Crate FXT120 volume fades out

[Dr. Brapp]

I have a guitar amplifier (Crate FXT120, entry-level solid state model from the mid-2000s) that is having a problem. Starting 3-15 minutes after turning it on and starting to play, the sound fades to almost nothing. Here's what I know so far.

• This happens over the course of a few seconds to a minute, with the volume erratically dropping some, coming back, dropping more, etc. until I can't hear it any more.
• It happens on all preamp channels (Clean, overdrive, and "Lead")
• There is never any popping or scratching sound that I would associate with a loose connection / open circuit carrying the signal.
• When fading out, the volume changes quickly but not quite instantaneously. I would guess it typically takes about 0.5 seconds to go from one volume level to another.
• The amp has an input jack labeled "Insert", which is intended for playing along with backing tracks. I believe it skips the preamp and goes straight to the power amp but I don't actually know that for sure. My problem does NOT occur with the "Insert" input; it keeps amplifying just the same while the volume on the main input drops.
• I have reflowed the solder on the input jack pins and thoroughly cleaned the input jack. This had no effect.
• When initially powered on it works fine. There is no excessive 60/120Hz hum and volume is normal.
• LEDs showing the selected channel stay on and do not fade as the volume fades.
• Looking at the PCB, there don't appear to be any bad solder joints or anything visibly burnt.
• If I tilt the cab back and forth I can hear something moving around. Is there a tilt shutoff switch like on space heaters?

Based on the above here are my thoughts:

• It's not the power amp or anything downstream of it (output transformer, speakers, etc). If this were the case, the "Insert" input would also be affected.
• It's not a loose connection in the signal path. I have experienced bad headphone cords and instrument cables and this is nothing like that.
• It's not filter capacitors. There's no hum from the power supply ripple.
• The problem is probably triggered by heat.
• Could it be the power transformer? How would I test that?

Thanks!

[Jazz P Bass]

The Insert jack itself maybe the issue..
Yes, by goingin there you do bypass the preamp.
Try sending the preamp signal to another amp.
It is a stereo jack.
The Tip is for the power amp while they use the Ring for the preamp.

[The Dude]

The most common problem with those amps is cold solder joints, particularly in high heat areas and larger cement resistors. Try chop sticking around the board and see if you can find a particular spot or component that makes the sound come and go. Or, just pull the board and inspect closely for solder problems. And FWIW, you don't need to worry about an output transformer. This is a solid state amp and doesn't have one. It also wouldn't be the power transformer, as you said the power amp section of the amp works as it should and this PT does not have multiple taps. That doesn't mean that you couldn't be losing your + or -15V supply after R95 & 96 or that the resistors have cracked solder joints. Check if supplies are still present at the op amps when it dies. Also, here's a schematic (attached).

[glebert]

The Insert jack itself maybe the issue..
Yes, by goingin there you do bypass the preamp.
Try sending the preamp signal to another amp.
It is a stereo jack.
The Tip is for the power amp while they use the Ring for the preamp.

If one has a TRS/stereo patch cord you can plug it into the Insert and then hold a piece of metal to short together the tip and ring at the far end of the cable. This is the same as using a mono patch cable to jump from Send to Return on a more conventional FX loop.

[Jon Snell]

Clean the contacts in the socket.
I use a piece of Contact Cleaning Strip https://uk.rs-online.com/web/p/elect...aners/1015564/
Take one strip, cut it to half width , lift the contact with a small screwdriver to allow the strip to close within the contact and pull the strip out.
Repeat again and then repeat the sequence in 10 years when it oxydizes again.

[Dr. Brapp]

If one has a TRS/stereo patch cord you can plug it into the Insert and then hold a piece of metal to short together the tip and ring at the far end of the cable. This is the same as using a mono patch cable to jump from Send to Return on a more conventional FX loop.

Yup, it was the Insert jack! I plugged the cable in partway, which I assume achieves the same thing as shorting the tip and ring at the other end, and the sound came back. I wasn't able to fix it by just spraying contact cleaner all over the inside and outside of the jack and working it around, but I'm on the right track. Once I saw where the Insert jack was in the schematic it all made sense. Thanks everyone!

[ZAnalogKid]

so i have one of these amps that was passing DC to the output and finally blew the main fuse
have checked all the output transistors and diodes, no shorted caps
originally it would work for about 5 minutes then start making nasty noises, now it just blew the fuse
this amp has gone from bad to worse
please help!!!

[The Dude]

To avoid confusion, please start a new thread for your amp.