Saturday, December 20, 2014
The Sequencer on the TVS 1 can get out of tune (not scale correctly). There are several areas to check... foremost you can check that the 14v voltage created by the 723 regulator on the sequencer is correct... failures could be around the 1% resistors, the filter capacitors, or the chip itself. Oxidation on the pins that connect the boards together could also be a culprit... the voltages have to be pretty exact for the unit to scale correctly.
Another issue could be with the DAC voltage... it is worth checking it with a scope to make sure it rises smoothly and over the full range of voltage.
The trim pot will also only get you so far... although it is for this purpose, in this case it was too far out of tune for the trim pot to correct the issue.
In the case of this particular unit, the issue was with the capacitors shown highlighted in the image below... these polyester capacitors had started to fail and lost their ability to hold the voltage they are required to hold (it's a type of Sample and HOLD circuit they are involved in). They are charged to a certain voltage with each DAC cycle and they must hold that voltage until the next DAC cycle (which comes pretty quickly). That voltage becomes the CV for the SEMs, so it is important that it is exact (as a mere 10th of a volt significantly changes the pitch. They leak different amounts of voltage for different CV values... hence the scaling issues.
Replacing those capacitors brought the scaling back 100%.
Another interesting point is that you can move the connector (coming out of the sequencer) from the A connector to the I connector, and bypass the quantized voltages coming from the A connector, and just get the analog voltages directly from the pots themselves. You would have to be more careful in adjusting the tuning pots on the sequencer, but you could bypass this problem, and also use this move as a test to see if the analog portion of the sequencer was working correctly.
It turned out the real issue was with capacitor 128... it was leaking and dragging down the voltage at that voltage divider. This caused the KCV (filter control voltage) to come out at 3 volts instead of 5, and caused distortion coming through the filters.
It is rare to see a ceramic capacitor fail, but clearly with an older instrument like this components that are usually reliable can ultimately go bad.
|In red are the OP amp VCC supply for the filter DAC|
|Here is where it is located on the actual PCB.|