Advertisement
verify 750mV DC (+/– 100mV) across them. Check for correct values of R2 and R25
(0.1 ohms, 3W) and verify 5mV (or similar to bias setting) across them when board is
powered. If any of the above is not true proceed to "Power Amp Test".
G) Amplifier Muting Test
1. Power up board and measure voltage at junction of D18, R43, and C24. The voltage
should be around 46V DC. If voltage is negative check polarity of D18.
2. Power up board while monitoring collector of Q19. Voltage should ramp from zero to
around –8V in around 2 seconds and eventually –15V. If collector remains close to zero
check: for collector to emitter short on Q19; for incorrect value R45; and for shorted SW1.
3. Interrupt power while monitoring collector of Q17. The voltage should quickly change to
+15V and eventually to zero volts. If it doesn't check C25 and Q19.
4. With negative voltage applied to pin 6 of U1B, pin 7 of U1B will be at 15V. If not, check
for short or open traces around pins 5 and 6 or replace IC U1.
H) Power Amp Test
1. Amp pulls excessive power when board is connected.
a. Verify correct values of Q1, Q2 (2SC3201), Q3 (2SD1763A), Q7 (2SB1186A), Q8, Q9
(2SA1302). Look for possible shorts in the areas where these parts are. All the above
parts form the "Current Amplifier" (Providing current gain). Everything from Q10, Q12 and
before, form the "Voltage Amplifier" (Providing voltage gain).
b. If output voltage is at ground potential (test point 4, TP4), and output leg is not shorted,
there could be a problem with the bias reference. Check transistor Q5 (2SD1763A), make
sure base is not shorted to emitter, check resistor R15, R14, R13 and potentiometer R17.
c.
If the output voltage is offset (+LF test point) from ground there could be a problem in the
voltage amp. Jumper across base of Q3 and Q7 and then bring this point to ground. If the
output voltage is close to ground potential, remove jumper and check transistors in the
"Voltage Amplifier" area, Q10 - Q15. If the voltage does not center, then the problem is in
the current amplifier. Check for shorts on all elements (base to emitter, base to collector,
collector to emitter) of Q1, Q2, Q3, Q7, Q8, and Q9.
2. No output from the amp when driven.
Verify signal at base of Q17. If there is no signal check the low frequency filter section
(U2A, U2B and U1A). Verify that junction of R41 and pin 7 of U1B is around 15V. If not
close to 15V refer to "Amplifier Muting Test". If all of the above is OK check the "Voltage
Amplifier" area.
3. Amplifier AB Crossover distortion in output.
38
Advertisement
