ACE-Bass circuits Power amplifier 2. TROUBLE SHOOTING TIPS B2.70 Does not work Fuses F501, F502 blow Automatic ON/OFF faulty B2-70 oscillates 3. ADJUSTMENTS Bias current in Power Amplifier Negative output resistance 4. SPARE PARTS 5. ENCLOSURES Circuit diagram 55 21295-01...
( 10-100mV ) on its output terminals under operating conditions. If the B2-70 is tested without any load on the Power Amplifier, or with fuses F501, F502 removed, the balance between +15V and -15V may be upset during on/off switching. In such cases the operatio- nal amplifier may start oscillating when power is disconnected, so that automatic ON is actuated.
This circuit is powered via diodes D201, D202 and capacitor C201. The power transformer and the Automatic ON/OFF circuit are powered at all times when B2-70 is plugged into a live outlet. The other circuits in B2-70 are powered only when relay K101 is actuated.
1.4 Filter The signal from Z301 pin 7 (test point C) passes a variable low pass filter including R401, R402, R403, C401, C402, and Z401 (9,10,8), and then on to a fixed high filter with kneepoint 20.8Hz. The output terminal is test point (E). Remaining components in the filter circuits are part of the ACE-Bass amplifier, and is described in section 1.5.
V510 and V511 filter the supply voltages to the input stages, removing hum and transients. NOTE: If there is a"click" in B2-70 when it is switched on, the cause could be a short circuit in one of the tran- sistors V510, V511.
2. TROUBLE SHOOTING TIPS Use J07 or J08 as common point for measurements of DC or AC voltages. 2.1 B2-70 Does Not Work Set push button switch in ON position. 2.1-1 Check that LEDs AUTO and ON are lit. 2.1-2 Check power fuse and fuses F501, F502.
2.3-3 Shunt R210 by 100k ohm shorten off - delay. Check that ON is actuated when signal from generator exceeds 0.5mV at 500Hz. 2.4 B2-70 Oscillates B2-70 Oscillates between ON and OFF with no audio signal on input. 2.4-1 Check DC offset at test point (G). Should be between +10mV and +100mV.
(no signal on input terminals of B2-70). Adjust trimpot (R525) till voltage reads 10-15mV. (This corresponds to a bias current of 50mA). Make final adjustment after B2-70 has been ON for a few minutes, so the bias current has reached steady state value.
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NOTE1:Voice coils of drivers must be at room temperature when Rrs is set. NOTE2:Voltages in 3.2-3 and 3.2-4 are correct. Negative output impedance of ACE-Bass amplifier makes output voltage larger when amplifier load increases. 3.2-5 Reset jumper in "RUN" position. NOTE: Do not touch trimpot Ra (R410).
4. SPARE PARTS Spare parts for B2-70 can be obtained from distributors and service centers. When ordering spare parts, please refer to component number per diagram 55 21295-01.
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