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The voltage at both bases of Q10 and Q11 should be very close to ground potential and
the emitters should be at –6V. The voltage drop across R23 and R26 should be around
50mV. Both drops should match within +/– 20%. If all above is not correct check Q10,
Q11, R23, and R26.
The voltage drop across R4 and R6 should be 1.5V for both devices +/– 20%. If voltages
not matched, or if they are higher or lower, verify values of R4 and R6 (3.3K), R5 and R9
(243 ohm), and check for open or shorted elements in Q10, Q11, Q4, or Q7.
2. Upper Class A Drive
Verify that voltage across R9 equals 870mV +/– 20%. If incorrect, check Q7 for open or
shorted elements.
Check the voltage at the base of Q6. This should be half way between the positive supply
voltage and the output node. V(base Q6) = (39 +V (output))/2. If the output is offset –10V
the voltage at the base of Q6 will equal (39 –10)/2 = 14.5V. If incorrect, check values of R8
and R10 (33K). If R8 and R10 are correct, replace Q6.
3. Level Shifter
Verify that voltage across R5 equals 870mV +/– 20%. If incorrect, check Q4 for open or
shorted elements.
Verify that the voltage at the emitter of Q3 is .6V. If incorrect, check Q3 and Q4 for shorts
and opens.
Verify that the voltage across R29 and R5 are the same (+/– 10%). If not check values of
R5, and R29 (243 ohms). Verify that D9 isn't open, shorted or reversed. If the above
checked OK, verify value of R21 (243 ohms), and check Q9.
4. Lower Class A Drive
Verify that voltage across R21 equals 870V +/– 20%. If incorrect, check Q9 for open or
shorted elements.
Check the voltage at the base of Q8. This should be half way between the negative supply
voltage and the output node. V(base Q8) = (–39 +V(output))/2. If the output is offset 10V
the voltage at the base of Q6 will equal (–39 +10)/2 = –14.5V. If incorrect, check values of
R20 and R25 (33K). If R20 and R25 are correct, replace Q8.
Remove jumper across bases of Q10 and Q11. Re-solder lifted end of R12 back into
board. Turn Generator A on and Power up board. Verify that the output is centered and
undistorted. Turn down oscillator and re-connect 4 ohm load. Bring up oscillator level and
verify that the output is centered and undistorted.
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