Ampeg G110 blowing fuses

[pontiacpete]

This amp is blowing a 1A fuse. When I first started working on it, it wasn’t blowing fuses, it just hummed and had voltage on the output. I changed the caps, C38. C35, C36, C32 , C37 and almost all other electrolytics. I tried a new pair of output transistors Q8 and 9, with TIP31 and 30 and it started blowing fuses. I put the old transistors back in and now it’s blowing fuses with them in place. I’ve only tested the rest of the transistors, Q5,6 and 7 in circuit and there doesn’t show any signs of a short.
I’m not sure where to go from here.
Does anyone have an idea of what to do?

thanks Pete

[g1]

When you say it had voltage on the output, which end of C35 ? The positive end will sit at half the supply voltage, the negative end will charge up if no load is connected.

[Jon Snell]

Insulators OK on the transistors?

[pontiacpete]

When you say it had voltage on the output, which end of C35 ? The positive end will sit at half the supply voltage, the negative end will charge up if no load is connected.

On the negative end of the C35. I have been measuring DC with my meter clipped to the 2 speaker wires, no load.
I just tried with a 4 ohm load and the C38 shorts but the fuse didn’t blow!

[pontiacpete]

Insulators OK on the transistors?

One the transistors Q8 it’s base is shorted to ground when I put the molex connector on it. I changed the insulator and checked that it’s not touching ground before putting the molex on. I wonder if Q7 is shorted to ground. I guess I need to pull that out.

[pontiacpete]

Oops it’s the collector of Q8 that’s goes to ground, the base is not grounded

[Helmholtz]

When you say it had voltage on the output, which end of C35 ? The positive end will sit at half the supply voltage, the negative end will charge up if no load is connected.

This is a good and useful comment.

I'm just not happy with the phrasing "will charge up" in the second sentence.
When one end of a discharged cap is connected to some DC voltage, it's other end must be at the same voltage as long as the cap can't charge via some DC path. No time delay involved here.
This follows from the fundamental capacitor equation Q = C*V, where Q is the charge and V is the voltage difference between its ends.
So, when a cap is discharged (Q = 0), the voltage difference between its ends must be zero.
This means that the voltage at the speaker output will drop to zero, when the cap has fully charged to the voltage between its positive terminal and ground.

[pontiacpete]

Ok this is surprising, the + sign for C38 is written on the wrong end of the cap. Reversed the cap and I’m not blowing fuse and bursting caps.
This just happened with the other repair I did last month on MEF. Weird. Ah still no sound but getting closer!

[g1]

Ok this is surprising, the + sign for C38 is written on the wrong end of the cap.

Do you mean on the cap itself, or on the board? From what I can see from your picture, I can't tell if that is a black bar running the length of the cap, or just a shadow.

I'm just not happy with the phrasing "will charge up" in the second sentence.

Agree. It later occurred to me that it was poor terminology.

[pontiacpete]

The + sign is definitely written on the ground side. Why I didn’t check with the meter before

[g1]

The + sign is definitely written on the ground side.

Are you talking about the cap or the circuit board? It's impossible to tell anything from that picture.

[pontiacpete]

This is another amp with a PCB that has a mistake. The + sign for the location of C38 needs to be ignored, it’s printed on the wrong end. Hard to believe I’ve come across two amps with this problem within a month.