Fender Stage 160 hum

[catstrat]

Thanks everyone. I just set it to the side while i work on something else that's giving me fits as well. But i'm back on the Stage 160 and i'll check everything all of you have suggested and see what happens. One thing i've never understood though is how a transistor amp can be working fine one moment and the next time you turn it on this nasty hum appears. To be truthful, i've never had success on fixing any thing transistor that has this type of a hum. As we all know you have to find the problem first before you can fix it.

[g1]

To be truthful, i've never had success on fixing any thing transistor that has this type of a hum. As we all know you have to find the problem first before you can fix it.

That's because it's not really hum, it's straight DC.
In solid-state amps, you have DC coupling between stages, lots of feedback networks, and no output transformer. This means basically any of hundreds of different faults can all result in DC on the output. It makes it very difficult.
In tube amps, having an output transformer means we never have to deal with DC on the output. Any hum in a tube amp is real AC hum. (with the exception of gross OT faults that usually blow fuses).

[Enzo]

One thing i've never understood though is how a transistor amp can be working fine one moment and the next time you turn it on this nasty hum appears.

Most anything that breaks will work up until that point. Everything works until it doesn't.

[Jazz P Bass]

"Most anything that breaks will work up until that point. Everything works until it doesn't. "

Ahh, Enzoism #97.

[Enzo]

I got more rules than Special Agent Gibbs.

[The Dude]

I imagine there have been numerous times you would have liked to slap someone in the back of the head, too.

[Enzo]

Oh I usually want someone to slap me around. I've been bad, I need to be punished.


(Safe word today is "banana")

[Jazz P Bass]

In the NCIS season 7 finale, Gibbs revealed 'Rule #51'.

What was it?

[The Dude]

Rule #51 = Don't use "banana" as a safe word?

[Jazz P Bass]

Aehnt.

Rule #51: "Sometimes you are wrong".

Here is all of them: Gibbs's Rules - NCIS Database

[Enzo]

And so many of them fit in amp repair.

[catstrat]

stage 160

As G1, i think it was, said the hum, as i call it, is actually d c voltage. I was wrong earlier in this thread, i do have 8.8 volts positive dc at the speaker out. That's the source of the noise. I checked the rail voltage again last night and the 147s now are about 4 volts less than the 142s. I'm starting to think this may be a bad solder joint so i'm going to reflow every connection on the board. At full 115 a c voltage it's pulling about 1 to 1.5 amps. In this amp what are considered the drivers and pre drivers? Last night i checked the 147s and the 142s with a meter and now the emitters, collectors and bases all test shorted to each other. What causes d c to appear at the o/p ? A shorted o/p transistor? I'll check the other things that all of you have suggested and get back to you. Thanks.

[Jazz P Bass]

Quote:"What causes d c to appear at the o/p?"

Short answer: an imbalance in the output section.

The shorts between the transistor leads has to be rectified.

Each side has three transistors in parallel, so any one shorted will show up as all three.

As to how it works:

The predriver is IC U6A, pin 1.
The drivers are Q10 (positive going sine wave) & Q11 (negative going sine wave).
All of this should 'setup' the Tip transistors to have a base voltage that barely turns them on. (@.600Vdc)
Q12 & Q13 are protect transistors. On extreme output signals, they will steal drive signal from the outputs.

The fact that the amp is pulling 1.5 amps at idle is certainly a cause for concern.

As an aside, you do not need all three (per side) output transistors installed to simply 'get it going'.
For sure, do not expect to drive the amp into full power.
Not that you are there yet, as at this point, you do not even have a stable idle condition.

Aside #2: If this amp really took a wack when it failed, one or both of the protect transistors may be bad.
Either one bad can lead to some confusing issues.

[52 Bill]

And don't forget the diodes in there. D68 and D69 are wired across the outputs and if one is shorted the outputs will test shorted as well.

It can be frustrating in dealing with a major blow out, but they can be fixed if you just take a straightforward approach to testing components and being certain that you have found all of the dead parts.

The worst thing is finding one shorted transistor and assuming that it is the only problem. If it isn't the only problem, powering up the amp with other bad parts will not only put the replaced transistor at risk, but other parts as well.

This is where using a lamp limiter or Variac comes into play. If it starts drawing too much current when first powering up after a repair, you back off and look for additional problems.

As Jazz noted, the entire output amp is basically a balanced system. One side pushes while the other side pulls. When one side is not working right the idle balance is disrupted and there is a voltage seen at the midpoint of the circuit.

Take your time, test everything that relates to the power amp and see what you find. If you get odd readings, try unsoldering one leg and retest.

[Enzo]

Before you do anything else, please verify that your 15v rails are both clean and up to voltage. If one of them is bad, that leaves the IC at the input of the power amp unable to center properly, which will throw the whole power amp off center.