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Im Moment arbeite ich im Rahmen einer Hiwi-Stelle an einem DC-DC Wandler Design. Das besondere bei der Schaltung ist die geforderte Ausgangsspannung 220 V bei einem Strom von 150 mA. Das sind mehr als 30 Watt und damit wesentlich mehr als bei anderen Entwürfen die ich bisher gemacht habe. Dabei ist mir wieder eine Schaltung eingefallen, die ich das erste Mal auf circuitsathome.com gesehen habe: Jim Williams’ Trigger Probe Amplifier (siehe Linear Technology Application Note 70). Die unten abgebildete Schaltung ermöglicht es mit einer in der Nähe der Spule des Wandlers montierten zweiten Spule ein Trigger Signal zu generieren. Der Aufbau ist dadurch galavanisch von der zu untersuchenden Schaltung getrennt, was die Messung unter Umständen stark vereinfacht. Außerdem kann auch der Stromverlauf durch die Spule auf dem Oszilloskop dargestellt werden.

Leider ist die Schaltung schon etwas betagt, das CA3036 Transistor Array ist nicht mehr zu bekommen und die zwei OpAmps und der Komparator sind alles andere als günstig. Daher habe ich mir einen MyLinear Account eingerichtet und die drei Bauteile als Sample geordert. Große Hoffnungen hatte ich nicht, laut Google ist Linear Technology bei Samples nicht sonderlich freigiebig (im Gegensatz zu Texas Instruments).

Knapp zwei Wochen später dann die Überraschung: in meinem Briefkasten liegt ein Umschlag von Linear Technology. Und tatsächlich, LT hat mir die drei bestellten Bauteile wirklich zugeschickt. Kostenlos und ohne Rückfragen. Im Sample Formular hatte ich genau das angegeben was ich oben geschrieben habe, dass ich die Bauteile für ein Uni Projekt benötige.
Für diesen Service vielen Dank an Linear Technology!
Nach dieser Erfahrung werde ich häufiger LT Bauteile einsetzen.

Nun noch ein paar Bilder vom Auspacken:

I just recently found a defect Panasonic RF-B45 All Band Receiver that had been lying in a box waiting to be repaired. The Panasonic RF-B45 is a great little radio that can be run of 6V DC or 4 Alkaline Batteries. It was actually still working, but could not be turned on via the ON/OFF switch, as it was broken. I disassembled the device and found that the keyboard uses cheap radial tactile switches that could be easily replaced. I ordered 30 new “ALPS” switches from China, reassembled the device and waited.
Finally the switches arrived and I could start replacing them. In the following I will document the necessary steps.

Step 1: Disassembling the receiver

The Panasonic RF-B45 is easily cracked open. Just remove the antenna and then six screws. In the above image I have marked the position of all screws. After the back is taken off the mainboard can be removed.

The switches are on an extra PCB that is connected via a ribbon cable. Remove the connector carefully and then the two screws holding back the PCB. Again, the screws and the connector are marked in the image.

Step 2: Unsoldering the switches

Having the PCB removed one can start the tedious task of unsoldering all switches. You can of course just replace the broken switches, but to be on the safe side I just replaced all of them. One pad lifted from the PCB while I unsoldered the switches, but all in all the PCB endured this procedure quite well. As it is neither plated through nor of FR4 material you have to be careful not to destroy the delicate pads and traces. The metal shield on the back of the LC display covers three switches and therefore has to be removed, too. This is time consuming work but not difficult at all.

Step 3: Soldering the new switches

After all switches are gone clean the PCB. Then start soldering the new switches. This again takes some time. I soldered one leg at first, then reflowing the joint and pressing the switch flush to the PCB. I then soldered the second leg. After finishing all joints cut the legs, drown them in flux and reflow them again. Do a short inspection under magnification and make sure that all joints are nice and shiny. Then clean the PCB again.

Step 4: Assembling the device and testing

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Now you can put the receiver back together again. Fasten the two screws fixing the PCB and then carefully connect the ribbon cable connector. Put the mainboard back in the front part of the case and put the back on. After all screws are fastened and the antenna is mounted the receiver should be working as normal.
Congratulations, you have rescued one more device from going to the landfill.

These days I swiped the Windows installation from a Shuttle XPC system and set it up with a clean install as I plan to give it away. While I was cleaning the mainboard from dust, I found the single SRAM module to be sitting in the slot DIMM1. On most boards a single stick of RAM is expected to be put in slot #0, so I swapped the module over to the first connector. I then updated the BIOS and installed Windows XP SP2, but not without difficulties. The installation would not finish with an AGP graphics card connected, and even after I removed it the network adapter was neither recognized by Windows nor a Linux system.
Downgrading the BIOS and clearing the EEPROM did not help, so in an desperate attempt to restore the LAN adapter I plugged the RAM module once again in slot #1. And you know what, Windows boots up and recognizes the adapter just fine. This is not mentioned in the mainboard manual and installation guide so I had to find out the hard way.
So if your Shuttle FS51 v2.0 (aka SS51G) acts weird try to change the RAM slots.

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Update 25.08.2011
Well, as it turned out the RAM wasn’t the problem. After I reinstalled Windows XP on the machine the network card did not show up in the device manager. In a desperate attempt to restore the network connection I measured the BIOS backup battery and voilà, it was empty (down to 1.5V). I replaced it with a fresh one and the network card magically appeared in the device manager. Everything seems to be back to normal now so I can finally finish the installation process.

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Update 27.08.2011
The network adapter disappeared again! A CMOS clear brought it back for some time, but then it disappeared again. The CMOS clear should be done after every BIOS update by moving the on-board jumper to its second position and back after several seconds – lesson learned. After checking the power supply (all good), the caps on the mainboard (also good) and a lot of hair tearing I finally came back to the BIOS settings. I hadn’t changed the “Reset Configuration Data” switch before so I gave it a try. And the network adapter finally came back. It even stayed after I changed back the switch to Disabled. I don’t now how long it will take until my RTL8139 goes back to its happy place but for now I’m confident that I solved the problem.
I read about the meaning of this BIOS setting and what the Extended System Configuration Data (ESCD) is, and it seems plausible that this was the problem. Time will tell…

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Update 28.08.2011
I give up. The network card has been and gone. Under the screen of night it left and I have been unable to bring it back. From the looks of it something is seriously messed up with this PC, but I won’t bother even further.

R.I.P.
RTL8139
2003 – 2011