|Re: Help! HP-19C printer gone mad|
Message #12 Posted by Philippe Lasnier on 13 Mar 2011, 5:17 p.m.,
in response to message #10 by Katie Wasserman
I just spent an hour scrapping off any trace of green oxidation that I could find, checking that tracks and solder joints were ok around each, and soldered a wire that fell off. I dusted off everything, and reconnected the power PCB with the keyboard.
Again, for a few seconds, things seemed to be slightly better; I was getting a few characters printed out. For instance the whole of "PRST" appeared as I asked to print the stack...but not the stack values. Quickly, the printing went back to as bad as it was before.
But I noticed something else (I had vaguely noticed yesterday, but hadn't paid much attention): every "bad" printing zaps Y Z and T, and the motor's pitch changes. So for instance if I am in TRACE mode, with 5 in X, and I press 4 and then +, then 4 stays in the display after the printer flounders. The printer only runs once, instead of twice (as it should to print "4 +" and then "9 ***"), and going to MAN and cycling the stack with RĀę I notice that Y has been set to zero. The addition operation obvioulsy didn't happen either. A similar observation goes for stack printing. If (in MAN mode) I load the stack with 1, 2, 3, and 4, then do PRST, then the printer runs twice (as if to print "PRST" and the subsequent blank line) then stops, instead of printing the stack. Examining the stack, only X remains at 4, with T, Z, and Y, having been reset to zero.
Considering that storage registers 16-29 and the stack registers Y, Z & T are not in permanent memory and are zeroed when the power is turned off, this looks like a power cut (maybe from overload) is effected somehow when the printer runs.
So here is my conclusion: the printing starts, and some pixel columns get printed. Most often though , a power "cut" occurs, which stops everything. The power comes back on effectively immediately, and because the printer's end-stop switch is open, the printer is driven all the way until the switch gets closed again (starting position). There's no printing that would happen during that "run back to start" stage, and that's actually audible - the pitch of the motor is different and the printhead speed is faster.
Now, as to what is causing this power glitch, I don't know. Sometimes it doesn't happen. If it's a bad solder joint somewhere, I haven't found it (it might help to know where in the circuit it's most likely to happen, but I don't know). If it's a bad connector, I don't know where it is; and considering that the power circuit and the printer are on the same PCB, a bad connector is less likely.
Could it be an (intermittently) faulty component? And if so, which one? It could still be the power supply, but I'm pretty sure mine is ok at the moment.