Crate RFX120

[raistlinmagic]

I have been trying to repair this amp. the rhythm and solo channels don't seem to put out much volume. I have replaced all the tl072's they looked burnt. Has anybody see this problem?

Thanks for the help

[Enzo]

You mention rhythm and solos not putting out much, does that mean there is yet another channel that does still work OK? Or is the complaint just the whole amp puts out little power?


Patch across the FX loop with a cord, any help?

I don't seem to have RFX120, can you link a schematic?

[raistlinmagic]

Sorry I wasn't very clear on that. The amp has 3 channels, Clean, Rhythm and solo. The clean channel works great but the Rhythm and solo channels don't put out much sound, but you can hear it if you crank it all the way up. I do have a schematic for this amp. I will post it later today when I get to my laptop.

Thanks again for the help

[raistlinmagic]

Here is the schematic diagram for this amp

[Enzo]

OK, so the clean works. Look at the schematic. The input branches at TP1. The clean channel branch works, so the distortion channel, the top strip, is the trouble. SInce both options on that channel seem equally distressed, we look for common parts. Suspects like IC1b, IC2b, IC3a. A broken, cracked, or unsoldered HIGH control in the EQ there could kill the sound.

[raistlinmagic]

Ok Sounds correct. I did already replace IC1 - IC5. I really haven't checked for cracks. I was looking at Q4 as a possible suspect I haven't had time to test this yet. I may need to examine the board very closely to check for cracks or broken leads on something.
I do get some distorted sound out of the amp but at extremely low volume if I crank the gain up on the rhythm channel and the solo channel all the way. I discovered today while examining the schematic of this amp, there is a page missing.

[Enzo]

What's missing? Page 1 is preamp, page 2 is power amp, and the two other files are little jack boards or something.

[raistlinmagic]

I think the second page would be the power amp I guess. I don't see any reference to the clean/rhythm button or the rhythm/solo button. Maybe I am over looking them. If you look at the schematic it says page 1 of 2 at the bottom. On the board there is a transistor with a heat sink on it that I can't seem to find in the schematic. Anything that has a heat sink, as you know most likely gets hot beyond the specs of the device and could be trouble. I also would like to know where connection point B2 goes in location F1 . Looking at the test point conditions wouldn't TP4 and TP5 Be a good place to start? Condition C on TP4 and tp5 show a flat line on the clean channel. I know that IC2 is good so it would have to be something after that correct? Most likely a diode or a transistor q6 or q8?

[52 Bill]

I think the second page would be the power amp I guess. I don't see any reference to the clean/rhythm button or the rhythm/solo button. Maybe I am over looking them.

The switch circuit is on the lower left side of this page (page 2 of 2)

I also would like to know where connection point B2 goes in location F1 .

It goes to the switching circuit (see above).

Looking at the test point conditions wouldn't TP4 and TP5 Be a good place to start? Condition C on TP4 and tp5 show a flat line on the clean channel.

Yes do you have a signal generator and an oscilloscope?

I know that IC2 is good so it would have to be something after that correct?

Yes, but I'd suggest starting by checking Q25 and Q29. I'd think it was unusual for both of those fets to die at the same time, but it's possible. My first suspect would be C24, as both of the distortion channels feed through it.

And don't forget to check for the broken connections, bad pot, etc. that Enzo suggested.

[raistlinmagic]

I think you might just be correct on c24. its a Electrolytic cap. I don't have any non polarized Electrolytic caps here to replace it with My meter tests caps but never used that function before and i am not sure it works on electrolytics. Unfortunately I don't have a scope or a signal gen handy but it looked like the logical place to start testing components. I am just stuck with my multimeter.

Thanks again for all the help.

[Enzo]

Those two test points show a flat line - really it means no signal - in the clean mode. Well, we would expect no signal in the distortion channel in the clean mode.

C24 is common to both yes, but so is C15, and C13, and R44, and the treble control.

Apply a signal to the input, if you have no signal generator, play a CD or MP3 into it. We don;t care if it sounds like a guitar at this point, you are mising the sounds completely. Now look for that signal at the various points along the circuit. If you have no scope, use your meter set to AC volts.

[raistlinmagic]

I believe I found the problem I believe it is q29. I can't seem to find one anywhere to replace it with so i guess I will just have to wait.

[Enzo]

If you think Q29 is bad, just remove it. The Solo will no longer work, but Overdrive should, along with the clean.


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FAIRCHILD SEMICONDUCTOR|J175|P CHANNEL JFET, 30V, TO-92 | Newark.com

[raistlinmagic]

Here is the full schematic with the power amp and switchs on it.

[raistlinmagic]

Ok I did some more troubleshooting. so at test points 1 -5 I get ac voltage on all 3 channels. on test point 6 I get voltage on all 3 channels. Mind you I am just plugging my guitar in and hitting a string to see if i get a reaction.
at test point 7 I get voltage on the clean channel and no voltage on the distortion or solo channels. I sure wish I had a scope and a proper signal generator.