Sequential Circuits SIX TRAK : problem on parameter value setting

[YannLu]

Hello,

I have here a SC SIX TRAK for repair.
When I turn the value pot, the max value is already reached when the pot is turned 3/4 clockwise. For the max values higher than 15, the reached max is only 30 (instead of 31), 60 (instead of 63) and 121 (instead of 127).

I first thought of a calibration issue but there is no trimmer for this section or maybe I need to replace the pot ? I will appreciate your help !

Thank you !

Yann

 

[zamproject]

Hello

I just have a look at schemo

There is a 4051 that seems to MUX all pot, so if at the wiper (or U108 pin 15) you have clean 0-5V range it's at digital side...
But if it's only one pot that have data read problem, it could be the pot.
As a start, easy to check...

Best
Zam

 

[YannLu]

Thank you Zam

At the counterwise end I have 4.28V it seems normal regarding the voltage divider R 2K + POT 10K...

 

[YannLu]

I swapped pots : same problem
I directly applied 5V on the pin 15 of the 4051 : same problem.
For example Max value on parametre 19 is 122 (instead of 127)...
Where would it come from ? The 4051 itself ?

 

[zamproject]

I swapped pots : same problem
I directly applied 5V on the pin 15 of the 4051 : same problem.
For example Max value on parametre 19 is 122 (instead of 127)...
Where would it come from ? The 4051 itself ?

Well... I never repair this one...
As say I just have a quick look at shemo, there should be a DAC somewhere.
Scope the data/multiplexed lines etc...
maybe it's just a feedback issue ?
Unfortunately I can't help more  :-\
Best
Zam

 

[YannLu]

Thanks dude !

I think now the issue is coming from a part of digital circuitry  :-\

 

[Monte McGuire]

You might have already done this, but check all of the power supply voltages first before you dive too deep. I worked on a Prophet 600 recently and the PSU regulators had some pretty iffy mechanical design that encouraged them to fail. These are old enough to be from the days of "poorly applied 78xx and 79xx regulators", so they can be fixed easily, but their haphazard mechanical design does not bring confidence. The 600 I worked on was a later revision of a clearly bad mechanical design that obviously failed enough to cause a design ECO, also carried out in a kludgey manner. Shotgun tantalum cap replacements and a lot of heatshrink tubing rendered the PSU safe enough for my tastes to be applied to the unobtanium ICs in the device itself. Best of luck, and proceed cautiously!!

 

[YannLu]

You might have already done this, but check all of the power supply voltages first before you dive too deep. I worked on a Prophet 600 recently and the PSU regulators had some pretty iffy mechanical design that encouraged them to fail. These are old enough to be from the days of "poorly applied 78xx and 79xx regulators", so they can be fixed easily, but their haphazard mechanical design does not bring confidence. The 600 I worked on was a later revision of a clearly bad mechanical design that obviously failed enough to cause a design ECO, also carried out in a kludgey manner. Shotgun tantalum cap replacements and a lot of heatshrink tubing rendered the PSU safe enough for my tastes to be applied to the unobtanium ICs in the device itself. Best of luck, and proceed cautiously!!

You are probably right : I measured the psu voltages: +5V and -6.36V (instead of -6.5V)
I am wondering if the issue could be from the LM337 providing the negative supply...

 

[YannLu]

Well it was from the comparator LM311 : I replaced it and it's working again !