Moog Music Forum

Hi all,

seems like my DFAM´s sequencer is somehow faulty.
After hitting Run/Stop it keeps playing the first step only and repeats it.
Also the Advanced button does not make the sequence to go further.
Same with using a external signal on the Adv/Clock input.
Any suggestions or do i have to send it for repair?

Thanks for any advice
Marxon

EDIT
In meanwhile I disassembled the DFAM to investigate the problem.
Maybe my electronic knowledge is enough to fix it.

I have made some progress with my troubleshooting but couldn’t fix it yet.
Here is what I figured out:
The eight sequencer leds are controlled by an 8 channel 74HC4051D multiplexer and two 8 channel DG408 multiplexer switches are used to route the 2x8 potentiometers.
First I checked the control inputs of all three multiplexeres while the sequencer was running and as might be expected: 0V.
But what’s the reason?
As it turned out, the multiplexers are controlled by an 74HCT393.
It is a dual 4-stage binary ripple counter.
Supply voltage, clock signal, reset pin, everything seemed to be OK
but no reaction from the 74HCT393. So this should be problem!
But hen I realized on my oscilloscope, the clock signal HIGH level
was only about 2V! I quickly desoldered the clock input pin from
the board, connected it to a signal generator: Voila!
The sequencer is stepping forward. Now I need to figure out what causes the voltage drop
Measuring the DFAMs trigger output shows instead a correct 5V level signal.
The source of 74HCT393’s clock signal is small 6 pin ic (maybe sot-363)
labeled with „TK“. The smd code reference gives my over 40 entries!
This will take some time for indentification.
I will keep you updated…

Best regards
Marxon

I'd start checking the patch bay, and my first suspect would be the tempo patch point.