Found black AC220V input fuse fuses to determine serious short circuit power transformer load, you must identify failure causes before replacing fuse. Pull cover check power transformer secondary to the rectifier bridge two black 15A fuse has blown, fuse block, plus copper wire burned off. In road measurement amplifier tube C3858, A1494 c-e junction on road electric group, found the right channel 3 of the 4 power amplifier tube resistance of 4 ω has been penetrated. For ease of maintenance, real-draw the right channel amplifier circuit, as shown in Figure 1. The power supply voltage is high (75V) is a classic single-difference, triode constant voltage bias, double-tube parallel current in the output stage of the OCL amplifier, circuit is simple and reliable. Remove the V7~V13 test, found a C2690 transistor V7 e-c breakdown, power tubes, V8, V9, V13 breakdown. To guarantee power tube parameter as consistent, author pick has two on BVceo, and hFE parameter basic consistent of on tube, respectively substitution V8, and V12 and V9, and V13, check correct Hou again replaced V7 and the insurance tube, power several seconds subsequent electrical sucking collection, measuring have ± 75V powered normal, output midpoint voltage for 0.13V, also normal; access speaker, entered VCD audio signal, speaker vocal normal. Slowly increase the volume level, about adjusting to a total volume of 1/3 when relays disconnect, speaker sound, turn down the volume and normal playback. Ask the owner learned of the machines before the volume protection. Test machine separately connect the left and right channel speakers are also found in protection, so suspect protection circuit problem, especially the sample components. The protection circuit is shown in Figure 2, check-point voltage sampling circuit, contemporary change audio bypass capacitors C1 and C2 after troubleshooting.JyMservice manual download site