changeset
[hp34970a] new blog post about the HP 34970A
Mon, 13 Jun 2016 22:14:30 +0200
- author
- David Douard <david.douard@logilab.fr>
- date
- Mon, 13 Jun 2016 22:14:30 +0200
- changeset 58
- 58e79464cd09
- parent 56
- 3d0bed1a79c8
- child 59
- cc8413dfcf14
[hp34970a] new blog post about the HP 34970A
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/content/hp34970a.rst Mon Jun 13 22:14:30 2016 +0200 @@ -0,0 +1,129 @@ +================================= + HP 34970A Data Acquisition Unit +================================= + +:Author: David Douard +:Category: Electronics +:Tags: HP, 34970A, DMM, repair, test equipment +:series: HP 34970A repair +:series_index: 1 + + +I recently "scored" a HP 34970A with the DMM included and 2 plugin +cards (HP 34901A and HP 34907A) for less than 200€. The plugins should +be in working condition, but the main switch unit was almost "given" +with the plugin modules since it is non functional. + +.. image:: {filename}/images/hp34970a/front-view.jpg + :alt: front view of the HP 34970A data acquisition unit + +It's in very good shape, the front panel and the push buttons are +clean. The knob is nice but it has a rough step when turning it for a +complete revolution. + +When powered up, however, things are a bit nastier... The VFD is, as +often with these equipment, very dim and the digits tend to puke on +the others. In fact, it's mostly unreadable, and the unit beeps +several times, signs of errors (which I cannot really read them see on +the display). + +Internals +========= + +The unit consists in 4 parts: + +- the main logic/cpu board on the bottom left side, +- the DMM module, juste above the main board, +- the frontpanel and +- the back plane where plugin modules are connected. + +.. image:: {filename}/images/hp34970a/bottom-view.jpg + :alt: bottom view of the HP 34970A + +.. image:: {filename}/images/hp34970a/top-view.jpg + :alt: top view of the HP 34970A with DMM removed + +.. image:: {filename}/images/hp34970a/dmm.jpg + :alt: DMM module of the HP 34970A + +The front panel hold the keyboard, the rotary encoder and the diplay system. + +.. image:: {filename}/images/hp34970a/front-panel.jpg + :alt: front panel of the HP 34970A + +The front panel is managed by a 80C51 (a 87C51 actually) +microcontroller. It's responsible for handling the keyboard and +feeding the shift registers of the VFD drivers (a pair of +HV518PJ). The communication with the main CPU is a serial line with +opamps to adjust the signal levels. + +The chips are PLCC44 located under the VFD are the drivers: + +.. image:: {filename}/images/hp34970a/front-panel-pcb.jpg + :alt: front panel PCB of the HP 34970A + +On my unit, the voltage levels for the VFD are fine. I suspected that +one of the 2 VFD drivers (the one responsible for driving the grids) +were partially failing, so I ordered a pair of new drivers, as well as +some PLCC sockets. It was my first real smd revork. I though that it +might be easier to cut the pins of the soldered chip, but it was not a +good idea, I peeled 2 pads on the PCB. Thanksfully, they were not +broken. So I've undoldered the other VFD with my new cheap hot air +soldering station, and it's been way easier and did not damage the PCB +at all... Lesson learned. + +After that first real SMD unsoldering experiment came the first SMD +soldering: install the plastic made PLCC sockets... I was not sure +wether to use the hot air gun or my Weller soldering iron, nor what a +correct temperature for the hot air station whould be adequate. The +informations on the socket datasheet were not very clear to me. + +So I gave a try (I think at around 280°C). The sockets seemed properly +soldered, but in fact, a few pins were not making contact. So I +decided to cut the bottom of the socket (which is essentially useless) +so I can more easily rework these "cold" pins with my soldering iron. + +The tricky part of installing these PLCC sockets is that decoupling +capacitors (C6, C7, C9 and C10) are a bit too close, so I had to move +them. + +.. image:: {filename}/images/hp34970a/front-panel-sockets.jpg + :alt: PLCC sockets for the VFD drivers of the HP 34970A + + +Now I had to try to rejuvenate the tungsten filaments. Like for old +CRT (and even for triodes), the filament, which is responsible for +emiting electrons toward the anode (coated with phosphor) tend to be +polluted and loose their emissive power. A common trick to "clean" +them is to apply a much higher voltage than rated so they shine for a +few seconds. One mst be careful not to burn them, otherwise the VFD is +definitively lost. + +.. image:: {filename}/images/hp34970a/vfd-back.jpg + :alt: back side of the VFD (probably dead) module + +So I tried such a rejuvenation on my VFD module. I wasn't sure what +color the filaments must be heated to (between a light orange to an +almost white yellow). I was doing this using my `Lambda PSU +<{filename}/LPD422FM.rst>`_ raising the voltage while looking at the +filaments. + +The results are very disappointing. The VFD is a little bit brighter, +but the spilling over between digits is now much worse... + +I guess this VFD module cannot be saved, after all... + +So I've been thinking of a plan B, and I think I'll try to replace the +VFD display with a blue 256x32 OLED module. It's smaller (I cannot +find a module near the size of the VFD display), but it should still +be quite readable. Indeed, that requires a bit of work: I plan to +build an arduino-based adapter (using an arduino micor) that will take +place of the VFD drivers (then be fed by the serial-to-shift registers +outputs of the 80C51). One other solution is to replace the 80C51 with +a newly programmed one, but I think reverse ingineering the serial +communication between the front-panel microcontroller and the main CPU +would require much more work. Since it's a 87C51, I doubt I can easily +dump it's ROM... + + +Meanwhile, next step is to take care of all these errors.
