# HG changeset patch # User David Douard # Date 1457645442 -3600 # Node ID 89689c6e9cf450e984510109bf52e66da0f4fa6c # Parent 9d2c5ded644179aa9c25ffc2b6bbfed65f93666a [hp8904a] add two blogs about the HP8904A and a small update of the about page diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/first.rst --- 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? + 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. diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/hp8904a_2.rst --- /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 + + diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/images/hp8904a/A2_U46.jpg Binary file content/images/hp8904a/A2_U46.jpg has changed diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/images/hp8904a/A2_bottom.jpg Binary file content/images/hp8904a/A2_bottom.jpg has changed diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/images/hp8904a/A2_top.jpg Binary file content/images/hp8904a/A2_top.jpg has changed diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/images/hp8904a/HP8904A_Q106.jpg Binary file content/images/hp8904a/HP8904A_Q106.jpg has changed diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/images/hp8904a/HP8904A_daughter_board.jpg Binary file content/images/hp8904a/HP8904A_daughter_board.jpg has changed diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/images/hp8904a/hp8904_psu_followme.jpg Binary file content/images/hp8904a/hp8904_psu_followme.jpg has changed diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/images/hp8904a/hp8904a_top.jpg Binary file content/images/hp8904a/hp8904a_top.jpg has changed diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/images/hp8904a/psu_schematic.png Binary file content/images/hp8904a/psu_schematic.png has changed diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/images/hp8904a/twin_HP8904A.jpg Binary file content/images/hp8904a/twin_HP8904A.jpg has changed diff -r 9d2c5ded6441 -r 89689c6e9cf4 content/pdfs/HP_8904A_Service_Notes_2.pdf Binary file content/pdfs/HP_8904A_Service_Notes_2.pdf has changed diff -r 9d2c5ded6441 -r 89689c6e9cf4 pelicanconf.py --- 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