Fender Deluxe 112 problem

[stokes]

I have a solid state Fender Deluxe 112 Plus amp. When I first turn it on, the amp sounds great with good bass, mid-range and treble. After about 5-7 minutes of playing, the amp abruptly goes all mid-range/treble, outputting a shrill sound.

Before I open up the amp, I'd like some opinion about what to focus on. The behavior started a few weeks ago and appears to be something breaking down (capacitor, etc) or heating up (semiconductor).

Thanks in advance to all who answer.

Jeff

This gentleman is having trouble registering on this forum,so he could not post this question himself,He pm'd me and asked if I could post it for him.Also if any of the administrators can help him with his registration issue,he has tried to email the webmaster,but gets no reply.Thanks
stokes

[Amp Kat]

Sounds like the filter caps solder joint may be loose or maybe some HF oscillation. Lots of stuff to take off to get to them so I'd wait for Enzo's response on this one

[Enzo]

Hah!

First, ball up your fist and whack the top of the amp hard - seriously. If that has any effect, there is a loose connection. Oh, and turn the reverb off before you do it.

Inside I would be wiggling things, in paerticular, yes, those filter caps standing up on the board. They love to crack their solder.

I would also resolder any of the cement power resistors. Especially if there is one in teh speaker return line. Fender like to put a .1 ohm or something in the speaker return, and then sample off it for feedback. if it comes loose, then it opens the speaker circuit except for some higher resistance parallel paths..

As the amp warms up, thermal expansion often affects solder connections mechanically.

Otherwise it COULD be a thermally sensitive part, so then heat guns and freeze spray are useful tools.

ANd there is also signal tracing. Pick some freq that the amp quits passing adn feed a sine of that freq into the front end. Now scope stage to stage to see where we lose it.

That should get him started.

[stokes]

Thanks guys,when it comes to solid state I got nothing.I never work on them,I am sure there are similarities to tubes but I have never had the need or desire to delve into any solid state gear.Enzo,I see you are a moderator here,do you have any suggestions for Jeff as to getting his registration issue cleared up?

[Mars Amp Repair]

Whacking the amp on the top

Yeah, couldn't agree more.
You should see (or hear on the phone...yeah, you can hear the look on their faces, too) the look on some customers faces when I ask them if they've tried pounding on the amp like that. When I was in consumer electronics repair I used to ask the same question with VCR's & camcorders (only I would say 'tapping' the VCR/camcorder, as you wouldn't really want to bash those).
The look/sound was the same...oh my god! I'd never think of doing that. I believe it's the good ole American way. Always got a kick out of that scenario (pun intended). After all, that's what we do with the amps when we have an intermittent like that, right?! Sometimes the balled up fist & bashing is the only way to get the intermittent to rear its ugly head...glen

Hah!

First, ball up your fist and whack the top of the amp hard - seriously. If that has any effect, there is a loose connection. Oh, and turn the reverb off before you do it.

Inside I would be wiggling things, in paerticular, yes, those filter caps standing up on the board. They love to crack their solder.

I would also resolder any of the cement power resistors. Especially if there is one in teh speaker return line. Fender like to put a .1 ohm or something in the speaker return, and then sample off it for feedback. if it comes loose, then it opens the speaker circuit except for some higher resistance parallel paths..

As the amp warms up, thermal expansion often affects solder connections mechanically.

Otherwise it COULD be a thermally sensitive part, so then heat guns and freeze spray are useful tools.

ANd there is also signal tracing. Pick some freq that the amp quits passing adn feed a sine of that freq into the front end. Now scope stage to stage to see where we lose it.

That should get him started.

[Enzo]

That moderator title is like an honorary doctorate - I don't know anything about the BB software. Talk to tboy for that. All I can do is edit posts or move them.

I've said this before, and whenever the tube/SS thing comes up I'll say it again. Troubleshooting is troubleshooting. If fuses blow, look for shorting output devices, or possible rectifiers, or shorted filters. If symptoms are intermittant and come and go, look for loose connections or check the effects loop jacks. And so on. I never mentioned tube or transistor in that short list.

AMplifiers have stages and powr supplies, inputs and outputs. We need to have good power supplies, and after that we trace a signal through and see where it leaves its path. Granted, if you have no idea what an op amp is all about, it can be daunting at first, but really, you can determine a 12AX7 is bad without knowing how it works. Stage by stage, signal in, and signal out.

Point is, if you can fix tube amps, you know a lot more about fixing solid state than you think.

[stokes]

I agree with you on the tube and solid state similarities Enzo,but since I have never, in all the years I've been working on amps,looked inside a solid state amp except once to change a reverb tank for someone,actually on a Deluxe 112,I stay away from the solid state questions.I am sure if I were forced to,I could find my way thru a solid state amp,as you say troubleshooting is troubleshooting and the basics of both type amps are quite the same,but never actually having done this I think it is best to leave these things to those of you who have.Thanks for the replies I am sure Jeff has some good info to go on now,when I hear from him again I will post his results.

[jeffkelly]

Thanks Tboy for activating my account!

I started playing today and like clockwork the amp abruptly went all mid-range/treble. This time I was armed with a 1 pound rubber mallet . A few whacks on the top and side brought life back to the amp.

When I get some time, I'll pull it apart and re-solder everything. While I'm in there, I'll look for leaky/bulging capacitors and replace them if found.

Thanks to everyone who responded and a special thanks to Stokes who was kind enough to post my issue after receiving a rogue PM from me.

J

[stokes]

No problem,Jeff.Glad to see you got your registration issue cleared up.

[Enzo]

Actually I would be surprised to see a bulging cap. Look for solder problems.