Crate FXt65 cuts out on clean channel only

[groovyfingers]

That's my story, and I'm in need of a direction to go in. Not an expert, and that's probably what is needed. I'm sure there are many causes of the loss of power/output in this amp. A new amp to me, and it had a bad input jack which I replaced. It cured the ugliness of an intermittent connection and I was happy for a moment. Then the volume went away. I tried plugging and unplugging the newly installed input connector but it made no difference. I noticed the tuner was no longer working, another problem too. But switching to the overdrive channel gave me full output with no cutting out. Put it back on clean and low volume reigned. I tried a smack to the top of the cabinet and volume came back. But left again shortly. Now the smack doesn't work, and I don't want to damage the cabinet with too hard a wack. Poked around the web, and here for a similar problem, hoping for the magic bullet. Elextroubleshooter said this in a thread about crate amp cutting out:
Had same the same problem with Crate TX15 Taxi amplifier and here is my fix. With guitar plugged in, power indicator goes from red to orange, and/or sound cuts in and out. Load capacitor in power supply may have failed. Even though battery shows 12v dc and power plug reads 15 v dc, the capacitor's purpose is to maintain a consistent load voltage for amplifier to function. To verify, measure power supply output during normal operation, two yellow wires. Plug in a instrument, observe DC voltage. If voltage fluctuates when instrument is played, unsolder two leads and replace load capacitor. Largest cap on power supply board, (4700uf 25 volt). Visual inspection of capacitor indicated bulging on top of can and leaking dielectric. Ensure lead polarity during install, negative lead of capacitor is circuit board ground. Appx cost of cap $3 US. Thanks.
Any input would be appreciated, even recommendations of a fine tech out my way, Yucca Valley, CA 92284 I've worked on my tube amps, but ss seems much more complex to my mind. Looking at the schematic printed out, I can see where I can check some voltages to see if things are somewhat right. I have a DSO, but I've never tried using it for Amps, just automotive use. I guess I should play with it some.
This looks to be a pertinent post:http://music-electronics-forum.com/t3057/

[g1]

So you have verified that it is a physical connection or (solder joint) issue somewhere in the clean channel area. There is not much specific to the clean channel so that narrows it down a lot. It could be as simple as a bad connection at one of the clean channel pots, like the high, or something in the switching circuit.
The FXT-120 schematic is also for the 65.

http://music-electronics-forum.com/a...05-fxt-120.pdf

[g1]

Here is the part that is clean channel only. To rule out the switching circuit you could just jumper across S to D of Q21.

[groovyfingers]

Trying to pull the circuit board out, should I just leave in place, hook up the speaker with some jumper wires, put an mp3 player in the input and start poking around first? It'll have to come out eventually. I can't remove the female blade connectors, is there a trick to getting them to release? Usually a good firm pull and many will disconnect. Don't want to break anything.

[groovyfingers]

g1, did as you suggested and jumpered across S and D of Q21 no increase of volume. It was in the quiet phase when I jumpered, expected an increase in volume but no effect. I have my mp3 player hooked up and I started moving the big ceramic resistors (?) to see if it changed. Nothing. Started tapping on IC's and got a volume increase. But could not be sure it was the IC I was tapping. Kept tapping and had the most consistant change on capacitors C32 and C34. Can make it soft or loud tapping here consistently. What do you think? Both are in the clean channel circuit. Am I onto something here?

[Enzo]

This problem has nothing to do with it being a Crate, and likelwise the Crate TX15 will have nothing to do with this amp. You already found out that banging on it can restore the sound sometimes. SO some conection is loose/failed. I'd start with the controls across teh front, solder their legs again, and anything you push on that seems to maybe want to make it work - resolder that too.

G1 put up the entire circuit for the clean channel, so if the dirt channel works fine, then everything else is OK.

SO we have a treble pot and a level pot, either of which can kill the sound. You have IC3, and then a bunch of small parts, and of course the copper traces that hook them all together. On a circuit board, when you push or hit a part, the mechanical shock is also going through other parts on the board. Resolder those caps, but I;d wager something nearby is the issue, like the level control they surround.

[groovyfingers]

OK, taking pliers to the female spade connectors and pulling like my life depends on it. Gotta get the board out where I can work on it. Will let you know what I find.

[groovyfingers]

Those female connectors did pull off the male blade but it took more pulling than I like. Didn't break anything thankfully. Got circuit board out (couldn't figure out how to remove ribbon cable so additional board came along). Will reflow the solder in the clean section and see if I don't do more harm than good.

[Enzo]

Generally to get the quick disconnect spades off, I put a small screwdriver under the edge of the female and pry upwards. Usually by turning the blade.

The four wire ribbon? Look at the little socket it plugs into. Hidden on the far side of it, is there not a flat surface to the connector, and does it have a springiness you can push down against? Push down that "bar" and the wires should then easily pull out of the connector.


I usually don't remove the board entirely, I try to leave the wires on, and free it enough I can flip it over where it sits.

[groovyfingers]

Enzo, I assume you do this or did this daily as your chosen profession. Your knowledge along with many others on this forum is deep where mine is the depth of a rain puddle, enough to be dangerous. Your description of the ribbon connector was spot on and I was able to remove that small board and place it back in the chassis. Looked at the main board in the sun with a magnifying glass and all solder points looked shiny and whole with the exception of one of the tone controls in the clean channel. It was probably the problem. Reflowed the solder in the clean section, I hit every pot on the clean side, left the gain 1 & 2 alone. No problems there. Reassembled and played for a couple of hours. No more dropping out/volume loss! Big thank you and g1 for being here and giving me moral support. A nice amp that I will gig with for a while. This forum rocks!

[Enzo]

Hi gf, yes I retired my pro shop after 30 years of it. before that there was touring, and in between field service in another industry.