Tweet
Laney DB300C - similar symptoms
Hi there,
still someone alive here and responding to this thread? Few days ago a friend of mine brought me his Laney DB300C bass combo for repair. Blown power amp. I have read this thread very carefully and found very similar symptoms as described here. Fuse FS1 (T5A) blown. There were fried and shorted a pair of power transistors MJ15003 and MJ15004. So I decided to replace all four O/P transistors (TS9, TS11, TS14, TS19). Another faulty parts were blown up 2 caps C14 and C12 in positive rail. Well I replaced all four caps 4700uF, also those in negative rail (C15, C17). Another blown up cap was C3, replaced as well. Then I tried to run the amp with bulb in series, bulb didn't glow or minimally, good sign, anyway the amp wasn't working. Then I focused my attention to the SMD transistors. As I don't have SO642/SO692 I have used BFN26/BFN27 as a replacement. So far I have replaced TS3, TS7, TS2, TS6, TS15, TS16, TS17, TS1 and TS8. As a replacement of TS4 I have used TO-92 MPSA92. Well since then when I switch-on the amp, the bulb (75W) is shining almost full, I'm getting the voltage on the rails max.+6V/-6V each. I removed and checked the drivers, they seemed to be fine, but replaced them with MJE15030 and MJE15031.I have checked and replaced TS12 with MPSA42 as original. I have replaced TS10 and TS13 with same type BC847B and BC857B, still the same, bulb is shining. I have checked C10, it's OK as well as R22. I have to say I'm baffled.
So I tried to power the amp using autotransformer and carefully raised the voltage to 100V and measure the current on each rail. At cca 100V the current was around 2A each rail. Too much of course. The power transistors started to heat. One NPN-PNP pair significantly more then the other one. I have noticed that the zobel resistor R20 gets really hot (speaker not connected). When I connect the speaker(s) it gets hot R21 as well.
Because under those stress conditions it is not possible to take some measurements, I decided to improvise and tried following again with the bulb in series:
- disconnected collector of TS4 -> bulb is not shining
- disconnected collector of TS8 -> bulb not shining
- disconnected collector of TS7 -> bulb not shining
Based on that I'm guessing the O/P transistors are OK, anyway tried to change them again, same behavior.
Any help would be greatly appreciated! Can someone point me where to start, how to measure? Any hints, suspicion?
Many thanks for any response!
Petr
laney_db300_sch.pdf
Hi there,
still someone alive here and responding to this thread? Few days ago a friend of mine brought me his Laney DB300C bass combo for repair. Blown power amp. I have read this thread very carefully and found very similar symptoms as described here. Fuse FS1 (T5A) blown. There were fried and shorted a pair of power transistors MJ15003 and MJ15004. So I decided to replace all four O/P transistors (TS9, TS11, TS14, TS19). Another faulty parts were blown up 2 caps C14 and C12 in positive rail. Well I replaced all four caps 4700uF, also those in negative rail (C15, C17). Another blown up cap was C3, replaced as well. Then I tried to run the amp with bulb in series, bulb didn't glow or minimally, good sign, anyway the amp wasn't working. Then I focused my attention to the SMD transistors. As I don't have SO642/SO692 I have used BFN26/BFN27 as a replacement. So far I have replaced TS3, TS7, TS2, TS6, TS15, TS16, TS17, TS1 and TS8. As a replacement of TS4 I have used TO-92 MPSA92. Well since then when I switch-on the amp, the bulb (75W) is shining almost full, I'm getting the voltage on the rails max.+6V/-6V each. I removed and checked the drivers, they seemed to be fine, but replaced them with MJE15030 and MJE15031.I have checked and replaced TS12 with MPSA42 as original. I have replaced TS10 and TS13 with same type BC847B and BC857B, still the same, bulb is shining. I have checked C10, it's OK as well as R22. I have to say I'm baffled.
So I tried to power the amp using autotransformer and carefully raised the voltage to 100V and measure the current on each rail. At cca 100V the current was around 2A each rail. Too much of course. The power transistors started to heat. One NPN-PNP pair significantly more then the other one. I have noticed that the zobel resistor R20 gets really hot (speaker not connected). When I connect the speaker(s) it gets hot R21 as well.
Because under those stress conditions it is not possible to take some measurements, I decided to improvise and tried following again with the bulb in series:
- disconnected collector of TS4 -> bulb is not shining
- disconnected collector of TS8 -> bulb not shining
- disconnected collector of TS7 -> bulb not shining
Based on that I'm guessing the O/P transistors are OK, anyway tried to change them again, same behavior.
Any help would be greatly appreciated! Can someone point me where to start, how to measure? Any hints, suspicion?
Many thanks for any response!
Petr
laney_db300_sch.pdf
Comment