|MAX232 chip in MP|
I found out that the 'CAT' port chip a MAX232CWE is not very robust. When testing the End Fed antenna at hi power the computer connection dropped out and this chip is a common failure. The CAT cord has home made and not well shielded for one thing. The chip is pretty direct to the cable on the computer side and they tend to fail in hi RF situations in other types of gear as well. The good thing is that they are inexpensive and I ordered two from DigiKey for about $3.70 each. Some experience with replacing SMD parts is suggested. You will need a jewelers hood and a bright light as well as a small pair of flush cutters, and a small enough iron to complete the job. The chip is accessible with removal of the covers only. No other dis-assembly is required. Unfortunately I did manage to pull some traces off the board due to not being gentle enough in removal. I was able to repair on trace with a single strand of copper from a CAT V wire and I wired two pieces of enamel wire direct to the connector pins. Not pretty but it works. I did not follow the advice of another repairer who suggested using a cutter to remove the chip. I cut one side off but I heated the other side with the desolder gun. Usually I have had good luck removing SMD parts this way but NOT this time! Using a cutter and then removing the pieces carefully is the way to go. The desoldering gun or braid can be used to assist removal of the pieces of leads that are still on the board. When the board is clean and smooth place the chip over the pads and solder it back on using the smallest iron you can get. Pins 8 and 9 pads lifted off the board too but there is no connection to these two pins so that is 'OK' Use the schematic and make sure that all the connections to the new chip are good. Here are some notes I used at the bench for my ohmmeter checks.
Pin 1 and Pin 3 C5011
Pin 2 and Pin 16 C5012
Pin 4 and Pin 5 C5018
Pin 6 C5018
Pin 7 RS162
Pin 8 N/C
Pin 9 N/C
Pin 10 Busy Pin 15 on Q5035
Pin 11 C90 CAT TX 8pinchip
Pin 12 C81 CAT RX 3pin
Pin 13 TX pin 3 RS-232 connector
Pin 14 RX pin 2 " "
Pin 15 GND
Pin 16 C5012
Check all these with ohmmeter to make sure the chip is soldered in correctly and that all the traces are good and it should work. I used no real static protection.
In addition to replacing the chip I also purchased a fully shielded RS-232 Cable with RFI hoods on the connectors and put some 'Beads' on each end of the cable. Not sure this will help but changing the chip is not going to be any easier the next time.
Thanks to Hal for these directions
Dear Paul, Unfortunately for me, I did the same thing on my MarkV,and had to
change the chip. There are web sites that have good explanations on howto
perform the surgery. The chip is a huge SMD with what looks like a billionwires.
The idea is to buy more than one replacement so you feelcomfortable if you screw
up. The good procedure is to clear a table top, put down layers ofterry cloth
towels, get some good high-intensity halogenlighting up above your head and
aimed from two differentdirections, plus a smaller halogen goose neck to getreal
close to the board. The next good idea is to fabricate a chassis ground gromthe
radio frame to the a.c. electrical ground. Then, obtain a static wrist strap and
secure it to thegrounded radio frame. Now you are ready to yank off the covers
to the radio. Start with all the top stuff, flip the radio over andremove t
bottom covers. Have the rear apron close to you and look down intothe radio near
the left edge as appearing upside down. The '232 chip is close to the rear
apron. Tie off as much of the cabling as you can to exposethe chip. Make sure
you identify what direction the chipnumber one lead faces! Use a flush-cut
dikes, ( a good pair, not junk) and cutall the wires off the '232 chip. The pull
the chip body away. Using a needle-point soldering iron like a Weller W60P andan
electronic tweezers, heat and pull up the remainderwires from all the pads. When
done, clean pads with 10 % MEK / 90% Alcohol mixture,and a toothbrush / rag. Use
a magnifying glass to absolutelyconfirm all the debris is gone. This clean up
process may takehours. Go slowly, don't overheat the board. The chip lands need
to bebright and shiny when you're done and the tiny spaces between the landsneed
to be clear of debris. A #11 scalpel lightly applied with blade upis a good way
to shovel this debris, followed by the toothbrush and rag. Tin the iron well.
Tin the lands, one by one with a tiny addition ofsolder to each one. Get yer new
chip, face it correctly and use 1 tiny drop of Barge cementunder it and lay it
down so all the lands and wires line up exactly. Holduntil you feel the chip is
settling into the cement. Wait 1 day. Recover your nerves. Back to it: Tin the
iron's tip well, and do this every lead. Heat every lead so it melts the solder
under it. Be quick about it, which is why the iron is important to beclean.
Inspect carefully. Use the toothbrush, scalpel, magnifier. Removedebris. Yer
done. Button up and continue hamming. HalW4HBM