Laney DB300C - power amp blown

[Petr]

Hi there,
Few days ago a friend of mine brought me his Laney DB300C bass combo for repair. Blown power amp. I have read this thread on this forum carefully and found similar symptoms as being discussed there. Fuse FS1 (T5A) blown. There was 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 measured 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

[MarkusBass]

Petr,

Since the amp is DC-coupled, there is not much sense in disconnecting collectors of TS4, TS8 and TS7. This does not prove anything except from the fact there there may be something wrong in that area (or may not).
I would rather removed the output transistors, solder a silicone diode in place of base-emiter of the output transistors - in correct direction (just to be sure that R22 is properly referenced to the ground/output) and I would start checking voltages on R22. The output currents depend on these voltages and they are probably incorrect. Then go back to the previous stages of the amp. For example, TS2 and TS6 form a current mirror. Do you have equal currents there?

It is also hard to believe that that many components have failed. Is it possible that someone by mistake reversed the power supply rails and switched the amp on? What could be a reason of C3 capacitor failure?

Mark

[Petr]

Hi Mark,

thank you so much for your advice. I took the steps you suggest and replaced power transistors with 1n4007 diodes in place of B-E, powered the amp again with 75W bulb in series. The bulb was low glowing, but I have found out that both drivers TS5 and TS18 were getting really hot. Zobel resistor R20 was hot as well. Anyway I took quick measurement on R22 (220ohms) – it had negative voltages on both ends (-0,6V and -2,2V), so suspicion on failure in positive rail. I measured voltage on TS5 driver and had negative voltage on its base. I followed the signal backwards as you suggested and discovered that TS8 was heating too. Then I started to check voltages on TS10 and TS13, followed their common emitter’s wire to the base of TS7. During checking of voltages on R16 and R17 I have discovered faulty resistor R15 that has caused all the trouble. R15 was optically OK, but showed resistance around 10k instead of 270ohms. Replacement of this resistor has fixed all the negative voltages in positive rail. The drivers stopped to heat, R20 got right voltages (+0,6 and -0,6V), anyway TS8 stayed hot, so I decided to replace SMD version with full TO-92 MPSA92. I returned the P/O transistors and adjusted offset and set the bias to +5mV. The amp is now working perfectly.

Well just side comment on how the amp was blown up. It actually happened being connected on stage to the main mix by using XLR line-out plug, but actually not played. After the band‘s performance ended, the sound technicians started to rebuild the stage and plugged-out lot of XLR cables and then replugged it to some new setup. It might be they connected something wrong, had grounding issues, or phantom power on somewhere, i don’t know, just guessing. Of course not all transistors I have changed were faulty, but as I started to solder those SMD little items, I tried to change always all set around.

So finally the amp is working again and I’d like to express many thanks to you Mark, as you put me to the right direction and helped me alot to ressurect this amp. A friend of mine (a combo owner) is thrilled and happy. Great forum here!

Best regards,
Petr