HP 8568B display problem - Page 1
Forum section: electronics / repair

author: Mrt12
author_rank: Regular Contributor
timestamp: October 10, 2017, 08:01:09 am
content: Hi,I have a 8568B spectrum analyzer (from 1984) which works very well, but there are two minor issues with the display:a) the font is clean and sharp, but the graticule is somewhat blurry. If I adjust the focus potentiometer, I can adjust the sharpness of the font, but not of the graticule. Is there some other potentiometer inside which allows me to adjust the sharpness of the graticule?b) If I place a marker on a trace and use the knob to move the marker around, the display flickers a tiny bit. Is this normal, or is this a real display issue? if the latter is the case, how can I fix it?

author: SaabFAN
author_rank: Frequent Contributor
timestamp: October 10, 2017, 06:54:28 pm
content: The flickering when moving markers around sounds to me like a problem with the Power Supply or Capacitors on the boards responsible for generating the graticule.If I remember correctly, the 8568-Series has an absolute ton of modules, of which many are responsible for generating the picture on the screen. Most likely one of those modules has several potentiometers on it to get the graticule into focus. Or some different board in the chain of components that lead from the processor to the CRT.I'd suggest you get the service manual and run through the entire process (as fas as your equipment allows that) of aligning and calibrating the instrument.I don't know exactly, but I think part of the picture is generated in the bottom unit instead of the top-unit with the CRT in it.

author: G0HZU
author_rank: Super Contributor
timestamp: October 11, 2017, 01:18:54 am
content: I fixed the fat/blurry graticule on both my HP8568B and HP8566B and the fault was on the Z axis board. I can't remember what resistors I replaced as it was a few years ago. But see below for a before and after picture. The CRT looks much better than this in reality when viewed with the naked eye. It really is like a new CRT now but I can't find a camera that can capture this properly.

author: Mrt12
author_rank: Regular Contributor
timestamp: October 11, 2017, 07:56:23 am
content: Hi,thank you guys. I have the service manual (unfortunately only in PDF form, the original folders are supposed to be somewhere but I can't find them at the moment).Unfortunately, for the display adjustments, a special adjustmends board is required according to the service manual, which I obviously don't have.However, I tried to adjust the display just by using the trimmer potentiometers (FOCUS LIMIT, ASTIG, FOCUS GAIN) and it got a tiny bit better, but not much.So you suggest I should take out the Z axis board and measure all the resistor values?What do you think concerning the flickering display, @G0HZU? the display normally does not flicker, but when a marker is placed on a trace and the marker is moved using the know, the trace flickers a bit. Is that normal, could you check with your spectrum analyzer as well?

author: G0HZU
author_rank: Super Contributor
timestamp: October 11, 2017, 08:40:26 pm
content: I don't see any flicker when I move the marker on my 8566B and I think the 8568B will be the same.It was about three years ago that I repaired the Z axis board on my two analysers but I recall that I found an erratic trimpot resistor and also a fixed resistor that had aged a bit. I replaced the same parts on both z axis boards (i.e. for the 8568B and the 8566B) and this cured both analysers. But I can't remember which trimpot it was. I think there are about 6 trimmers visible on the top edge of the board and it was one of these.See below for before and after on my HP8568B analyser. You can see it is the same analyser because you can compare the top edge of the old and chipped sticker just below the CRT.

author: cncjerry
author_rank: Unknown
timestamp: October 15, 2017, 02:52:02 am
content: you can tweak a few of those pots on the z axis board without any special boards or generators and correct that issue by eye. I did it on two of my 8568b's and an 8566b. Just remember where they were before messing around. take a picture in case things go south.

author: Johnny10
author_rank: Frequent Contributor
timestamp: November 03, 2019, 06:56:13 pm
content: Thanks Jerry,Took your advice and adjusted the pots on the A1A2 Board.Was very conservative with Intensity controls.Sharpened the graticule and stopped the intermittent graticule blooming.I found R516 adjustment had the most effect.

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 06, 2019, 10:03:19 pm
content: I just tweaked mine and it made a significant difference. I thought I might have CRT issues, but it only needed some minor adjustments.Despite the weight and size issues, this really is a lovely instrument.

author: Johnny10
author_rank: Frequent Contributor
timestamp: November 07, 2019, 05:31:21 am
content: Cool.