Peavey microbass blowing fuses

[Lownotebb]

Just acquired peavey microbass amp in non-working condition...powered up, slight static from speaker though no output. Opened chassis, output transistor quite destroyed, looks like it caught fire!...cleaned everything up, replaced output, turned on and blew fuse...don't quite know where to look now....need someone with more circuit experience than me...thanks

[Enzo]

Unless there is a newer Microbass I do not know, the Microbass does not have a transistor, it does have a five-leg thing that looks like a transistor, a TDA2040. Does that sound like yours?

it is bolted to the heat sink and soldered to the board. There MUST be an insulating wafer between the IC and the heatsink. If you removed the old IC and the wafer stuck to it, you may have installed the new one without insulation. This makes a dead short across one of the power supplies.

[Jazz P Bass]

To add.
I do not know where you sourced the TDA2040 but they have been out of production for a while now.
If it is a Chinese knockoff then it is a toss of the dice whether the ic will work or not.

Rant: if you are going to 'knockoff' a part, why not make it better or at least as good as the original?
What is with these people?

[Lownotebb]

I did swap insulator over and also at bolt hole...but did not anticipate the possibility of replacement being bad....hmmm....anything else I should look at? Thanks guys

[Jazz P Bass]

Remove the output ic from the PCB.

If it still blows fuses then it is another issue.

[Lownotebb]

Got it!...👌Thanks Jazz P Bass

[Enzo]

Rant: if you are going to 'knockoff' a part, why not make it better or at least as good as the original?
What is with these people?

Because they are scammers, they are not trying to sell quality parts.

You still can get REAL LM1875s from REAL suppliers like Mouser. $3.16 each.

[Lownotebb]

Is the LM1875 a drop in for the TDA 2040?

[Enzo]

20 watts, handles 30v rails, has same shape and leg arrangement. So it drops in place of TDA2040.. It might make a few less watts, but they will be real watts.

[Lownotebb]

Ok...new development, TDA 2040 was shorted to ground, insulator had seen better days...made a new one from an old output transistor insulator, reinstalled, no longer grounding pin 3....fired it up through a dim bulb tester, bulb stayed dim a few seconds then went out, came back on dim then smoke started to leak from power supply/rectifier area...unplugged then removed ic amp chip , same thing happened, in circuit it appears 1 diode is shorted...?

[Enzo]

Well, you had one power supply shorted to ground, so it is easy to believe a rectifier was damaged. If one diode is shorted, just replace all four, they are dirt cheap. And when one shorts it leaves its mate dead across the transformer. If that doesn't kill it, it sure stresses it. Drawing says 1N4003, but 4004, 4007, or similar are fine too.

[Lownotebb]

Ah...yes, I will straighten out that power supply and let you know what happens....thanks for the quick informative replies Enzo..

[Lownotebb]

Well...hoping I would have all good news at this point...but...repaired rectifier, replaced all diodes, came up to full power, no problem, checked input, checked for dc on output, hooked up speaker, got a signal!...sounded as if speaker was blown. Unplugged amp to move towards aux speaker, plugged in and turned on, now nothing but medium loud hum, not affected by volume, no other signal pass through, even bypassed speaker out jack all together....nothing but hum....could it be my Chinese power amp chip?....don't get it...

[Jazz P Bass]

If you have a good signal on Pin #1 of the TDA chip , both power rails (+18Vdc/ -18Vdc) are good & still no sound, then the ic is dead.
Here is the schematic: microbass.pdf

[Lownotebb]

Is pin #1 the preamp output? What am I measuring there? And can I check 18v rails anywhere labeled on schematic? Thanks for putting up with my eager noobness....🥴