Fender Rumble 500 hisssss

[drewl]

Replaced a bad input jack on this thing and now I'm getting a nasty hiss after the master volume and before the FX loop return.

Anyone ever find a schematic for the 500?

If it's like some of the other models, the tone networks are after the master so it may be one of those IC's unless I knocked something off while repairing the input jack.


Thanks.
Happy Thanksgiving everyone!

[J M Fahey]

Is this one of the modern Class D ones?
If so, itīs basically "a Fender preamp driving an Icepower module", where preamp is basically the same and only power module and maybe a couple minor frills change.
A couple Rumbles have been posted here, compare what you have to them.
I think Rumble 300 or 350 should be real close.
I mean the modern ones, not the old Class AB Rumble 300 heavyweight.
Post a gut picture or two.

[catalin gramada]

Hi. usually hiss is asociated with very high gain amplification stage. There are some tricks to tame it, in order: change the tube (some are very noisy), turn the tube (usualy in a double triode one is quiter thean other, put second section in highest gain amplification position), reduce the value of grid stopper to minimum-meant oscilation limit, and cut extremly hights in other way., use a very good quality grid leak resistor (eg.metal film one) or even reduce its value if no impedance problems concern. , twist input signal wire and run it sticked to chassis, run grid leak resistors directly to cathode ground point, reduce the filament heater voltage to nominal value or slightly under..there are many to do , I still thinking...to be continued...

[drewl]

Uh, yeah.
This is a newer all solid state amp.

The 500 seems quite different from the others. I looked at the 350, and this has a master volume.

As mentioned it seems to be after the master and before the loop.
Except that the quad opamp the master is part of goes right out to the ribbon connector to the loop board.
No coupling cap either, straight to the ribbon connector, unless the coupling cap is shorted, but so far I haven't found any other places it's connected to.

So yeah, with a tightly packed preamp board that's different from the other models it's kind of a pain to track down where the hiss is coming from.

The tone controls do seem to effect the level of hiss, but master volume does not.

[catalin gramada]

I shut up myself. Sorry

[drewl]

No worries, you were trying to help!

[drewl]

And actually Catalin, your diagnosis was correct.

That final amp stage has intermittant feedback, causing way too much gain at that stage.

Flexing the board must have caused a bad connection there.
As it is okay right now, but I need to rework that stage to make it right.

[Jazz P Bass]

500 schematic: Rumble 500 V3 Schematics-Diagrams.zip