Old Peavey Minx sputtering at high volumes

[acorkos]

This 80's Peavey Minx 1x10 30 watt bass amp behaves well until the volume is raised very high....a loud static, sputtery noise interrupts the Bass sound. I tried a different speaker....no change. I resoldered everything on the PCB, thinking bad solder joint....no change. I had replaced 2 input jacks, the preamp out jack and the stereo headphone out jack. Resolderd those as well. I just checked and sputtering is there in the headphones, too. It sounds like a loud speaker rattle. But it's not vibration related. Some component must be bad. Any ideas on where to start looking? Thanks!

I just tried a line out into another amp....sputter is present at the other amp as well. So it seems the bad part is before the line out jack

[soundguruman]

if you are sure that all the parts are soldered, and the controls and jacks are very cleaned, you can try changing the 4558 op amp. I would put a socket in, if there is not already.

[Enzo]

So you isolated it to the preamp section. Look at the schematic, the entire preamp is run by a single dual op amp IC. I'd swaap the IC and see what happens, but before that, look at the input, pin 5, upper leeft on the drawing. See diodes CR1, CR2? If one of those leaks, it upsets your op amp, so measure for voltage on pin 5. You should see about zero volts DC, but if you get several volts, then either a diode is bad or the IC is.

The ICs are in sockets, aren;t they? If you pull that IC from the swocket, if any voltage remains, it is not the ICs fault.

[acorkos]

yes, the ic's are socketed. I measure .037V at pin 5 of the IC...pulling the IC, i measure .001V at socket pin 5. it's not a lot of voltage when the IC is in place...is it enough to cause the problem? i'll swap it tomorrow and see if that fixes it. thanks for the info.

[nickb]

Do you hear the problem if you take preamp output to another amp? The headphone out comes from the power amp so that only eliminates the speaker, not the power amp.

[Enzo]

he did say it was on the preamp out.


If there is no DC voltage there, then I just suspect the IC itself. The voltage test was just something that only took a second and would tell us a story either way. The story is that your IC is the main suspect.


OOPS, I am overlooking the obvious. Make sure the +15v and -15v supplies are present at the ICs. Pins 8 and 4 of the IC in question.

[acorkos]

it was a bad 4558

with a new one installed, there is no sputtering....and the overall amp volume increased dramatically.

thanks for the help!