no reports of the problem with it. The 25C and 29C use some CMOS technology and some NMOS, however the 29C (at least) seems to have an entirely different set of problems. The following solutions have been found to fix this problem on the 22/25/25C/27 so that the calculator works as it should initially:

There are two clock signals on the 22, 25, 25C and 27, one is approximately 175Khz the other is about 1.6KHz. On the 22, 25 and 25C the problem seems to be with the lower frequency clock signal having excess noise while on the 27 it's with the higher frequency signal being doubled.

On the 22 I found that the 175Khz-clock signal was coming through the unused pins on RAM/ROM chip (the one closest to the battery compartment 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 ACT chip (the ACT chip is the one at the bottom of the circuit board.) and there is a convenient ground just across the chip on pin 12. One fellow collector has reported that this same fix worked as well on the 25C.

On the 27 what I found was that when the RAM/ROM worked, the 1.6KHz line was really 1.6KHz. But when it was not working it was exactly twice this frequency (i.e., 3.2Khz)! I isolated the clock line and found that the ACT chip is the one that generates this clock and outputs it on pin 20. For some reason that I don't understand, depending on the load on this pin it runs at 1.6KHz or 3.2Khz. I found that if I resistively couple this clock line to the rest of the circuit I can get it to run reliably at 1.6Khz. What I did was cut the trace from pin 20 to the rest of the circuit and inserted a 20K resistor. The calculator now runs perfectly regardless of the ambient temperature (well, within reason at any rate).

Notes on the HP 27 circuit power supply:

Hook up an external 2.5 volt power supply with an ammeter in series. If the power supply is working, you should see around a 100 ma (blank display) to 200 ma (most segments on) current draw. Assuming that you're getting that, you should next check the output voltages from the inverting supply. If you measure them relative to the negative battery terminal you should find three of them: -3.3, +6.5 and +12.5. The easiest place to find these is on the non-component side of the main circuit board on 16-pin IC pattern that doesn't have a chip installed in it (labeled U5). Pin 9 = 6.5v, Pin 12 = -3.3v, pin 15 = +12.6v.

Edited: 15 Feb 2005, 3:10 a.m.


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