Laney bassamp DB300H

[Stewen]

Hallo !  Am working with this bassamp and it´s not easy for a "tubeampguy" like me...... haven´t been soldering transistors
much since the 60-ties !    Wonder if its important to select matched powertransistors and /or drivertransistors ?  And , can for example the drivertransistors have been damaged even if they is OK when controlled with an ohmmeter ? Will a hfe
test tell more about their condition ?  About ten or so transistors on this board is the type "surfacemonted" . I must say , I don´t like them !!!!   Can I change at least some of them to TO92 types with the same data but higher voltagecapacity ?  For example :  BC 847B  to  BC 546B ,  and   BC 857B  to BC 556B   ?  I have ordered some books to
learn more about solid state amps but not sure they will answer my questions so I´m happy if you can give some advice.
Regards  Stewen

[J M Fahey]

I like Laneys very much, but I hate their using (for many years now) that mixed through_hole/SMT parts mix.
I would not touch original parts unless actually damaged, and if replacing them , would stick to ooriginal ones; SMT if they are so.
That said, on a Murphy's Law late Saturday "urgent" repair work, I have sometimes replaced an SMT one with a TO92 version, with closely cropped, pretinned legs , a bench mounted loupe, very good light and steady hand, but do not exactly *recommend* that.
I have a small stash of general purpose BC¿846/856? SOT23 case (SMT) transistors just in case.
Hook your dead amplifier to a series lamp bulb limiter, no speaker attached, and measure voltages looking for "impossible" ones: you should have your power rails, although somewhat lower because of the lamp limiter , your +/-15V , *no* DC on the output, more than 5V across transistors with close to .65V from base to emitter, etc.
Good luck.

[Stewen]

Thanks Fahey for Your answer !  I really appreciate that !  Today I got Randy Slone´s book by the postman !  Have just done a quick look , and it seems to be a good book .....  talks about poweramplifiers ..               Regards   Stewen