HP 85662A CRT to LCD mod - Page 1
Forum section: electronics / repair

author: Rémi
author_rank: Regular Contributor
timestamp: December 11, 2019, 03:28:41 pm
content: Hello,I will soon receive a HP85662A display that suffers from a dim CRT. Solutions are available online to change the CRT to an LCD but I have to admit that I find them pricey.I'm wondering if it would be possible to mod the unit by myself in order to work with a LCD display.I had a look at the service manual (https://www.febo.com/pages/hardware/8566B/85662A%20Service%2085662-90085.pdf) and I'm wondering if by using the X,Y, Video signals the mod would be possible ?I saw other topic on the forum about this :https://www.eevblog.com/forum/rf-microwave/hp-8573-vna-version-c-vs-d/25/where they use CGA, EGA YUV to VGA converter.What do you think ?Rémi

author: andy2000
author_rank: Frequent Contributor
timestamp: December 11, 2019, 04:08:24 pm
content: That other example is a raster display, so no problem to convert to VGA. The 85662A uses a vector display which will be a lot more difficult to work with.It looks like there is at least one company offering a kit:http://simmconnlabscom.ipage.com/store/page2.html

author: MadTux
author_rank: Frequent Contributor
timestamp: December 11, 2019, 06:42:19 pm
content: Another problem is that 85662A display uses 2 vector modes, a slow one for slow sweep (digital storage) trace, grid, readout and a fast one, that is in use when using the bottom box in downconverter mode, with zero span.Then the vector display is used more like a analog scope with up to 1MHz, depending on the video bandwidth.With a slow vector display, you loose the downconverter scope mode. Don't use that one much, but I guess that's why they used a vector/scope CRT instead of a magnetic deflection monitor CRT.

author: bsdphk
author_rank: Regular Contributor
timestamp: December 12, 2019, 10:15:58 am
content: The HP 85662 is a very complicated X-Y display, by any standard, so you basically have four options:1) Find a replacement CRT (or try to "regenerate" the one you have.)2) Buy a CRT kit where somebody did all the hard work (and loose both screen resolution and detail)3) Do all the hard work yourself, and it is a LOT of hard work (and loose both screen resolution and detail)4) Buy a cheap analog oscilloscope or XY monitor and hook it up to the XYZ outputs.

author: Gyro
author_rank: Super Contributor
timestamp: December 12, 2019, 10:59:06 am
content: It looks as if the CRT is behind a window - try cleaning the CRT front face and the inside of the window (seriously, it's amazing the amount of fine black dust that can accumulate over the years!).Tweaking the presets on the video board may well yield an improvement too.The seller might have already tried both of these of course.

author: woodchips
author_rank: Frequent Contributor
timestamp: December 12, 2019, 03:10:36 pm
content: The 85662 I have seen quoted as 5MHz bandwidth to cope with the vectors, quite fast. Can't think of a scope that works over 4MHz.On the back are a bunch of BNC connectors with the X, Y etc signals out and then in so using an external XY monitor is feasible, such as the HP143 scope with a 12" screen. I have also seen mentioned that these connectors don't have either the static, grid, or dynamic, trace, information so one might be missing.Just hook a scope in XY on the back and try first.Other minor problem is the 85662 CRT is nearly square, so might have to fiddle with the standard 10x8 scope display.

author: xwarp
author_rank: Frequent Contributor
timestamp: December 12, 2019, 04:28:20 pm
content: Tag because I've personally seen two 8566b's with these new displays. They are pretty sharp, but I haven't had a chance to play with them, so I can't really offer any input to functionality.

author: orin
author_rank: Frequent Contributor
timestamp: December 12, 2019, 07:27:52 pm
content: I have an 85662 with a VGA resolution Simmconn display. It works amazingly well IMO and I find the resolution just fine.I find the argument that the resolution is less than the CRT to be moot. If you are taking a measurement on one of these instruments, you should be using a marker, not reading off the display.The Simmconn ismuchbetter in zero span mode with a fast sweep. The CRT in this case was dim and the sweep did not start at the left edge.The only 'problem' with the Simmconn kits is the need to machine the bezel. I have a micro-mill so I did my own (I can help anyone in the Seattle area with this).Overall, I definitely recommend the Simmconn Newscope kit and would not convert back to a CRT even if you gave me a new CRT.