diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/hp8904a.rst --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/content/hp8904a.rst Thu Mar 10 22:30:42 2016 +0100 @@ -0,0 +1,191 @@ +=============================================== + HP8904A Multifunction Synthetizer Restoration +=============================================== + +:Author: David Douard +:Date: 2016-02-20 +:Category: Electronics +:Tags: HP8904A, repair, test equipment, signal generator + + +What Is This About +================== + +I have a couple of HP8904A multifunction synthetizers for years +(bought them on ebay around 2007 IIRC). One of them is working +properly, but the second one had very inaccurate output levels, +generating a non-symmetric signal when output level was set above 5V +or something like that. + +.. image:: {filename}images/hp8904a/twin_HP8904A.jpg + :alt: My 2 HP8904A synthetizers. + +I knew for a fact that the PSU was the problem (thanks to the fact I +have 2 units, I could easily swap elements between the units). Back +then, I spent a bit of time trying to figure out what was going wrong, +and detected the +16.3V output of the PSU was failing at around 9 or +10V. I did not had a chance to investigate more then. + +For some strange reasons, I've recently decided to fix this PSU +problem. + + +The PSU Problem +=============== + +.. image:: {filename}images/hp8904a/hp8904a_top.jpg + :alt: Top view of the HP8904A + +The PSU board is the one at the rear of the unit, on the upper part of +the case (the lower part hold the analogic boards). The PSU is not a +custom HP board, but is a Computer Products switchmode supply (90 +Watts, Model XL51-5601). + +It delivers 4 voltages on 2 separated ground planes: + +- a +16.3V and -16.3V rail (with their own ground plane), +- a 5.1V for logic devices, +- a -15V (same ground as the +5.1V) which powers at least the fan. + +There are not too much material on the web about the possible PSU +problems of this equipment. A few threads on forums and +`this very interesting blog by Simon Schrödle`_ + +.. _`this very interesting blog by Simon Schrödle`: http://www.simonsdialogs.com/category/repairs/hpak-8904a/ + +But his problem was different from mine. + +On my device, the problem was with the +16.3V output falling at around +9V *on load*. The level was at his proper 16.3V level when the main +board as not plugged in. + +I first gave a close look at the caps, looking for some evidence of a +failing capacitor... which is always the culprit to think of in this +kind of situation. But I could not find anything obvious. + +So I started to reverse engineer the circuit to try to understand a +bit the possible causes. + +The PSU consists in 2 PCBs, the main one which is a double-layer PCB, +and a small daughter board with several op amps and zener +regulators. Obviously the regulation stuff. + +In order to be able to follow the tracks, I had to remove the heat +sink and split these two boards apart, since it's not easy to deal +with this: + +.. image:: {filename}images/hp8904a/hp8904_psu_followme.jpg + :alt: part of the +16.3V PSU rail + +So I removed the heatsink and disassembled this small board from the +main one (which was quite painful, since I don't have a disoldering +station. Disassembling the 3x 8 pins angle connectors from the double +sided PCB was not a piece of cake...) + +.. image:: {filename}images/hp8904a/HP8904A_daughter_board.jpg + :alt: the small daughter board of the PSU + +After that, I noticed a possible leak of a small cap, but this was on +the 5.1V rail. Should not be related to my +16.3V problem. + +But I decided to take advantage of having disassembled the PSU to +replace a few of these caps, starting with this dubious one. + +I also decided to replace the soldered assembly for the two PCBs by +some connectors so I can easily install and remove the daughter board. + +After having replaced the caps, the PSU was still failing. So I really +had to understand the schematic to investigate more. + +The part of the schematic involved in the +16.3V rail is the following: + +.. image:: {filename}images/hp8904a/psu_schematic.png + :alt: part of the schemtic of the PSU + +At first glance (the schematic was then a bunch of poorly made +"Dave-CAD-like" sketches on several sheets), the exact role of the +LM392 opamp was not clear to me, but when I redrew the schematic a bit +more clearly, it became obvious it was some sort of current limitation +circuit. And, sure, my problem could very well be a current limitation +system being a bit zealous. + +So I started to probe the resistors around the LM392 to check their +values. And guess what? R7 was dead. It's a 20k 1% resistor, and it +was not letting any current pass through. + +What are the odd of such a failure on a PSU like that? Unbelievable! I +was expecting the failure of a cap, of course, maybe an opamp or a +diode, but not a resistor. + +I have not yet bought a proper 20k stable 1% resistor (which I don't +have laying around), but I've installed a pair of 1% resistors in parallel +(can't remember the exact values I've used) to get a close 20k. And bingo! +the HP8904A can output a nice 10Vrms again! + +Of course, while I was digging around my PSU problem, the Schaffner +power line filter died in a violent smelly smoky way (took a while to +get rid of the smell). + +Battery, RAM and software options +================================= + +One of my goals when I looked into my two HP8904A was to check the on +board batteries responsible for keeping the content of the memory. + +This is a critical thing, since the content of the RAM must NOT be +lost otherwise the device won't work anymore (the official procedure +to change the battery involeve sodering a power source in parallel to +ensure the RAM chip is always under power). + +The HP8904A seems to be the first HP test equipment to have +software-enabled options. The option activation depends on the serial +number of the machine, which is also stored in this "non-volatile" +RAM. + +There is mainly one option one does not want to loose, it's the +Option 1. It enables the Channel Configuration Mode Enhancement. It's +the one that make the unit so handy (allowing to mix up to 4 +waveforms). + +I already cheated a bit back then. In fact, one of my 2 HP8904 came +with no option, the other with option 1 and 2. Option 2 means it have +a second analogic board allowing two separated outputs. So I used the +serial number of the unit coming with option 1 on the other one, and +bingo, this latest had then the option 1 activated also. + +But I recently found on the EEVblog a +`thread pointing to an HP service document`_ describing how to modify +the correct byte in the RAM to activate all the options! + +And the magic trick is:: + + Power-Cycle/Power-On + SHIFT, SERVICE, SHIFT, f4, 0, 9, 2, 4, 4, 8, SHIFT, SERVICE + NEXT, NEXT, NEXT, f1, 3, F, F, C + f2, F, F, f4, f4 + Power-Cycle + +That is: write the value 0xFF at address 0x3FFC. + +The procedure to restore the serial number can be found in the service +manual of the HP8904. + +And in case of failed battery, the memory can be restored following +`this document <{filename}pdfs/HP_8904A_Service_Notes_2.pdf>`_ + +.. _`thread pointing to an HP service document`: http://www.eevblog.com/forum/testgear/hp-8904a-options/ + + +Next steps +========== + +The first thing I have to do is to replace my temporary fix with a new +stable resistor. + +I'll probably replace the power line filter of the other signal +generator before it dies in a smoky way. + +Then, I need to recalibrate these puppies, but I need to build a bit +of confidence in my HP3456A DVM for this. But that's an other story. + +I also want to replace the fans with some more recent and quiet models.