OldSynths

--- In oldsynths@yahoogroups.com, "rorymcd@..." <rorymcd@...> wrote:
>
> Hi All:
> 
> I have a Sequential Circuits Pro-One that needs work.
> Cosmetically it is a 9, but when I power it up I get the following:
> 
> 1) Constant pitch to output- no controls affect it

 Check the power supply first - you should be seeing + and - 12 to
15vdc and probably a +5vdc output. Check for ac leakage, indicating
blown or leaky rectifiers and/or filter caps. Higher than normal volts
means regulators that are blown. Use an oscilloscope to determine the
health of the output voltages. Look for ac leakage, glitched DC and 
any high-frequency hash or leakages. Multimeters won't give you the
ability to diagnose power this completely.

 Check your filter, it's either stuck wide open or stuck in
self-resonance. Try removing the input, if that silences it
you may have a defective oscillator as well. Check the filter's
controls, defective or broken pots can cause either condition.

> 2) Gate LED lights on keyboard input or LFO- does not affect output

 Gate circuitry inoperative - check the keyboard outputs, look for 
shorts. Try removing the keyboard input to the gate generator.

> 3) Volume control (Master) does not affect output

 Inoperative VCA or final buffering amp. Check all controls and look
for shorts or opens in the output stages. If it's a CEM/SSM vca chip
you might need a replacement. If the design uses discrete transistors
then a replacement of them might be indicated.
 
> 
> Can anyone here give me some tips?
> I have the original manual (and even the box) so I want to get this  
> unit back up in shape.
> 
> Regards
> Rory McDonald
> OldSynths Moderator
>
 

 Best tip: invest in a good O-scope! The use of a scope can give you 
the ability to track the signal path from source to output; as well
as being able to completely examine the output of your power supply,
the waveforms of the oscillators, the output of the LFO, envelope
generators, and the VCA.

 Cheers!
 -Erik