[sdiy] ARP Sequencer running when touching

[Antonio Tuzzi]

with the oscilloscope probe...

Hello friends,

I have a nice question for you much more experienced than me...

I'm repairing an arp seq 1611 ( long time n ot orking and stopped...)

I changed a lot of ICs...

I'm finding many faults:
dead led drivers, dead multiplexer, dead nor gates (those pesky CMOS 4xxx!!!!)

one 3904 and 2 3906 (dead all 3)

finally I had all the led working
and the step and reset button working

but the inner VCO is always stopped.

the inner VCO should charge a cap (0.1 µf) and discharge following the voltage
the voltage enter a lm3086 where if the voltage drops under 7.5 volts,
there is a retring of the circuit

the IC 3086 is the marked Z5 on the schematics...

BUT

A)
when I touch the Z5 on pin 4 and 5 (they are connected)
the VCO starts! (and happy leds are dancng!)

Do you know why this is happening ? (I changed 3 3086 in the same place!)

B)
I tried to use a cap 0.47 nF cap (and others) connected to the same
pin and GND (far GND, not the GND connected to the 3086)
and the seq is running again


What I'm doing wrong ?

Thank you
Antonio