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Fender Stage 185 with low power

Started by 1woodhippie, April 22, 2013, 10:10:54 PM

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I've got a Fender Stage 185 with low power overall on both channels.  It sounds OK and doesn't crackle or hum, just doesn't get NEAR as loud as it should.  I would suspect something in the power section since it happens on both channels.  I would guess Q17 - 20...  am I on the right track?

schematic attached.
Any idea how to narrow this down anyone?  I'm a tube guy so this SS stuff is a little new to me.

thanks in advance!


Possible, I suppose, but no, in my opinion those are about the last thing to suspect.

Your problem is not a lack of power, it is a lack of volume.  Yesm if your 100 watt amp only puts out 1/8 of a watt, it would be a lot less loud.   But that would not be a very likely failure scenario.

If those transistors were bad, you would likely have blowing fuses, or VERY severe distortion.

You are right to observe that it affects both channel.  SO many things involve both channels.  Not least of which is the low voltage power supply - the +15 and -15 for the ICs.   Always good to start by verifying the power supply.   But before you even open it up, you can start to isolate the problem.   

Turn the amp on and set up as if to play.  Turn the reverb up half way.  No instrument.  Now rock the amp to crash the reverb springs.  Is that loud?  Or is that also really reduced in volume?

Plug the guitar into the POWER AMP IN jack, play.  Is that strong and clear?  Or is it still showing the problem?   Likewise, you can plug into the regular input jack, and run a cord from PREAMP OUT to some other amp with speaker input.  Is that preaqmp signal good or reduced?

Play with the level control switch, any difference.

All this helps isolate the problem to the power amp or the preamp.  The preamp has a lot of circuitry that is common to both channels.   If it is the power amp, I'd first suspect the input sections first.  In fact of the power amp parts, it is little Q21 I first check.  It is a power up mute, and if you just remove it, you can see if the sound returns.   It is a J111 type, just to the right of the little level switches on the print.

Once I knew which half of hte amp had the issue, I'd run a signal through it and follow it along the signal path to see where it disappears.


Quote from: 1woodhippielow power overall on both channels.  It sounds OK

..then it won't be any of the output transistors, and like Enzo I'd be looking at the power supplies first, then at the muting FET Q21 and associated components.
If you say theory and practice don't agree you haven't applied enough theory.


  And you need to verify that it is not a speaker issue.


Thanks guys!

Think I got it!  Started troubleshooting further and messing with the 3 position Level switch on the back for the FX Loop.  When I did the vol came back.  Cleaned the switch good and it seems to have the vol back.

Thanks to all you guys!