BBK AB217 Service Manual
               | |
7.1 Power amplification sectionThe R channel signals are coupled by R401 and C401 and sent to the base of differenceamplification section V401. V401 and V402 comprise the difference amplification circuitof single input and output. The sound signal is sent from the collector of V401 to the baseof the voltage amplification section V404. The amplified signals reach the compound poweramplification section. V403, V405, VD401 and VD402 constitute the mirror image constantcircuit. VD401 and VD402 provide a constant base current to V403 and V405. The emitterresistor of V403 determines the working current of the difference amplification section andthe V405's emitter resistor determines the working current of the voltage amplification section.V430, V407 and V409 constitute the upper tube (NPN) of the compound power amplificationsection. V430 and V407 are first parallel-connected to function as a triode (To raise the power)and then compound V409 to constitute a NPN type compound tube (To make amplificationmultiplied). V431, V408 and V410 constitute the bottom tube (PNP) of the compound poweramplification. Its circuit construction is the same as that of the upper tube except that it's PNPtyped after compounding. The temperature compensation section V406 has the following twofunctions in the circuit: First, it is composed of the voltage reversed triodes of the sameparameters, so its working mode determines the static working current of the compound poweramplification section. That is to say, we can set up the static working point of compound poweramplification section through adjusting the V406 conducting level. The usual way is to changethe base resistor of V406. Second, it functions as automatically adjust the working mode of thecompound power amplification section when temperature rises. The adjusting process goesas follows:TOTAL OUTPUT CURRENT = WORKING CURRENT + LEAK CURRENTWhen temperature rises, the leak current increases thus the total current increases (un-favorable condition). At the same time, the base current of V406 increases and Ucedecreases thus the output section's bias current decreases. Therefore, the working statuschanges and the back working current decreases. The total current is limited in a certain range.7.2 Protection CircuitThe protection of the power amplifier's output section is performed by a relay series connect-ed between the output end and the speaker. The power amplifying circuit is not stable whengetting started and an impact current output will occur. If the output end has already beenconnected to the speaker when getting started, BOO sound will come out of the speaker.This is very harmful to the speaker. Therefore, we serial connect a relay between the outputend and speaker. In this way, when this unit is getting started, the relay is switched off andthe output end and the speaker will not be connected and thus the impact current will notoccur. The relay will open only when the circuit works stably. Therefore, the protection isrealized. In a similar way, when the circuit goes wrong and a high voltage and large currentwill occur in the output end, the protection circuit will also cut off the relay to realize theprotection function. AB217 boasts its three protection functions: delay switch-on protectioncircuit, midpoint over-voltage protection circuit and short circuit over-current protectioncircuit. The working power of the protection circuit comes from a half-wave commutatingcircuit composed of VD408 and C418. It's about +26V.- 13 -7.2.1 Delay switch-on protection circuitWhen this unit is getting started, +26V passes through R447 to charge C417. The positiveend voltage of C417 increases slowly. When the voltage supersedes 12V, VD409 (12Vvoltage regulator diode) is penetrated and its negative end outputs high level which makesthe compound tube composed of V426 and V427 conductive. Therefore, their collectors'potential is dragged down and there is current in the relay Y401. The relay is switched on.The delay time depends on the constant of R447 and C417 charging time. The positive endvoltage of C417 is a key point through detecting which whether there is the voltage over +12Vto judge whether the whole protection circuit is started. When there is a voltage over +12Vand the relay is not switched on, it indicates that the problem only exists in the back compon-ents VD409, V426, V427 and Y401. When there is a voltage below +12 and the relay isswitched on, it indicates that the protection circuit is started and you need only to check thecorresponding circuits.+26VR447chargesC417PenetratesVD409V426 & V427conductedY401switchedon7.2.2 Midpoint Over-voltage CircuitA midpoint over-voltage sampling resistor R462 is connected to the output end of ChannelR (Channel L is R461). Because the power amplifying circuit is provided by twin powersupplies, the output end usually has two conditions: positive or negative voltage. They willbe analyzed respectively as follows. This protection circuit's protecting range is the voltageabove +4V or below 4V.When the output end voltage surpasses +4V, the base receives a voltage above +0.7Vdue to the voltage division by R462 and R445. V424 is conducted and the collector'spotential is dragged down.When the output end voltage is below 4V, the base of V423 receives a voltage below 0.7Vdue to the voltage division by R462 and R445. V423 is conducted and the collector'spotential is dragged down..According to the above statements, the collector's voltage will be dragged down whetherthe output end potential is over +4V or below 4V. This low potential passes R444 andmakes the base's voltage of V425 decrease. V425 is conducted and thus its emitter'svoltage decreases. That is to say, the positive end voltage of C417 decreases and therelay is shut off, thus the protection function starts.Over +4V V424 ConductedOutp ut End V425ConductedRelay ShutOffBelow -4V V423 Conducted- 14 - |
Related manuals for BBK AB217
This manual is suitable for:
manualsdatabase
Your AI-powered manual search engine