EB5AGV's Workbench: Hewlett-Packard 54111D repair

This is one of the more challenging repairs I have attempted so far. The HP54111D is a final 80s 1GSa/s 500MHz oscilloscope. I got it with some faults and am trying to fix it.

Please, let me know if you can help me with parts for it or any hint. Thanks!


I got the unit from eBay with a known fault. It passed all tests but then generated an INPUT OVERLOAD error and got stuck. This problem cleared itself and I have found the way to duplicate it, just removing a connector. So, beware with the multiple connections to the input modules.


Since I got the unit, horizontal convergence was really bad. White lines were really three slightly sepparated color lines, all around the display. It was very annoying to see duplicate traces also, when they were not of a basic color.

So I got to the Service Manual and followed the directions except that I managed to access rear panel controls without removing display module from frame, as you can see here:

I tweaked a bit H_STAT control, on the back of the display module:

And got a nice,converging display:

Once back to its original position, I generated this 4-trace display for demo purposes ;-):


After getting unit back to working condition, once the INPUT OVERLOAD problem was gone, I checked CHANNEL 1 at different attenuator settings. And then I found it was not working properly. It does weird things and real attenuation does not follow the settings. Luckily, CHANNEL 2 works perfectly.

I have tried to find the culprit assembly but, so far, have not yet found it.

This is the message I sent to the hp_agilent_equipment list at Yahoogroups:

Date: Fri, 21 Dec 2007 18:26:41 +0100
Subject: [hp_agilent_equipment] Puzzled by my HP-54111D


Well, the more I dig into the problem, the more puzzled I am :-(!

Let me explain what I have found... well, CHANNEL 1 (CH1) has an uncommon behaviour: when you go through different attenuator positions, signal does not follow attenuation as it should (there are discontinuities). I have found that it works fine only at some positions.

So I thought it had to do with input assemblies. I have got out the input module for CH1 and have dismantled and visually checked it. Mechanically, all seemed OK. So I have put it back and of course, failure was still there. I have then tried to use the control lines (what the manual calls CHANNEL SOLENOID CABLE and ATTENUATOR POWER CABLE) to the input assembly (which includes attenuator and preamplifier) from CH2. And, yes, then CH1 worked fine!. So this has proved that CH1 input module and cabling is OK.

Then I have thought that problem should reside on CH1 ADC CONTROLLER BOARD, so I have swapped it by the CH2 counterpart. But then problem has continued in CH1!

So, summing up, if I use the control signals from CH2 to manage CH1 input assembly, it works fine. But if I swap comtrol boards from CH2 to CH1, problem continues in CH1.

Any hint?

Best regards,


PS: HP-54111D service manual is available at Agilent site, if someone is curious about this.

UPDATE 23-December-2007

This message, sent to the hp_agilent_equipment list at Yahoogroups, summarizes my last works:

Date: Sun, 23 Dec 2007 15:25:37 +0100
Subject: [hp_agilent_equipment] Christmas came earlier in Valencia :-) (was Re: HP-54111D parts needed)

Hello John and all the List members,

John wrote:
..I have been successful in repairing the input modules on the 54111D. The
..best resource on these modules was published in the HP Journal:

Great article!. I recalled having seen it but had not read it in detail. It has been sure of big help. Thanks!

Well, I have given the unit a last chance before storing it waiting for replacement parts to come... I knew for sure problem was on the input attenuator / preamp assembly of CHANNEL 1, so I have, again, checked all and every attenuation setting. I had a table with the failures I found in a previous check. To my surprise, some of the ranges had fixed themselves!!!

Thinking about it, I recalled yesterday I run some calibrations, one of them vertical calibration... could it be related?. Sure it looked suspicious... I have also checked both 10X attenuators by going up and down where solenoids activated. And, yes, one of them (which engages at 50mV/div setting with single display) did weird things, as sometimes signal was attenuated a bit more than it should. So, thinking a bit, I have thought that it could be related with vertical calibration accuracy / results. So I have exercised a bit more the attenuator (still failed but less often) and have redone vertical calibration... BINGO!!!, now all ranges which still failed (which were 10mV, 100mV and 1V/div) were right most of the times (you guessed it, when attenuator worked fine!)

So, following John's advice, I have taken it apart (I am starting to master getting them in and out of the unit without removing front panel ;-) ) and carefully cleaned switches (I have taken pictures of the process and plan to add to my WEB page soon). One important thing: it is a very good idea to label all parts. I have just put a 1 to 3 number on all attenuator and impedance switch related parts with permanent red marker (even if they are black, you can see it). This way you have position and orientation clear when you reassemble the unit.

Once I have cleaned it, I have put it back in the 54111D. Now all times attenuator worked fine and so ALL ranges were OK in CHANNEL 1 :-)!!!

I have gone again through the vertical calibration process, just in case, and result has been perhaps an slight better cal.

All in all, my parts-unit (and so cheap ;-) ) 54111D is now a fully working unit. As I said, Christmas time has been a bit early in La Canyada :-)

Thanks for all your help!

Best regards and Merry Christmas,


Here you have some pictures of the attenuator / preamplifier works. It is a delicate job but can be done!

This is the assembly:

And this is what you get with the aid of a screwdriver:

Beware with the small spring contact, as it can fly and dissappear!

This is the hybrid signal conditioner:

These are the tiny slidding switches:

And these are the contacts and my index finger, for size comparison:

These are the contact pins:

General view of the disassembled unit:

Mounting it back:

In the top cover there is a nice wiring diagram, just in case you forget where every cable goes ;-)

And this is the scope after the works. Red trace is CHANNEL1 - CHANNEL2 and, as it should be, is a flat line.

Just a detailed display picture:


More to come soon... keep tuned!