more midi2cv...

[Daniel Gendreau]

> The midi2cv works ok, with one exception: it seems like the 8 ouputs bleed
> into each other _far_ more than they should.

BINGO! I remember this one! It was really annoying too.

One or more of the 4 transistors(Q4-Q7) that control the 4051 mux gate/ports
is probably zapped.

CAUTION: The following is very precise work. Be extremely careful NOT to
short adjacent pins together! Proceed at your own risk. I zapped several
transistors and a pin on the 8031 CPU by not being careful enough! Learn
from my mistakes please. :)

Step #1 ---
Use a logic probe or an oscilloscope to measure the 4 control signals at the
mux pins. Note: A VOM will simply not do here. You must be able to detect
logic pulses. Check the following:
IC13:8  Mux Inhibit: Should be a square wave at about 6400Hz (800Hz Mux
refresh rate * 8 outputs)
IC13:11 MuxA: Should be a square wave at about 3200Hz
IC13:10 MuxB: Should be a square wave at about 1600Hz
IC13:9  MuxC: Should be a square wave at about 800Hz

The Hz amounts are from memory, so I could be wrong. The point is, there
should be pulses on all 4 lines. If they are stuck at logic 1 or 0 or they
float between, something is wrong with the signal.

Step #2 ---
If a signal is stuck, proceed to measure The CPU MUX control output pins:
IC3:11 CPU Mux Inhibit:
IC3:12 CPU MuxA:
IC3:13 CPU MuxB:
IC3:14 CPU MuxC:
These signals should be the same (though inverted) as those reaching IC13.

If a signal is stuck at IC13, but its okay at IC3, this is good news. You
only have to replace the transistor for that signal. But if its also stuck
at IC3, this is bad news, as you will have to replace IC3 (the CPU!) because
the CPU output pin has been zapped. By the way, you dont have to order a
replacement transistor from Paia. you can just pick up a standard NPN
transistor from radio shack (cant remember the part number right now).



You gotta be really careful with transistors:
#1 They are EXTREMELY touchy about being shorted. Its really easy to
accidentally short one out with an oscilloscope or logic probe and Poof! its
dead. Or worse yet, it could be in some freaky zombie state where it works
partly, but not the way it should.

#2 Be VERY careful about heat when soldering them. I usually clip a little
metal a heat sink across the transistor's pins on the component side, and
then quickly solder it from the solder side. This helps draw off some of the
excess heat.

#3 You must observe Electrostatic Discharge precautions when handling
transistors, or any semiconductors for that matter. Walking around on your
carpet in your socks while holding a transistor in your bare hands is just
asking for trouble.


Hope this helps. :)
-Dan G.