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Re: HP9804A backlight?

 

Bruce,
I am still perusing the service manual, and am not yet sure how/where disconnecting it would be possible, but part of the issue here may be the unit it on just one or two PCBs. The adjustments for the LCD are all over the main board - in my unit (2948A+ serial number), the contrast adjustment?(R32) is way deep inside the unit, and not near the LCD, where I'd expect it.

Point being they seem to have kind of intertwined the LCD controls and power?supplying throughout the main board. The service manual seems kind of "end of an era" transition type, in between the exceptional service manuals from early days and the non-disclosure of later times. I, for one, am still looking for schematics for my HP54522A scope.?

If anyone has found disabling the 80V supply is the route, it'd be very appreciated sharing insight on the process.?
Radu.?

On Mon, Mar 7, 2022 at 9:33 AM Bruce <bruce@...> wrote:
Would disabling the 80v source be a big deal ??

Cheers!

Bruce

Quoting Radu Bogdan Dicher <vondicher@...>:

> Doug and all,
> I just acquired one such unit (HP8904A), and indeed, it is?
> exceptionally accurate. Its 10MHz internal reference measures at?
> 4.1ppm off on my oven conditioned HP5328A, and the DC levels are?
> almost as accurate as I can measure them with my Fluke 8840a (for?
> instance, the 10V DC level reads at 10.0003V). AC amplitude levels?
> read with my HP54522A are at 0.00000V off for the 0.1 and 1000Hz?
> signal. Wherever I am seeing deviations, they are usually an order?
> of magnitude less than the allowed tolerance.
>
> Anyway, VERY impressed. This is a unit which, as far as I could put?
> it together, was in a maintained lab until some part of 2020, but?
> since has been on eBay at least twice. Funny thing is it's been last?
> looked at by a metrology lab in my literal neighborhood, then spent?
> these 1-2 years somewhere in AZ, then came back to the neighborhood?
> in CA to join my herd.
>
> Now, on to the issue at hand - I'm very interested in replacing the?
> LCD with a hopefully far more legible unit. Can anyone indicate some?
> clear directions, if available anywhere, and maybe LCD units to use?
> for this? I've seen Doug's recommendation, but I'm a bit weary the?
> shield doesn't seem to fit behind it. The other concern is with the?
> 80V source for the old unit that seems to need a load of sorts to be?
> happy.
> Radu.
>
>
>








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