Need schematic for Crate FXT120

[fixitrightthefirsttime]

Anyone have a schematic for a Crate FXT120?

Signal is weak at the output and a schematic would save me a ton of time.

Thanks.

[Enzo]

I can send it to you, but first, check these things: The rectangular cement power resistors and the insert and external speakers jacks.

The insert jack is an effects loop. Plug something in and out of that jack a few times. Helps? Plug a TRS plug into the jack and short tip and ring together with a wire. Helps? Or pull the board and measure resistance between the tip contact of that jack and its cutout contact. When they are touching, the resistance should be under half an ohm. And inspect the solder.

Plug a signal into that jack and see if it comes out the speaker OK. It should. That was the power amp in. Likewise, plug a cord into the jack half way - to the first click - and send that to another amp. SOund OK? Should. That is the preamp out.

The external speaker jack unserts the external speaker in series with the internal one. The cutout contacts in that jack carry the current for the internal speaker when the jack is empty. Measure the resistance of the tip contact to the cutout contact. SHould be under that half an ohm. And look at the solder.

All those cement resistors? Check the solder.

Verify the speaker is OK.

[fixitrightthefirsttime]

tks

The signal is uniformly weak at the base and emitter at each of the TIP... trannys. It is very good at several of the op amps. Voltages at the finals are +/- 39 VDC with negligible ripple. Clipping is minimal. So my guess is something is attenuating or suppressing the signal.

[Enzo]

So did you inject a signal; at the insert jack? Just plug a guitar in there even. That will instantly tell you if the power amp is working or not. There is a mute at the power amp input as well. But a bad insert jack, and believe me this is a VERY common problem, will have a great signal on the preamp sice and very weak on the power amp side. And that would be your signal; suppression. And if the mute is on, that too would kill the signal there.

If it is not the jack, then Q16 is that mute. It should have something like 7-8 volts on its gate. The little JFETs are ON until turned OFF by their gates. No gate voltage, and it acts like a 200 ohm resistor or something like that. That wull surely attenuate the signal. If no gate volts, find out why, the circuit controlling it is simple. If there are gate volts, then remove Q16. AMp levels restored? I also suppose you could check it another way too. With the amp ON, measure resistance across Q16 source-drain. If it measures low, that is the trouble. It ought to read like the 27k that parallels there.

[fixitrightthefirsttime]

Yep, a 1004 Hz -22 dBm 600 Ohm output Z signal is what I use, typically...but I prefer to use Diana Krall CDs (pre-Elvis Costello, that is.)

Your 2nd paragraph is an awesome set of info. It will save me a lot of time.

THANKS!

[Enzo]

Well, assuming that the mute is your problem anyway.

I leave my genny set on 100Hz. Hell of a lot easier to listen to that 1000, at least to me. And you are right, I mostly use a feed from the shop stereo. It's tuned to the country station. Rock music is always full of distortion, country is clean. I suppose Diana is not so distorted either. SOmetimes I find the jazz station. I like rock, but I want a clean test signal.

[fixitrightthefirsttime]

Oh, Diana Krall is a superior jazz musician with a pure, clean, natural, well-intonated voice, and plays jazz piano just as well as she sings.

That she is also physically beautiful as to be in the goddess category notwithstanding, her music is an enjoyable way to to evaluate speakers, linearity, etc. I think I have all her CDs memorized now.

Geeze, I gotta get back to work. Peaveys await. Thanks again!!

[BGR]

I know this thread is a bit old but i just bought a used Crate FXT120 and my volume drops a few minutes after the amp is turned on. I have to turn the amp off for a while and then it recovers. Anyone have a guess as to what could cause this?

[Enzo]

Dirty FX loop jacks. Plug a spare cord from FX send to FX return, any help? or if the amp has one INSERT jack, spray some cleaner in it and poke a plug in and out a few times to spread it around. Of course it also could be any number of other things inside.

[Jazz P Bass]

Crate FXT120 Schematic

Hmph.
The schematic was never posted.
Here it is.

[BGR]

Thanks for the replies.

I was a bit worried that the problem was a flaky transistor because the symptom the OP posted is similar to my issue but on mine it did not happen until the amp warmed up a bit. But after surgery on the brain I discovered it was a much easier fix. When the amp warms up the main board must flex a bit from the heat buildup. When I turn the power off and let it cool the board returned to its normal position. There was a bad solder joint that would connect when the amp was cold and disconnect when the amp warmed up. Problem solved

[Enzo]

Great!

[oldtgeezer]

Desperately Needed Schematic

Hmph.
The schematic was never posted.
Here it is.

Thank you so much for posting the schematic. Will now be able to repair one that has a burned resistor (now can actually get the value of said resistor) and find out why the component cooked. Quick check shows the power output transistors are shorted, but really need a schematic to fully understand what caused THAT! Great post for a simple request. Thanks again.