Analysis of repair: owner says the machine has been repaired elsewhere, have switched power tubes, after changing to normal. But it didn't take long for trouble relapse, and brought the last replacing power tubes C3858 and A1494.CmLservice manual download site
Speaker test and sure enough on the sound quality is poor, such as an amplifier in a push-pull output amplifier stage tube does not work when the symptoms. In the road to measure power output tubes and push tube (C2073, A940) PN junction resistance and found no abnormality; measuring the bias voltage of the tube, and were within the normal range. For ease of maintenance-real-draw left circuit, as shown in Figure 2.CmLservice manual download site
CmLservice manual download site
After splitting the circuit board testing machine, 5s, sound quality is good, then the sound quality becomes poor, judging circuit connection is bad. When using a wire parallel resistance RA C3858 emitter of negative feedback amplifier tube (0.5 ω/5W) ends when the sound quality gets better, remove the wire sound worse, seems to point of failure on the R1. Carefully observe the R1 pin Black oxide, remove the R1 shaved the PIN and then welding test machine, troubleshooting.CmLservice manual download site
R1 after a cold solder joint, equivalent to C3858 did not work, and only power amplifier tube A1494 amplified audio signal negative half cycle, so the sound quality becomes poor.CmLservice manual download site
Why last only for power management troubleshoot it? That is because Rl cold solder joint PIN pads and C3858 emitter pads connected C3858 welded at the same time, the heat emitter pads quickly passed on to the R1 PIN pads, indirectly by welding the R1, the virtual welding conditions have improved, resulting in malfunction "disappear" the illusion.CmLservice manual download site