no distortion on fender champ 25 tube amp

[bkemker]

I have a fender champ 25 tube amp and the distortion channel doesn't work and has no sound. The clean channel works just fine but when I hit the channel switch for distortion there is no sound. I've checked the input jacks, the channel switch, and the pots. I've switched the tubes out with ones that are ok and still nothing, and I've cleaned the tube socket pin contacts. I need help.

[52 Bill]

Is this a Champ 25 SE? If it is, there are two IC chips that could cause your problems U2 and U3. IC U2 is the preamp chip and IC U3 is the channel switch.

[bkemker]

reply thank you

Is this a Champ 25 SE? If it is, there are two IC chips that could cause your problems U2 and U3. IC U2 is the preamp chip and IC U3 is the channel switch.

Yes it is a champ 25 se. I have the schematic and the parts list so I know where the location of the ic chips and the part number. How do I test them or is it easier to just replace the? I would assume digikey would have them. Also, do you know of anywhere that would have the input jacks that would fit my amp? Again thank you for responding.

[52 Bill]

Yes it is a champ 25 se. I have the schematic and the parts list so I know where the location of the ic chips and the part number. How do I test them or is it easier to just replace the?

The guess about the ICs was just that, a guess. You need to do some tests to find out what the actual problem is. It could be as simple as a broken lead on a capacitor or something.

I will assume that when you switch channels, the front panel light changes, correct? When you are in distortion mode can you hear any sound, hiss, hum? If you can hear something, do any of the distortion channel controls change the sound of the noise?

If the sound changes with the controls, then the channel switch chip U3 is probably working and U2 should be tested. If there is no change to the noise, U3 should be tested.

To test U2, read the voltages on each of the two output pins #1 and #7. You should find little or no dc voltage to ground on either one.

If the problem is U3, you already know that half of the chip is working as the clean channel is turning off and on. The only other problem would be that the distortion side of the switch is not working. You will need to test the continuity from pins #1 and #15 when the distortion is turned on.

Also, do you know of anywhere that would have the input jacks that would fit my amp?

If I am remembering that amp correctly, the jacks are the older NALCO style jacks that have not been used by Fender for a number of years now. So exact replacements may not be available. What is wrong with yours?

[bkemker]

no sound at all on distortion channel

The guess about the ICs was just that, a guess. You need to do some tests to find out what the actual problem is. It could be as simple as a broken lead on a capacitor or something.

I will assume that when you switch channels, the front panel light changes, correct? When you are in distortion mode can you hear any sound, hiss, hum? If you can hear something, do any of the distortion channel controls change the sound of the noise?

If the sound changes with the controls, then the channel switch chip U3 is probably working and U2 should be tested. If there is no change to the noise, U3 should be tested.

To test U2, read the voltages on each of the two output pins #1 and #7. You should find little or no dc voltage to ground on either one.

If the problem is U3, you already know that half of the chip is working as the clean channel is turning off and on. The only other problem would be that the distortion side of the switch is not working. You will need to test the continuity from pins #1 and #15 when the distortion is turned on.


If I am remembering that amp correctly, the jacks are the older NALCO style jacks that have not been used by Fender for a number of years now. So exact replacements may not be available. What is wrong with yours?

When the distortion channel is selected there is no sound at all. I tested the u2 chip the way you said and there was little to no voltage from pin 1 to 7. I tested the u3 chip with distortion channel selected from pin 1 to 15 and it read 52 to 53 ohms. I have the data sheets for u3 if further testing needed.
My next plan was to test all resistors and capacitors one by one. And recheck all solder connections.

[52 Bill]

I tested the u2 chip the way you said and there was little to no voltage from pin 1 to 7.

I just want to make sure that you read the voltage from pin 1 to ground and from pin 7 to ground here and not from pin 1 to pin 7. And just because there is no voltage on the output pins, that doesn't mean that the chip is working.

Checking all of the parts and connections are a good thing, but you can limit your search to the parts that relate to the distortion channel only.

If you have a signal generator you could run a constant signal into the amp input and then follow it through the different stages to find out where it stops. The schematic shows the values of a test input signal and ac voltage readings along the signal path at different Test Points (TPs).

If you don't have a signal generator you could use a radio or mp3 player as a signal source, but you will not have simple voltages to follow through the amp, but it will work. If you make a signal probe from a wire with a blocking cap, you can use it to listen for the signal at various points in the circuit.

[bkemker]

I have a signal generator but I'm not sure if it will works or how to use it.

I am setup to check the test points that are shown on the schematic but I am not sure where to position my leads. Do I measure the voltage across each component or do I place the negative lead on the ground or the negative on the power cable?

[52 Bill]

I have a signal generator but I'm not sure if it will works or how to use it.

I am setup to check the test points that are shown on the schematic but I am not sure where to position my leads. Do I measure the voltage across each component or do I place the negative lead on the ground or the negative on the power cable?

Well, the generator will have some sort of output terminals. You will connect this output to the input of the amp with a shielded cable. The generator will have controls to set the frequency and the level (volume) of the signal. Depending on the unit, it may also have options for sine, square or triangle waveforms.

For these tests, you will want to use the sine wave output or setting. The schematic probably has the specific input frequency and level to use for these tests. It may also list where to set the volume and tone controls for these test readings. Fender typically uses 1K hertz and maybe 30mV level. I don't have the schematic in front of me, but it's pretty simple really.

Turn on the amp set to the clean channel and if the generator is working you will hear a steady tone coming out of the amp. Next take your voltmeter, set to read AC volts and connect the black lead to the chassis ground. Use the red lead to read the AC signal voltages at the different test points listed on the schematic. These voltages are usually shown in oval shapes (I think). You should be able to follow the AC signal through the amp by way of the different test points. Then you can switch the amp to the dead channel and look to see where the AC signal stops.

If your signal generator doesn't work, you can signal trace through the amp by sending any signal into the input of the amp, like a MP3 player or radio and then use either a second amp or the input to the power amp to listen to the signal as it goes through the different stages of the preamp. If you do this, you will need to make a simple probe cable with a blocking capacitor at the input, so that you can safely touch the probe to different points that may be dc potential.

[g1]

Champ 25SE schematic available here: http://ampwares.com/schematics/champ_25_se.pdf
See note 4 regarding set-up for controls etc., connect sine wave generator to input jack 1. Adjust generator for 1Khz frequency and set output level so you get the specified voltage at TP1.
Then go through and measure the other TP's with meter set for AC volts. Check only TP's that have voltages in oval, TP's with levels in rectangular boxes are DC readings not related to input signal.

[52 Bill]

Thanks for posting the schematic here. I wasn't in the shop when I wrote that last reply, so I wasn't sure about the details.

[bkemker]

I have hooked my sine wave generator up to the first input jack and have all the settings set on the amp according to #4 on the notes on the schematic page. I noticed that the notes said something about the foot switch, I can't locate mine so will that be a problem? And when testing do I keep the speaker wires hooked up or can I use a 1 ohm resistor inplace of the speaker so I don't have to listen to the tone from the generator? Or is that part of the process lol. And I want to thank everyone for all the help, this really is helpful and fun I might add.

[52 Bill]

I have hooked my sine wave generator up to the first input jack and have all the settings set on the amp according to #4 on the notes on the schematic page. I noticed that the notes said something about the foot switch, I can't locate mine so will that be a problem?

That won't matter for the tests that you need to do.

And when testing do I keep the speaker wires hooked up or can I use a 1 ohm resistor inplace of the speaker so I don't have to listen to the tone from the generator?

You don't need to have the speaker connected for these tests, and a 1 ohm resistor is way too low to use for a dummy load.

STOP! I forgot that this is a tube amp and it does need a load for the tests. The voltage figures on the schematic are for power testing the circuit, in order to test to see if the amp is putting out full rated power. If you use a load resistor that is too small wattage wise, it will overheat and start to burn while doing these tests. And if the resistance rating is too low it will change the voltage readings that you get.

For testing to see where the signal stops, you don't need to run the amp up to full output.

[g1]

Another thing you can do is just stick a plug or cord into the power amp in jack. That will kill the sound. It's a preamp problem so you shouldn't need any power amp voltages for now.

[bkemker]

I just wanted to say thanks to everyone's help I was able to determine that the U2 ic chip was bad which was an 80 cent part from Digikey. I got the chip in the mail today and with already having the bad chip removed was able to have the new chip in place and up in running in 5 minutes. Now all the amp needs is some new tubes and it will be screaming once again. Thanks everyone and keep rockin.