Peavey microbass blowing fuses

[Lownotebb]

If so, seems to be clipping at much too low a level....any reason why that may be?

[Enzo]

No. FInd out. Are all power supply rails up to voltage? and clean? Follow the signal path through the amp, apply a signal to the input and see where the signal fails.

This is a simple amp, but it does have a limiter. U4. U4 is an 87478 type - PV's number - Should be in a socket. NOTE which way it goes, and pull it, the amp should work without it, though no longer clip limited. Does it?

Dos it sound OK on headphones, or have the same issue? If so, the speaker is suspect. The speaker and phones come from the same signal, but the phones have a resistor.

Signal path is three op amps to the power amp IC, look/listen to the output of each.

[The Dude]

The TDA2040 is only rated for 25W at 4 ohms. The amp's not going to get real loud, especially doing bass frequencies. Is there distortion even at low volumes, or just when the amp is pushed?

[Lownotebb]

Distortion even at low volume level, not expecting much from small amp, but definitely breaking up well before it should, even with passive pickups....

[Enzo]

Well, let me know when you try the things I suggested.

[Lownotebb]

Please be patient...I can solder, use a DVOM, and read a resistor, but not too well versed in tracing a signal and checking for dirty rail power....but I'll forge through it....thanks again

[Enzo]

Well, we don't know you yet, so speak up when you are missing some particular skill or equipment. Sounds like you do not have an oscilloscope. A signal tracer is simple, google it. Basically all it is is a probe you make on the end of a guitar cord, and plug that into some other amp. You can then probe your circuit and listen to what a particular point sounds like coming out that other amp.

Power supply? It makes four voltages: +18v and -18v for the TDA2040, and +15v and -15v for the other small ICs. We want to find all four at more or less those voltages. Clean means smooth DC voltages. In fact a signal tracer can be used to check for clean. SMooth DC won't make sound through a signal tracer, other than maybe a pop as your probe touches. Dirty power will make hum.

[Lownotebb]

If it's not too much trouble, would you mind maybe pointing out points at which to trace signal as per schematic, just to confirm I'm heading in the right direction?.....much appreciated

[Enzo]

Did you try simply removing that 87478 IC?

The signal path is pretty easy to follow across the top row of the schematic. it flows U1a, U1b, U2a, and finally U3, which is the five leg amp chip. U1a is pin 1 on that IC, U1b output is pin 7 of that IC, then pin 1 of Us is its output

[Lownotebb]

87478 chip appears to be soldered in place. May still attempt removal...thanks for the signal path..will report

[Enzo]

Sorry, my memory said the ICs were in sockets. Must be thinking of other models.

[Lownotebb]

hey guys, few more questions if I may...have confirmed speaker is good, even tried external speaker on unit, same breakup..The rail voltage seems good where measured. Having a little difficulty correlating schematic with circuit board. Schematic labels op amps U1A, U1B, U2A, U2B, U3, and U4. Board seems to differ slightly, U1,2,3,4. Found board diagram that supposedly goes with schematic but layout is definitely different than mine and some smaller components I can't seem to locate.Is there more than 1 layout of this amp, being schematic is dated '87 and my board is dated '02? Thanks for steering me through...

[Jazz P Bass]

U1, 2 & 3 ic's are an NJM4558 dual op amp.
There are two separate op amps in one 8 pin package.
By convention the first op amp is labeled A & the second one is labeled B.

[Lownotebb]

Starting to become clearer....what about 175 millivolts dc on output...is that acceptable?

[Enzo]

Like Jazz said, U1, U2, U3 are dual section parts. So U1a and U1b are just the two halves of U1. Look at the schematic, note U1a uses pins 1,2,3 and U1b uses pins 5,6,7.

WE MUST get on the same page. I have the 1987 schematic. If you have a layout from 2002 it could be different.

Output voltage, what is that, about 1/8 of a volt? Unless I had other compelling reasons, I would not be concerned by it.