Laney bass amp blows output transistors!

[spy]

ok! Finally the amp IS working!
After changing very carefully one SMT, I liked the procedure, and started to change the possible faulty ones until the amp was fine! I started from TS15, TS16 & TS17 and ended with TS3, TS7, TS2 & TS6. One of the last ones were the faulty one! No dc at the output. I rebiased the amp and did an offset adjustment for 0mV at the output and the amp now is on my bench with an 8 Ohm resistive dummy load and a signal generator at the input and delivers clean 100W.

But how can I be sure that the amp won't blow any other pair at the future? Last time it blew the output stage plus C3 cap after a few weeks. Coincidence?
Was it because I changed only one pair and not a "matched" quartet? What was the problem?

[Jazz P Bass]

When amps blow how can you "know" what got fatigued?
What is recommended is highlighting on the schematic which components actually failed.
From this data set you can get an idea of the failure current path.
From that path you can make "educated" decisions.

[spy]

Of course you're right! Just wondering what I didn't do right at the first time I repaired it. I didn't expect it to blow the output stage again, especially after a few weeks and not after a few hours.
Hope now is everything ok and the amp will play more than a few weeks! After a few hours with the dummy load at full power and with various frequency input signals and nothing weird on the screen. Thanks for your help, guys!

[lalunatech]

I have one of these Laney power amps here for repair too, an RBG500 with 9075_3 power amp board. I have had to break off and do other jobs in the meantime but I was determined to crack it because it was such a wierdy! After busting ten output transistors at about three pounds sterling each and spending a couple of days time on it, I am now confident that I have fixed the fault.

The faulty part was TS4 (SO692). This is a PNP transistor, 300V VCEO 100 mA 250 mW. It is the SMD version of the popular HV transistor MPSA92 but is only max 250mW as opposed to 625 mW dissipation. Herein lies the problem. TS4 is the current source for the differential input amp and also provides the voltage reference for the main voltage amp section current source TS8. TS4 carries 5mA at a VCE of about 50 Volts which multiplies to give 250mW dissipation - the absolute max for that dinky little device. The PCB tracks have not been given any area to aid cooling and help the little bugger out. For all it's life it has been running very hot so it is not surprising that it has died. I replaced it with a 625 mW TO-92 MPSA92, bending the leads appropriately and sticking it down with some silicone rubber.

The symptoms of the fault were odd, I have never come accross a transistor that failed in this way before. That's why I wanted to persist with it even though any profit from the job was wiped out ages ago. It would run steadily for quite a few minutes and amplify a signal properly. I could set the quiescent current for the output devices and it would stay steady on about 12 mA. Then with no warning the current though R1 would rise rapidly pushing the output stage into heavy conduction till the O/P transistors fried. After the second set of power transistors had blown I belatedly replaced the fuses with 22 ohm 7W resistors to stop it happening again while testing. I normally test amps with a variac, it has been good enough in the past but this one caught me out with its wierd ramdom behaviour.

The parts I replaced were:
2x MJ15003
2x MJ15004
R22 22 ohm
TS10 BC847B SMD
TS13 BC857B SMD
R23 10 ohm SMD
TS4 SO692 - replaced with TO-92 MPSA92 (300V 500mA 625mW)
TS5 SM3178A NPN driver on heatsink. Not actaully faulty but replaced with MJE15030
TS6 SM3177A PNP driver on heatsink. Not actaully faulty but replaced with MJE15031

good luck!

[spy]

After a year now and the amp is still working in a daily basis! As I remember I had a similar problem with an smd type MPSA42/92 transistor but I don't remember which. But from the time I replaced one SMD type to a TO-92 type problem solved.

[lalunatech]

It's poor design to run a transistor at full dissipation continuously, don't you agree. Whoever decided to use surface mounted transistors in this amp should be surface mounted themselves.... by a randy buffalo!

[spy]

True!

[snowblind]

Hi,
first of all sorry for my english ...hope you still read here, i have a problem with my laney rbg500, in past it blowed c19 that i tried to replace but not working so i sent it to a man that fixed it but after a few months now started to make a big noise (like white noise really high), even with all levels off, no input and even if i disconnect the preamp board so i guess that noise comes from the amp board 9075_3. I don't want to spend more money to send it to fix but if someone here can help me i can try to change some components (I am quite good at soldering). I have read the issue about TS4 but where is it on the board? I spent lot of time but i see much less components on the board than on the schematics.
At this address you can find a picture of my board... Forte fruscio in ampli basso ? Il Forum di ElectroYou
I asked help there but no replyes. Thank you very much if you will help me

[snowblind]

Hi,
first of all sorry for my english ...hope you still read here, i have a problem with my laney rbg500, in past it blowed c19 that i tried to replace but not working so i sent it to a man that fixed it but after a few months now started to make a big noise (like white noise really high), even with all levels off, no input and even if i disconnect the preamp board so i guess that noise comes from the amp board 9075_3. I don't want to spend more money to send it to fix but if someone here can help me i can try to change some components (I am quite good at soldering). I have read the issue about TS4 but where is it on the board? I spent lot of time but i see much less components on the board than on the schematics.
At this address you can find a picture of my board... Forte fruscio in ampli basso ? Il Forum di ElectroYou
I asked help there but no replyes. Thank you very much if you will help me

[J M Fahey]

Hi snowblind: I've read your Italian thread.

If the "hiss" comes from the output stage, it's quite probably oscillating.

One possible reason would be problems with the Zobel network, made out of R20 (10 ohms, 2.5W dissipation) and C8 (.1uF x 400V)
If the resistor blows open or the cap opens or solder cracks, etc. , the amp will still work, measure beautifully with a resistive load ... and become unstable with a speaker.

Just in case, replace them and check that solder and tracks are fine.

*Or* some transistor might be overheating, use the "finger tester" for that.
Touch only the plastic case, not the legs.

[snowblind]

Thank you very much juan manuel, I don't think the issue is for overheating cause the noise start when i turn on the amp...I can try to change R20 and C8 that looks fine but can be damaged. What about the TS4 issue that wrote lalunatech? I spent 15 minutes looking the board but I can't find ts4! by the way what else I should change to be sure to fix it and not go the the component shop many times? thank you again

[J M Fahey]

Thank you very much juan manuel, I don't think the issue is for overheating cause the noise start when i turn on the amp...I can try to change R20 and C8 that looks fine but can be damaged. What about the TS4 issue that wrote lalunatech? I spent 15 minutes looking the board but I can't find ts4! by the way what else I should change to be sure to fix it and not go the the component shop many times? thank you again

Ok.
Start with the Zobel, easy to check.

Being "noise", next suspect is a bad contact, either actual metal to metal such as a connector (HDR1), or some cracked solder.

Check them under good light and redo any suspect.

Lift one end of C4 , any change?

Without reconnecting C4 yet, short with a small piece of wire across R6 .... any change?

Another suspect mechanical contact is the offset adjust (PS2) .
This is not a critical adjustment (it won't blow your amp if you touch it) so note its position, move it end to end, previously apply a little contact cleaner to it, any change?

Then restore it to original position and resolder C4 .

Next in line are noisy semiconductors, specially at the input.

Buy half a dozen each of those MPSA42/92 in SMT size (S0642/92) and another half dozen of the oerginal TO92 version.

Cost peanuts and less than the gasoline you will spend going there, so, it never hurts.

Working with SMT is pesky, so get whatever's needed (tweezers , fine tip soldering iron, a pair of 4X reading glasses to be used as loupes, etc)

Unsoldering them is not easy, so read something about it, or watch some YT video.

Good luck.

EDIT: I almost forgot: to find TS4 follow its connections from known parts:
> R11/R12/PS2 wiper
> TS1/R8/R10
> R4/R7

[johnpaul]

hi everyone,1st posting, I have just repaired my laney concept mixing console which uses the same power amp, after blowing several output transistors and the 10uf supply caps I decided to remove all the smd transistors and substitute for standard types. using mpsa42 for so642 and mpsa92 for so692, bc547 for bc847 and bc557 for bc857, the drivers were replaced with mje15030/31 types. all are now mounted on the top side of the pcb by drilling with a 0.6mm drill through the solder pads where the smd devices were and soldered in place. unfortunately the bc546/557 were under the heatsink so had to be soldered on the track side and bent to fit & fixed with a blob of silicone sealant. the 10uf supply caps were replaced with 400v rated, all now biased and works fine, this has been an ongoing project for about 6months, thanks to other members who have posted info about this amp it has been very helpful.

[J M Fahey]

Way to go.
Laney's mixed SMD/thru hole construction always gives me the creeps.
I always thought about doing what you did, will probably try it on the next Laney that lands in my bench.

[johnpaul]

amp still works fine after previous mods, used everyday now for about 1 month. keeping fingers crossed!