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1、? ? RadioFans.CN 收音机爱 好者资料库 ? ? ? ? ? ?! ?#?$?% ? see power supply troubleshooting procedure. 2)LEDs work but no output from power amp: Could be preamp or power amp problem. First, turn power switch off for 3 seconds, then back on without input signal to clear shutdown feature. Set all knobs to 10 a
2、nd put in a signal at 5mVrms. If there is still no output, look at either end of the red and white wires going between the preamp and power amp with a scope probe. If there is a signal there but no output, then there is most likely a problem with the power amp. If there is no signal, the problem is
3、in the preamp. Check the wires to make sure there is a good connection at either end. 3)Distortion at the Output: Follow procedure from #2 above (may want to start with output level on 0 and gradually increase it). This time, note whether the signal is a clean sine wave like the input, or if it is c
4、lipping off part of the signal or has any distortion. If the signal is clean, the problem is most likely from the power amp. If the signal is cut in half, check the 15V supplies (any LF353- pins 8 no setup of the power supply is necessary, since the characteristics of the transistors are very unifor
5、m. When replacing BUKs, cut off some length on the leads and flair the outputs slightly. Always install the BUKs with the board firmly in place to assure proper alignment of the heat sink holes. When bringing up a power supply that has a blown power amp, it mat be necessary to disconnect the outputs
6、 using the clips mentioned before (see Power Amp-Ohm check of Outputs). Oscillator Circuit Visually inspect U506 for cracks. Whenever replacing IC s, it is always a good idea to install the new piece in a socket. Check that U515 is OK by measuring diode drops from base to emitter and collector. R509
7、 sometimes opens up under severe power supply failure, check that it is low ohms. After failed parts are replaced, bring the supply up slowly with the variac as noted in the Start Up Procedure. Microamplifier Series Circuit Description of Output Disable GK Document # 450-0062-01 A. Mihalka, 2-13-91,
8、 Rev. 9/10/99-SW Scope This document describes the shutoff feature of the microamplifier series. Some boards may be stereo, but all boards work on the same principle. There are 4 conditions whereby the output is disabled, each of which are described below (see Circuit Descriptions). Disable Modes Po
9、p Elimination with Power Switch The first type of output disabling is for the elimination of speaker pops when the unit is either turned on or off. In the case of turn-on, the output is clamped for a couple of seconds and then released. At turn-off, the output is disabled and clamped until the entir
10、e power supply decays. This takes about 10 seconds, after which a low thump may be heard if listening close to the speakers. Output Overload The second type of output disabling occurs when an unsafe output operating condition exists. A circuit senses overload conditions by comparing output voltages
11、and current. The unit is able to drive 4ohms per side stereo or 2ohms mono without tripping off. Some conditions that will cause output disable are shorted cords, blown speakers, heavy loads, and plugging into the output jack while the unit is on. If an overload condition is detected, the outputs wi
12、ll be clamped indefinitely. In this condition, the LEDs on the front panel will remain on, and the preamp and its outputs will function, but no output is heard. The power switch must be turned off for 3 seconds before the unit can be turned back on and again produce output power. Both outputs of a s
13、tereo unit are clamped if the condition is detected in one side. Thermal Shutdown Finally, if the heat sink becomes too hot the unit will shut off. This is also considered an unsafe operating condition and will disable the outputs indefinitely. A circuit senses the temperature of the case of the out
14、put transistors. Both sides of a stereo unit will be disabled if one side detects a problem. The power switch must be turned off for 3 seconds before output power is again enabled. Circuit Descriptions Output Disabling The outputs are disabled by Q648 and Q647, an A06 and A56 respectively, which are
15、 either both on (disabled) or on (enabled). When on, these transistors effectively clamp the output drivers. When off during normal operation, the base of Q648 should be low and that of Q647 should be high. The bases are driven by U605 (LM324), a low speed quad op amp. If the unit has no output, U60
16、5/7 will be low and U605/14 will be high. If these points are not OK, check the power supplies for U605. Pin 4 should be should be about + 5.3V and pin 11 5.3V. Q722 and Q723 clamp the other side of a stereo unit. Turn-On Pop U605 is powered from the rails by 6.8K resistors and 5V zeners. This IC st
17、arts operating at about q3 volts. When the unit is powered up, pin 5 follows the + 15V on its way up to 3.14V. Meanwhile, pin 6 is climbing up to 4.8V, but is slowed by C821. Therefore, the output of the op amp, pin 7, is initially high, disabling the outputs. Note that pin 14 is an inversion of pin
18、 7 and drives the opposite polarity transistor. After a delay the voltage on pin 6 exceed 3.14V, forcing pin 7 low and enabling the outputs. Turn-Off Pop Pin 5 is connected to pin 3 and is 3.14V. Pin 2 is 3.05V. Note that these voltages are obtained with 1% resistors, plus a 470j to give a higher vo
19、ltage on pin 3. The output pin 1 is always high when the outputs are enabled, and D802 is back-biased to enable pin 6 to function. When the unit is turned off, the + 15V supply drops and pin 3 follows, but pin 2 lags because of C803, a 10uF cap. This causes pin 1 to go low, forward-biasing D802 and
20、pulling down pin 6. Also, R801 provides feedback to clamp pin 3 low and keep Turn-Off Pop (contd.) it there. All this causes pin 7 to go high, disabling the outputs. This condition persists for many seconds until the rails can no longer power U605 through the zeners, at which time a very faint, low
21、rumble is heard. Output Overload The overload circuit is developed around U604, another slow, quad LM324 op amp. The voltage and current are summed to determine if a safe operating point exists. The unit is allowed to operate down to 2 ohms total load (4 ohms each side). When an additional 8 ohms is
22、 paralleled, giving 1.6 ohms, after a second or two the unit shuts off indefinitely. This protects the outputs from shorted cabinets and from driving too many speakers. This condition may also occur while the speaker jacks are moved in and out. The following circuit works for positive waveforms only
23、. The voltage is obtained from a divider on the output, R784 and R777. Current is derived from R604 and R605, and is inverted and amplified at U604/1. These signals are now out of phase and subtract from each other at pin 9. When current is too large for a given voltage, pin 9 will tend to go more n
24、egative. This is because pin 1 is negative during a positive waveform excursion, and large loads keep the voltage from going high and demand more current. When pin 9 goes lower than the 2.5 2.65V reference on pin 10, the output pin 8 will go high. D766 is now forward-biased and starts to charge up C
25、741, a 10uF cap, through R743, a 220K. When this unsafe condition persists long enough to charge C741 and turn on Q742, U605/3 is momentarily pulled down. This causes U605/1 to go low, and R801, a 100K, provides hysteresis to keep pin 1 low by providing enough current to keep pin 3 lower than pin 2.
26、 The outputs will now be turned off indefinitely. The unit must be turned off for 3 seconds to allow the system to bleed down before output power can resume. Since the overload sensing circuit is not made from precision resistors (current sense resistors are 10%) and is somewhat sensitive, occasiona
27、lly a 1K resistor must be paralleled with R777. Also the original boards did not have a spot for R741A, which bleeds C741. Without this bleederC741 may charge up slowly through excess solder flux around the pins of Q742. Whenever output is inhibited for no apparent reason, check Q742 and find out wh
28、ich of the diodes feeding its base through R743 is responsible. Thermal Overload The final method of output disable is provided by U602, which happens to be a fast LF353 dual op amp, although there is nothing fast about the circuit. Q807 is situated on top of one of the output transistors. As Q807 h
29、eats up with the output, pin 2 will drop from about 2.75V. Pin 1 is low until pin 2 drops below the 2.06V reference on pin 3. Pin 1 will go high and stay high enough for the outputs to be disabled by the same mechanism using Q742 described above. Once again, the LEDs will remain lit and the preamp w
30、ill continue to function while the output is off. During this time the heat sink will continue to cool. To resume power at the instant of shutoff, not only must the power switch be turned off for 3 seconds, but the outputs must cool off as well. Q782 senses the temperature of the other channel in a
31、stereo unit. Defeating the Disable Circuit There will be situations where the auto shutoff feature of the microamplifier will be triggered by the noise from other equipment that is either connected to the same AC line or is operated in close proximity to the amp. An easy way to prevent this circuit
32、from tripping is described. This method also disables the thermal trip circuit; however the thermal trip circuit does not fire under most operating conditions. Procedure: Remove the bottom cover of the amp. Look through the side of the amp with the heat sink down and the front panel on the left. On
33、the left side of the lower circuit board you will see an MPSA06 transistor labeled Q742. Directly to the right of Q742 and also on the board edge is capacitor C741. To defeat tripping, solder a wire across C741. Microamplifier Series Lead Preamp Circuit Description GK Document # 450-0063-A Preamp Bo
34、ard #s- 206-0063-E treble to 0- Fig.3. B)Hi-mid to 10- Fig.4; hi-mid to 0- Fig.5. C)Lo-mid to 10- Fig.6; lo-mid to 0- Fig.7. D) Bass to 10- Fig.8; bass to 0- Fig.9. BOOST / SHAPE TEST 1)Engage Channel select switch to go to lead channel. 2)Change scope-B to 5V/cm 1ms/cm. 3)Increase shape to 10- comp
35、are to Fig.11. 4)Increase boost to 10- compare to Fig.12. FOOTSWITCH TEST 1)Connect RFG2 to footswitch jack (J6). 2)Engage switches on RFG2 so that both LEDs are lit. Lead channel LED should also be lit. 3)Output should look like Fig.12. 4)Disengage boost switch on RFG2-compare to Fig.11. 5)Disengag
36、e channel switch on RFG2- compare to Fig.1 (change scope range to 2V/cm, if necessary. Clean channel LED should light. 6)Remove footswitch. RETURN STEREO/MONO TEST 1)Set AC-VM to 10V scale. 2)Set oscillator to 100Hz sine wave at 26dBV (50mV). 3)Insert plug from oscillator halfway into return jack (J
37、3). 4)Set output level to 10. R Channel = 7.1Vrms (stereo). 4V for mono amp. Scope switch should be set to load B. 5)Push in stereo/mono switch (S2); signal goes away. 6)Disengage stereo/mono switch; signal comes back. 7)Push plug in the rest of the way. Signal disappears again on stereo unit, measu
38、res 4Vrms on mono unit. 8)Flip scope switch to down- L Output = 7.4vrms. 9)Engage stereo/mono switch: L Channel output = 7Vrms. NOISE TEST 1)Remove oscillator input. 2)Turn all front panel knobs to 10, switches out. 3)Connect speaker to Left Output (switches out) and listen for noise, should be smoo
39、th with no crackling. 4)Change AC-VM range to 1V. 5)Noise should measure less than: 400mVrms, 500mV-mono. 6)Change speaker to Right Output (switch up) and listen for noise. 7)Also change scope switch (up) to measure noise in R Channel as 400mVrms. 8)Engage channel select switch to change to lead cha
40、nnel. 9)Noise in speaker should be smooth, with no crackling and should measure: 250mVrms, 400mVrms-mono (on AC-VM). RETURN KNOBS TO ZERO- READY FOR BURN-IN Microamplifier Series-Lead Effects Preamp Turn-On Procedure GK Document # 420-0063 A Model #s: ML120E, ML260E- All options Board #: 206-0063-E
41、(ML/E preamp) 5/18/90 Rev.9/15/99-SW NOTE: Italics refer to stereo power units. SETUP 1)Power switch off- Connect power cord. 2)Connect power 3-pin connector from power amp to P1 on board. The colors should read (bottom-top): Black, Violet, and Brown. P1 is closest to front. 3)Connect signal 3-pin c
42、onnector from power amp to P2 on board. The colors should read (bottom-top): White, Black, and Red. The colors are clearly marked on the board for both connectors. 4)Connect outputs to load box- left to load-A, right to load-B. 5)Resistance loads open (switch in center). 6)Load box to scope-B and AC
43、-VM. 7)Set scope switch on load box to look at load-A (down). 8)Probe (1:1) to scope-A and DVM. (No gnd lead required.) 9)Connect oscillator to input. 10) Oscillator on 100Hz sine wave 1.6Vrms (+4dBV). 11) DVM on 20V DC range. 12) AC-VM on 30V range. 13) Scope-A on 5V/cm, Scope-B on 10V/cm. 14) Scop
44、e sweep on 1ms/cm, trigger on A. 15) Boost, gain, lead master, shape, and output level to 0. 16) Clean volume, tones 10, all switches out. 17) Reverb, rate, and depth to 0. INPUT SETUP AND LEAD CHANNEL TEST 1)Turn on power switch- clean channel LED only should light. 2)Engage channel switch- lead ch
45、annel LED only should light. 3)Look at U1/1 with probe. Adjust R5 (1k trim pot) until signal hits the rails on both sides evenly. Should be about 22Vp-p. 4)Look at drain of Q50 (square pad-J113): +7.3Vdc. 5)Adjust R67 for 4.0-4.5Vdc on drain of Q67 (J113). 6)Adjust R87 for 5.8-6.2Vdc on drain of Q78
46、 (J113). 7)Increase gain to 10 and watch the signal become a square wave- it should clip evenly and reach about 9Vp-p. 8)Decrease oscillator level to 66dBV (.5mVrms). 9)Change DVM to AC (20V) scale. 10) Adjust R86 (50k trim) for 2.0Vac at drain of Q78. CLEAN CHANNEL TEST 1)Change scope-A to .5V/cm.
47、2)Disengage channel switch so that the clean channel LED lights. 3)Set oscillator to 46dBV (5mV) at 100Hz. 4)Look at tip of send jack (J7) with probe: 0.47Vrms. 5)Engage compressor. Send = 0.94Vrms. Disengage compressor. (Monitor both signals to make sure they are clean sine waves. 6)Change scope tr
48、igger to B. 7)With output level at 0, there should be no output. Gradually increase output level and it should hit the rails at about 1 oclock and = 30Vrms. 8)Flip scope switch to load B (up) and see that it also is 30Vrms and hits the rails at 1 oclock on output level. 9)Look at tip of headphone ja
49、ck (J5) and continue increasing output level. Headphone output should be about 4.4Vrms with output level at max (may not even hit rails with output all the way up). 10) Repeat 9) looking at ring of headphone jack. TONE CONTROLS TEST 1)Set front panel knobs to center (12 oclock) except: boost, shape reverb, rate and d
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