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9700 VCF hot transistor
http://www.paia.com/talk/viewtopic.php?f=8&t=226
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Author:  PAiA-Scott [ Wed Aug 18, 2010 2:10 pm ]
Post subject:  More tests, 9710 L and R VCAs

Before sending the last reply, I'd made some notes for a method of testing this 'upper' section of the 9710 using DC voltage as the input, but thought it was important to get the last reply out to you first.

These tests use two alligator-clip test-leads in addition to any that might be used to hold the tester probes attached to the 9710. Reference the 9710 assembly manual illustration supplement pages with figures 6, 7 (schematics), 3 (panel wiring), and 1 (printed-circuit-board parts locations).

One lead will jumper a 5v reference voltage node over to the L or R VCA input and the other will jumper voltage over to the gate input to keep it active and the ADSR on or held in its sustain phase/state.

The circuit node for the 5v dc reference includes IC7pin7, R65 and C14 and it will probably be easiest to clip on to the '5' end of R65.

This same 5v node (or the '4' end of C14), or, the top (near point B) end of R1 (V+)) can be used to gate the adsr, when testing for output of the normalled L+R to VCA A.

Start with the 5v jumped to the R VCA input since the the L has the Noise Source normalled to it and it would take a plug in this input to interrupt the link.

Measure for DC voltage at the R VCA output, J11. With the Pan control ccw the voltage should be zero, at mid-setting 3.7, and with Pan cw 6.8v. The increase is from gain through the vca to compensate for loss through the following passive-mix.

The DC voltage at the L+R, J12-T should be range from 0 to 3.3 as the Pan control is adjusted from L to R (and about 1.8v at the mid setting).

Gate the adsr with the jumper from +5 or V+ to the Gate-trigger input, J9-T and the DC voltage on the L+R output should appear at the VCA A output (J13-T).

If you notice the voltage reading here to be slowly dropping. don't worry, it's OK. When the adsr is first gated, the reading will be at a maximum value but over about 30 seconds will drop to about half. When gated by V+ it will be more than for a 5v gate (maximums of 7v and 4.5v). This is a short-coming/compromise and for the normal or usual use of 'framing' a sound for a key-pressed on the keyboard, it would not be noticeable (a simple upgrade to minimize the effect is the substitution of a TL 084 op-amp for the LM 324 op-amp at IC7).

Moving the 5v over to the L VCA should provide similar results, but the input will need to be via the tip of a plug into this jack/socket and the output of the L VCA measured at J10-T.

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