|Re: HP-85 Error 23|
Message #11 Posted by Tony Duell on 15 Nov 2005, 8:07 a.m.,
in response to message #9 by Etienne Victoria
The service manual diagram posted a couple of messages back confirms that there are 3 supply rails to the RAMs, as I thought. There was a rarer 16K bit DRAM that used +5V only, I wanted to make sure those were not used in the 85A.
However, you should still check the power supply voltages at the RAM chips. These don't come from the RAM controller IC (which handles the logic signals), they come from the power supply board. If one is missing or incorrect you need to troubleshoot that area.
Of course the RAMs might be good, and the RAM controller IC the problem. That is an HP custom part, alas.
I hate to say this, but you really need to do a lot more tests before replacing chips. I know the 'modern way' is to replace components more or less at random until the fault goes away, but that's never worked for me. I would (at least) check all power supply outputs, the clock signals, the address and data lines to the DRAMs, etc. Only when I had a pretty good idea as to the fault would I change parts.