The Museum of HP Calculators

HP Forum Archive 13

[ Return to Index | Top of Index ]

Wierd HP-87 Problem
Message #1 Posted by Mike on 21 June 2003, 10:28 p.m.

I have an HP-87 that has a sweep problem. I've looked around for obvious damage and have also looked for horizontal sweep adjustments. Can find neither.

Anyone have a schematic for the HP-87?

Problem: Each line starts about 2" from the left border with the cursor shown in the first photo.

Then as characters are typed, the letters are shown reversed and move from the cursor position toward the left border. After 4 characters, the charcters are displayed left to right, from the border. These characters overwrite the first 4 characters. The 2nd photo shows the result. I typed "Sweep problem".

I have used the down arrow, after the 4th character, to show what the screen is doing.

Here is the way it looks, if I don't use the down arrow after 4 characters.

It really looks like the horizontal sweep starting position is moved way to the left. Typing a whole line ends too far from where it should on the right.

I can just poke around till I find the problem but a schematic would help locate this problem.

Anyone ever see this before?

Edited: 21 June 2003, 10:40 p.m.

Re: Wierd HP-87 Problem
Message #2 Posted by Ellis Easley on 23 June 2003, 5:36 p.m.,
in response to message #1 by Mike

Is there a horizontal hold control, maybe just an internal one? It is usually a coil with an adjustable slug, on any monitors and TV's I've seen. Sometimes it's called "horizontal phase" and that's a clue why it can cause this. In the vertical sweep circuit, the vertical sync triggers a new vertical scan but the horizontal is a phase locked loop and the control adjusts the phase relationship between the sync pulse and the horizontal oscillator. It looks like your horizontal sweep is starting too late for the video.

Re: Wierd HP-87 Problem
Message #3 Posted by Tony Duell (UK) on 23 June 2003, 6:37 p.m.,
in response to message #2 by Ellis Easley

First rule of electronic repair (Well, almost) : Adjustments do not tweak themselves, so a fault is almost never sured by making an adjustment! In other words, tweaking the horizontal phase control might improve matters _for the moment_, but it's masking the problem, rather than fixing it, and it'll come back. In any case, I am not conviced this is a horizontal phase problem. It might be, but it might also be due to the beam taking too long to fly back, and thus some of the video is displayed during the flyback. I am trying to remember what causes this -- IIRC a deffective capacitor in the horizontal output stage is one possiblity. This assumes the 87 uses a conventional design, of course.

I was thinking cap too
Message #4 Posted by Mike on 23 June 2003, 7:49 p.m.,
in response to message #3 by Tony Duell (UK)

I have a schematic for the HP-85 and it might be similar. I looked around for something that looked smoked and couldn't find anything.

I guess I will try changing some likely caps, based on the 85 design. Not in any big hurry though. I'll report back when I find the problem.


Re: I was thinking cap too
Message #5 Posted by Vieira, Luiz C. (Brazil) on 23 June 2003, 11:50 p.m.,
in response to message #4 by Mike


I have already seen this sort of horizontal sweep (or "shifting") when using old-fashion SVGA monitors and changing O.S. graphics resolution. In many cases it was necessary to re-adjust horizontal "position" and sometimes horizontal "length". As the HP87 has everything built together, I believe there is a part of the circuit where the both image signal and horizontal scan share the same active circuits.

I just want to figure out the capacitor suspicion. What you mean is that part of the signal is getting mixed with horizontal scan signal or there is a DC component "shifting" initial position? If so, chances are that you may have a voltage/current divider with bad component, too. If there is any sort of current or voltage source used as a reference, even a failling resistor or diode may cause it.

I do not have the HP87 circuit; is there anything wrong in my analysis? I tried to consider not a general purpose monitor, but an specific design, as HP designers used to do.

Please, let me know if I'm too off-target.

Luiz C. Viera - Brazil

Edited: 23 June 2003, 11:57 p.m.

Re: I was thinking cap too
Message #6 Posted by Tony Duell (UK) on 24 June 2003, 6:54 p.m.,
in response to message #5 by Vieira, Luiz C. (Brazil)

I don;t know much about the 87 (I will have to dig one out and investigate), but I have the HP85 schematics (from the MoHPC CD-ROMs, of course!). The monitor design is _very_ conventional. It's just like every other single-frequency monitor I've worked on. The video chip outputs a vertical sync pulse, a horizontal sync pulse, and video information. These are kept sepearate (there's no reason to combine them into a composite signal and then separate them out again), so actually, there is no common stage that handles both video and hsync. The horizontal output stage of the 85 is just like that in a TV or simple monochrome monitor, and presumably can be debugged and repaired the same way.

[ Return to Index | Top of Index ]

Go back to the main exhibit hall