[volt-nuts] 3458A Questions - The Sequel
wpgold3637 at att.net
Sat Oct 29 22:35:36 UTC 2011
My favorite tool for desoldering devices from a PC board is the Edsyn Soldapullit Desoldering Tool. I have used this and ones like it for over 30 years. You can get these at MCM Electronics and many other electronic tool suppliers.
I use this with a Weller temperature controlled soldering station using a PTA7 1/16 inch wide tip to heat the solder on the device pin. This is a less "messy" process than solder wick. But it does require some skill to get 28 leads desoldered, loose from the holes and pads and then remove the device without damaging the PC board. I first heat the pin until the solder becomes liquid and then trigger the desoldering tool, which sucks the solder into the tool. I repeat this process for all pins and then examine each pin to see if there was any solder that didn't get removed cleanly. I repeat the process for those pins that need further attention. Then I check each pin with a small pair of needle nose pliers to make sure that each pin is loose and can be wiggled in the PC board hole. If a pin seems still soldered to the hole I will reheat the pin and get it to move to the center of the hole so that it will not stick to the plate through. Then I apply a small screwdriver or lifting tool to the device and apply very gentle upwards pressure to see if the device is loose. If necessary I again check that all pins are loose. When done correctly the device will lift right out of the PC board.
Be very careful when "prying" the device out so as not to damage the PC board hole plate through. A plate through is the metal connection between the top and the bottom of the PC board, through the pin hole, and also connects any inner layers of a multilayer board to the hole. I would suggest that you use a scrap PC board to practice on to develop your technique before working on your 3458A boards.
I have two 3458A meters right now. One has a single PROM and the other has 6 PROMs. Both return the same version of the firmware when queried, version 7. From what I see in Agilent service note 3458A-12C there is very little reason to replace version 7 with a newer version unless you are replacing the A5 board, or experiencing a specific problem as noted. I have removed the NVRAMS in both my 3458A, and the VFD display on one of them using this technique as well as thousands of other IC devices over my career.
I have also seen others simply "butcher" a PC board beyond repair when not using this technique correctly. I have seen so called technicians use a 1 foot long screwdriver and apply brute force to pry the device out along with the plate throughs and PC board traces.
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