Tweet
This little bass amp, for which I can't find a schematic just now, breaks into a fuzzy sound when the output goes over 4 vAC/2 watts. It uses an LM3876. The input to the chip scopes clean until it clips then non-fuzzy thereafter, whilst the output can be seen breaking first into crossover distortion then the curves break up into lots of HF fuzz. The supply voltages drop from +_27v to +_22v as you turn it up, dropping as it goes into fuzz but on a curve, not dropping straight off. I wondered whether this fault mode spoke to anyone of anything before I start going around looking for schemos or anything. I've never known these chips to go bad in this way, just to burn up and short - could it just be the chip?
-
-
Are you checking it into a load or the speaker? Unless you have a speaker that may be cooked, the only way to REALLY troubleshoot (and likely fix) this issue is to replace the LM3876. It sounds like there is a soft short in the output stage. Hey, ANYTHING is possible. -
OK here goes for posterity. I put in a new chip and when I started to bring it up on the variac there was a short. I had reused the mica insulator rather lazily, so I took it out and replaced it with some of that thermal grey woven stuff, though the mica looked ok, and then the amp was fine. So I'm a-wonderin' whether the bad insulator was the problem in the first place, or the chip, but I clipped the legs off the old chip so can't be sure now...Comment
Comment