[sdiy] Davolisint Glide (T1/T2) Problem

Benjamín Velasco guatis at gmail.com
Fri Sep 8 18:36:21 CEST 2023


I'm currently restoring a Davolisint and came across a problem when
activating the "bending/glide" switches (T1 and T2). When either is
activated, the pitch rises more than slightly (~1 semitone). According
to the schematic, T1 and T2 independently switch on and off a
capacitor (.22uf and .47uf) at the base of what seems to be a Sziklai
emitter follower, forming a simple RC network (and thus, creating the
temporary bend/glide effect). This junction is also the voltage
divider of the keyboard notes (which ranges from ~30V to ~10V). The
voltage increase at the base junction is near 100mV when activating
either "T". The bigger the  "T" capacitor, the bigger the voltage
increase when activated and thus the bigger the pitch raise..

I tried replacing the transistors of the Sziklai pair and the problem
persisted. Then I played with the bypass emitter capacitor (0.01uF)
and realised that removing it, or replacing it with a much higher
value (ie 0.100uf) eliminates this voltage increase. Is this an
expected behaviour? I understand that in this case the emitter bypass
capacitor purpose is to shunt to ground anything above DC but maybe
this value by design is too low? Usually i see 1-10uF in amplifier

Any ideas?

Benjamín Velasco Ossandón
Cel. +56 9 96344468
Casa. +56 2 27162400
Oficina. +56 2 29254154
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