Polytone burning molex cable

[JLA]

Hello all.
Long time reader here. This is my first post.
I have a problem with my Polytone Mini Brute lll. Yesterday I changed the speaker to a Eminence Beta 15c and when I powered it up it suddenly began to make a strange noise and it smelled like something had melted.
I took the power amp out and the .3ohm 10w cement resistors had burned the pc board under them. Also, the center cable of the molex conector that goes from the power transformer to the board melted.
I changed the four cement resistors and the rectifier diodes and this time the molex conector burned again. But another wire. Any ideas as to where I should be looking at?
Thanks.

[mozz]

Is the speaker the same impedance as the original? I am taking it this is solid state amp?

[JLA]

Yes. This is a solid state amp. Previously it had a 8ohm speaker and this happened when I put a 4 ohm speaker in there. White more correct , as the amp seems to want to see a 3 ohm load , some already weak component might have stressed.
Another option is my stupidity.

[Enzo]

Leave it all connected as it is now. Measure resistance across the output. You should see the DC resistance of the speaker.. I am wondering if in doing the work you might have shorted across the output. Wires crossed, solder blob, something. See if it reads as shorted.

[JLA]

Hello Enzo.
Thanks for your answer. I´ve fixed three Peavey amps following your advice (VTX MX, Studio Pro 50 and Bandit 65), but I lack a lot of knowedge about electronics. I just learn from here, but can´t read a schematic. I just use a multimeter. Unfortunately there is much less information about Polytone amps in the net.

When I read your post I already had disconected things and I dont know what resistance across the output means...I´ll reconect things and will try to find out.

In every photo and diagram that I have seen, there is a bridge rectifier (CR1) in the centre of the board. Mine has no CR1.
I bought this amp one year ago. Ive changed speakers four times . Nothing wrong happened. The new to me speaker reads 3.4 Ohms.
I changed three diode resistors and I might have messed it up.

Now I´ve bought another molex connector . I changed D2, D4 and D7. Also R13. And in the proces I lifted two traces..
I see that with the rectifier diodes into the board , with the diode tester D4 measures 0.095 and D7 .620 in one direction and .690 in the other.
And I find continuity between D2 and D7.Should there be? Thanks.

Polytone_PA378B_PCB.pdf

[Enzo]

Your new speaker measures 3.4 ohms, but does it still read what when the wires to the amp are connected to it?

You schematic link doesn't work. Can you link to a different source?

[JLA]

I was mistaken with another speaker I also tried.

This speaker: alone it reads 3.7 ohms. With the wires conected but no outlet:3.8 ohms. Whith the outlet conected 4.1 ohms.

POLYTONE SERVICE INFO

[DrGonz78]

D4 measures 0.095

That D4 looks shorted to me. Must test it out of circuit to make sure the short is the diode itself and not something else. Probably D4 shorted out though.

[g1]

D4 has a 47 ohm resistor in parallel so it is probably ok.
Schematic and board layout attached:

[DrGonz78]

D4 has a 47 ohm resistor in parallel so it is probably ok.

I even looked at the schematic . I should have looked closer at the 47 ohm resistor because in my head I think I thought it was 47k. Nice call G1!!

[Dave H]

...and D7 .620 in one direction and .690 in the other.
And I find continuity between D2 and D7.Should there be?

That doesn't look right. Perhaps Q6 is shorted? Disconnect the orange wire to Q6 collector and check D7 and Q6 separately. I'd also check Q4.
D2 is connected to D7 via R18 (0.075ohms)