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Re: Need some more schematics for an HP-9825B

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  • joshbensadon
    ... Sorry, I was talking here in general terms without really studying the system. ... This is a very bizzare system. PrtClk is also shifting the register as
    Message 1 of 6 , Jul 23 10:24 PM
      --- In midatlanticretro@yahoogroups.com, "dfnr2" <dfnr2@...> wrote:

      > > I wouldn't say it's PWM modulation, but rather it's firing the
      > > print head as needed while the paper is advancing.

      Sorry, I was talking here in general terms without really studying the system.

      > I think the transistors are part of the PWM circuit. The ramp is being compared against a trigger voltage by U11A, to produce a fixed frequency, variable duty-cycle pulse train, gated by U11D from the "PrtClk" signal. This is used to modulate four inputs to the printer. The trigger voltage is adjusted using the 5k pot, presumably the "density adjust" that was originally referred to. It also looks like there's some feedback from a thermister in the print head, which appears to be intended to reduce the PWM duty cycle as the head warms up.

      This is a very bizzare system. PrtClk is also shifting the register as well as gating this PWM signal to create the Prtheaden signal. So, when PrtClk goes high, the next PrtData gets shifted in, then after a delay caused by the diode/capacitor/resistor leading to U11d the PrtHeaden is allowed to pulse at the PWM. Now, what if PrtClk didn't stay high? PrtHeaden wouldn't go active. Further, the last bit of the register activates the solinoid when low, but we can write bits faster than the solinoid movement, so only after a deliberate write 6 zeros and pause, would the solinoid activate. Also, only after deliberately leaving the PrtClk line high, would we actually be generating a prtheaden signal to modulate those 4 lines. There are 2 more control lines to the print head chip, Hdstb1 and Hdstb2.

      So, we are controlling the whole printed line with 6 signals comprising of 2 strobes and 4 modulated signals that must activate the heat. The line is 16 characters by 5 dots wide each. 80 dots if you don't count 1 or 2 dots between characters (if they exist?).
      Then the only way must be to demultiplex those 4 lines based on these two strobes.

      Now back to the problem, intermitent lack of printing. Either that demultiplexer chip at the other end of the 14 pin ribbon is acting weird... or it could be a bad connection in that ribbon header? or we are loosing this PrtHeadEn modulated signal?

      The part I'm a little stumped on, is the Headtime signal that seems to interfere with that ramp generator???

      PS. The reset pin of the '174 is definitely just tied high, confirmed on my board.

      That solinoid delivers a nasty impact, I wouldn't be suprised if it's a bad contacts between the ribbon and that chipboard (what ever it looks like, I can't tell because I don't want to dismantle my whole printer to take a better look).

      :) Josh

      > A failure of one of those transistors could kill the PWM circuit, resulting in no output., or, failure to reset the integrator, resulting in maximum print-head heating. Since the PWM modulates (or alternatively is modulated by) the data, it really can't cause an all-black like I said.
      > > The print head doesn't move, it's the paper that gets stepped
      > > through the print head vertically. There must be only 1 row of
      > > dots. If you are getting all black, then I guess all drivers must>
      > You are correct, I was thinking of the paper advance solenoid, not the print head.
      > I just was taking a look at the next page, with the printhead interface. It's a little confusing to me. Since the whole line must be sent, and there are only 14 pins, then the signal has to be multiplexed somehow. I see that the four "data lines" which appear to be the result of straight-shifting the "print data" line through the register, get modulated. However, I find it strange that the solenoid and "head time" are at the tail of that SR, with no clear. I could see them as part of a multiplexing scheme, but I'd expect a counter rather than a shift register for that sort of setup. perhaps the four lines are somehow part of some clever multiplex logic, and the two "control" lines are actually data and clock? It's hard to guess what's happening from the schematics, because so much function is encapsualted in the large control chip. I'll have to get my machine out and poke around.
      > Dave
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