Fender M80 Pro hiss/static

[armuss750]

I've been searching the boards, but haven't found a post with the same problem, so any help would be appreciated.

I've got a single 12" 8ohm speaker mounted in a cabinet, wired to a mono jack, then plugged into the 8ohm output of the amp with a speaker cable. With no instruments plugged into the amp and all volumes on all channels at 0, I get a loud hiss. With an instrument plugged in, the hiss is still there at any volume. I've tried replacing the speaker, replacing the wiring from the speaker to the mono jack, replacing the jack, replacing the cable from the speaker jack to the amp, and nothing helps. This problem has developed over the last few weeks, and gotten progressively worse. This amp does have an effects loop (preamp out, Power amp in) and two inputs. Not sure if it helps to know this or not, but throwing it out there.

Thanks for any suggestions...

[DJPhil]

It seems pretty unlikely that your hiss is related to your speaker wiring. By the time a signal reaches your 8Ω output it's been amplified dozens to thousands of times and somewhere in there something is going wrong. Often the hiss is generated in an input stage and is grown by amplification across the whole signal chain, but this would respond to playing with the volume and tone controls. If it's not responding to any controls then you've already narrowed it down (probably) to the final power amp stage. This is a good start!

I think I've found the schematic to your amp, but I'm not completely sure. You may wish to cross check the board drawing in the pdf with your amp.

I'd start with a visual inspection of the board, both sides, and see if you see any leaking caps, bad solder joints, or anything else fairly obvious. If it were me I'd then be poking around with a scope and a meter looking for trouble, but the pros here might have better advice on where to start. Pictures always help!

Hope that gets you started.

[armuss750]

DJPhil - Thanks for the ideas and the schematic. It helped a lot. When I tore the amp open and looked at it, I could see two diodes (CR25, CR26) that have overheated at some point. On the back of the circuitboard, the solder to the negative sides of these diodes had come undone, and this is what was causing the hiss. I re-soldered the board, plugged the speaker in, and it was quiet again. Now I'm wondering what caused the overheating in the first place, and whether it will repeat itself over time. With the amp plugged in, the two resistors nearest these diodes (R94, R95) get hot fast. They are 330ohm 10% resistors, and are reading 310 and 319 when metered. That seems to be within spec. What kind of heat is acceptable or too much?

Attached is a photo showing the overheated diodes and resistors.

[J M Fahey]

what caused the overheating in the first place, and whether it will repeat itself over time

Poor, just on the brink of destruction design (at least on that area), so probably repeating in the future.
On a hunch, I would replace those 330 ohm, 5W resistors with same dissipation 470 ohm ones.
My reasoning? simple: if those zeners heat up so much, it's because they are needlessly passing too much current, which the preamp itself is not using.
If the preamp were using that much, the resistors would be very hot, but the zeners much cooler, which clearly is not the case.
Replace the power resistors as suggested and check that you still have those +/-15V. I think you will.
Both resistors and zeners will still be hot, but not solder-melting board-scorching as before.
If yiu do it, post results.

[phatt]

Hi, Having repaired a Performer 1000 (similar circuit) I thought I should add my observations.

At least 10 Watt resistors if you want it to survive. 0:)
I mounted these off the board to help keep heat away from the the tracks and the Zeners certainly don't need more heat beside them.
Oh and watch out for those tracks as they will have delamanted from the PCB by now.
Yep those Fendurh tecks will never get a job at NASA.  :duh

Oh while I'm at it,,, the Ext speaker jack gives a slight difference in output.
It can be used even without the ext speaker in place simply by Inserting an empty 1/4" Jack.
Try it see if it changes anything.

Phil.

[J M Fahey]

Yep those Fendurh tecks will never get a job at NASA.

Or maybe they were fired from Nasa after that exploding Shuttle launcher. :trouble

[phatt]

Yep those Fendurh tecks will never get a job at NASA.

Or maybe they were fired from Nasa after that exploding Shuttle launcher. :trouble

Humm? Now there is a new idea for a stomp pedal.
Exploding *guitar player* launcher.

You only get to use it once 0:)
Phil.

[J M Fahey]

Well, you can save it for the last show in your "Goodbye" World Tour.
Of course, on the end of the last Bis.
*Then* the announcer can say, in Truth: "Please leave in Peace, **Elvis** has left the building "
Replace the name between **__** with the approppriate Star's name.
PS: no Sound Rehearsal or tests, of course.