unwanted distortion and crackle in laney bass amp

[ukuleledude1]

hi there, i am new to this forum!

well i bought a laney 400watt bass amp with blown output transistors. they were push and pull with four mj15004 on one side and four mj15003 on the other. i replaced them and the amp worked fine! then after a few days it suddely went fuzzy and distorted and notes died quickly!
i could still hear the origional clean note at the same time and also after further test realized that the mj15004 was working properly.
what could the problem be? are any of the mj15003's blown? it still makes loud nioses. does this mean it's a stage before? or could it be the power supply? i really dont know. i do know that it's deffinitly not the preamp!
any help would be much apriciated!!!

[Enzo]

Get the schematic and post it or link us to it, and the model number.

If the outputs were blown, there should be a ballast resistor for each power transistor. Those would be the power resistors with values under 1 ohm - .22 or .33 being very common. Often they are the cement rectangler types. Make sure none are open.

SOmething drives those transistors. ALways check drivers when outputs blow. And there will be smaller resistors associeted with the drivers, look again for opens there. You could be missing half the waveform due to open resistors.

SCoping the output usually reveals tons.

ANy op amps in that power section? Look for DC on an output pin.

[ukuleledude1]

well i found the resistors you mentoned and tested them. they all appear to be fine!
i did find that on the non working npn side of the amp, the transistor that drives the mj15003's has very low resistance between two of the pins. the resitance is around 70ohms. the transistor is a sm3178a.
could this be the problem? could this transistor need replacing? when the signal is high enough it does force sound through, but it is distorted and dies quickly

thanks!

[Enzo]

Measured in circuit or out? If in circuit, how does its friend read on the MJ15004 side? SImilar or not.

If a bipolar transistor measures 70 ohms on any two legs out of circuit, I doubt it is OK. EVen darlingtons measure higher than that.

Unless I am sleepy, the sm number sounds like a house number

Have you contacted Laney for a schematic?

[ukuleledude1]

yeah, mesured in the circuit! i mesured it's pair and it has nothing like that at all! only really high resistances.

i couldn't find much on the transistor itself, but i have seen similar power amps using tip29's and 30's in their place.

i bought the schemitic from a site and i doubt i can post it here can i? could i email it?

[olddawg]

Measure it out of circuit. Compare it to the known good transistor out of circuit. Do you know how to test bipolar transistor junctions with an ohm meter?