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+===============================================
+ 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.