Thanks Katie!

[Harald]

For this article:

http://www.hpmuseum.org/cgi-sys/cgiwrap/...i?read=267

I seem to have just "repaired" my newly acquried HP 27 by inserting a 20k resistor into the 1.6kHz clock line.
When I received the calculator, at first I thought it was dead. I hooked it up to my power supply and started measuring all the supply voltages. To my surpris after turning it of and back on it worked. I put it back togather thinking that I had probably fixed it by unknowingly resoldering a bad solder joint. Well, next time I turned it on it did not work anymore.
Then I found your article and could reproduce the problem. Cooling the calculator doubled the clock signal on pin 20. I inserted the resistor (by the way, is it 20k because it is on pin 20 ;-P or how did you get to that value?) and now it continues to work even when I cool it down.
I still can't really belive this solved the issue, but it does look like it so far. I recon it is noise getting into the ACT that causes the problem. Red is the signal at the ACT, blue at the other end of the resistor:

I wonder where the noise comes from, maybe in reality this is a bad capacitor somewhere that causes it.
Did you try and replace capacitors to solve the problem?
Anyhow, I seem to have a working HP27 now and am happy

Thanks, Katie!

Harald

Edit: I just had to open the calculator again and check. There was quite a lot of noise on the battery voltage. I replaced the cap and it did get a lot quieter. No change on the clock signal (I think this is actually the sync signal) though. Some of the pins on the RAM/ROM chip are unconnected and have noise on them. Grounding these didn't change anything either. So I am non the wiser where the noise is coming from....