Mercurial > whatever / changeset

--- a/content/first.rst	Sat Feb 20 16:56:53 2016 +0100
+++ b/content/first.rst	Thu Mar 10 22:30:42 2016 +0100
@@ -1,6 +1,30 @@
-:Title: My First Review
-:Date: 2010-12-03 10:20
-:Category: Review
+:Title: Why
+:Date: 2016-02-20
+:Category: About
+
+When I am not in front of my laptop doing some software related stuff,
+I do sometimes find the energy to do things with my own hands... I've
+enjoyed to hack eletronics since I am 12 or something like that. At 14
+I had a scope in my room. I also started to hack my Amstrad CPC-6128
+back then. Beginning to write code, first in BASIC, then in Z-80
+assembly code.
 
-Following is a test...
+I've never made something really serious in electronics, mostly
+repairing stuff, built my own Nelson Pass hifi power amplifier, but I
+finally made my life hacking computers. The discovery of the FOSS
+movement changed me, and is now an essential part of my life.
+
+More recently, I began to play with more substential
+hardware. Starting with a very small chinese milling machine (a Vario
+BF16) and a cheap inverter welding machine, I began to learn (mostly
+by myself) some elements of machining.
 
+Then I had the opportunity to acquire and install (thanks to my
+sister offering me a part of a newly acquired grange to welcome a
+couple of more serious machines) a few machinist toys I am, very
+slowly, reconditionning.
+
+I'd like to try to write kind-of journal of my experiments with my
+"grouwn-up toys", being electronics or machinist related. An maybe
+some other subjects as well, who knows?
+

--- /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.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/hp8904a_2.rst	Thu Mar 10 22:30:42 2016 +0100
@@ -0,0 +1,69 @@
+========================================================
+ HP8904A Multifunction Synthetizer Repair: 10MHz output
+========================================================
+
+:Author: David Douard
+:Date: 2016-03-06
+:Category: Electronics
+:Tags: HP8904A, repair, test equipment, signal generator
+
+After having `fixed the PSU of one of my HP8904A`_ signal generator, I
+discovered that the other unit had its 10MHz reference output
+dead flat.
+
+I've disassembled the main logic board, but unfortunately, the
+schematic of this later A2 board (as it is referenced in the servce
+manual) is notavailable in any available HP's manual...
+
+The board I have is the second revision:
+
+.. image:: {filename}images/hp8904a/A2_top.jpg
+   :alt: The logic board (A2) of the HP8904A signal generator.
+
+And the bottom side of the PCB:
+
+.. image:: {filename}images/hp8904a/A2_bottom.jpg
+   :alt: Bottom side of the A2 board.
+
+According to the presence of soldering flux, it looks like the U46
+chip (a 74ALS73CN) on this board has been replaced:
+	 
+.. image:: {filename}images/hp8904a/A2_U46.jpg
+   :alt: The U46 chip seems to have been replaced.
+
+After having moved my scope probe around the 10MHz output (J102), I've
+found that the most probable culprit is the Q106 transistor (which
+seems to be a bipolar one). The 10MHz arrives to the base signal, the
+emitter is grounded, but nothing on the collector...
+
+Unfortunately, HP had the awful habit of taging most of the parts in
+their test equipments with custom references, even if most of them are
+in fact regular parts.
+
+The problem was then to identify a reasonable replacement part for
+this puppy. It presents itself as a Motorola with weird reference
+``3 0659``:
+
+.. image:: {filename}images/hp8904a/HP8904A_Q106.jpg
+   :alt: The failing transistor.
+      
+Having no luck searching the net, I've asked the `wonderful EEVBlog
+community`_ for some help identifying this transistor. My question
+obviously was answered very quickly by several fellows. First I've had
+the correct reference for the transistor. Its HP part number is in
+fact 1853-0659 (notice the 3-0659 as stamped on the transistor case),
+and it is i fact a simple 2N3906.
+
+But another forum member also noted that a wire was broken, as it is
+clearly visible on the picture above.
+
+So I did a very quit fix for this problem by simply resoldering the
+broken leg of the transistor, and bingo, the 10MHz output is working
+again!
+
+This EEVBlog community really is awesome!
+
+.. _`fixed the PSU of one of my HP8904A`: {filename}/hp8904a.rst
+.. _`wonderful EEVBlog community`: http://www.eevblog.com/forum/repair/repair-of-an-hp8904a-signal-generator
+
+

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--- a/pelicanconf.py	Sat Feb 20 16:56:53 2016 +0100
+++ b/pelicanconf.py	Thu Mar 10 22:30:42 2016 +0100
@@ -33,3 +33,9 @@
 
 # Uncomment following line if you want document-relative URLs when developing
 #RELATIVE_URLS = True
+
+STATIC_PATHS = ['images', 'pdfs']
+
+PLUGIN_PATHS = ['pelican-plugins',]
+PLUGINS = ['better_figures_and_images',]
+RESPONSIVE_IMAGES = True