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Post subject: Champion 40 Hiss
Posted: Thu Mar 05, 2020 6:03 am
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Dear All, first of all I'm new in the forum and I'm from sunny Italy, therefore sorry for my English.
I've received a 2nd hand Champion 40, almost brand new. The amp shows a constant "hisssssss" noise in the background. The noise starts immediately after switching on, even not plugging any guitar, neither changing channel, gain, volume, settings, effects... I tried different options: power cables, different rooms, different time of the day... always the same annoying hiss. The level of the hiss is significant. I mean, if I play with volume over 2 the guitar sound level is much louder than the white noise, but overall the hiss is audible and much higher than my other tube amps noise level ! This is not hum from 50 or 60 Hz cycling, this is what I'd define like white noise: a constant "sssshhhhhhhhhhhh". Opamp gone ? switching power supply and high frequency interference ? Any suggestion/troubleshooting is welcome. By the way I'm experienced with basic electronics... next step is to start changing/welding some part.
Any experience ? I read about some other users having similar experience with Fender solid state combos...


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Post subject: Re: Champion 40 Hiss
Posted: Sun Mar 08, 2020 5:03 am
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Looking at schematic, I don't know how to fix this hiss. Look a Fender bad desing......

Many forum talk about same hiss from Champion 40 :(

https://forums.fender.com/viewtopic.php?f=13&t=105605

https://www.reddit.com/r/Guitar/comment ... ound_hiss/

https://www.ultimate-guitar.com/forum/s ... ?t=1727462

I went few times in Italy (I'm from Canada ) I love your country and peole, sorry about Covid19


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Post subject: Re: Champion 40 Hiss
Posted: Sun Mar 08, 2020 7:39 am
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Thank you Stratele for your comments. I've already read those posts describing similar white noise "hiss". This weekend I took some time opening the amp and checking every components with the "wooden stick" approach and I couldn't find anything wrong/damaged... the "hiss" level was present and with no changes touching and "ticking" all the visible components. All the welding points are perfect... Obviously I can't check the surface mounted smd components. At this point I've no solutions and I guess why I've got it "new old stock" with a such reasonable price... Is there anybody out there with a deadly silent Champion 40 ? My all tube Blackstar has zero "hiss/hum" even with high gain settings. It's really a pity situation that such nice amp and tones from Champion 40 are barely usable with this design weakness.


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Post subject: Re: Champion 40 Hiss
Posted: Sun Mar 08, 2020 9:25 am
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You can break the daisy-chain of the signal path in a few places to find the source of the hiss. First, listen to the headphone output to see if it originates in the front end or digital sections.
You can mute an op amp by shorting its two input pins together. First, measure to make sure there isn't rail voltage on either pin. I use a small insulated screwdriver to accomplish the jump. Pins 2 & 3 and pins 5 & 6 are the input pins on most dual op amps. You can go down the daisy chain, silencing op amps as you go. A hissy op amp may not mute by shorting its inputs. But if its upstream and downstream circuits are all right, it points to the hissy one.
If its clean there, the problem is likely in the main digital amp stage. I've heard a lot of PWM amps that hiss. Try plugging into the Power Amp input to see if the noise goes away. That will disconnect the entire preamp section from the power stage. If that doesn't change the hiss, try replacing the main amp chip. The TDA8950 is only about $7 from Mouser. If the output inductors have been overheated and shorted out, it could cause hiss. Measure the low voltage AC around both of them and look for cracked ferrite from overheating.


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Post subject: Re: Champion 40 Hiss
Posted: Mon Mar 09, 2020 1:17 am
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THANK YOU TimsAudio ! The same "hissss" I have from the main speaker is also on the headphones output. Does it means automatically that the problem is located somewhere in the preamp board and therefore the power stage pcb should be OK ?
The procedure of short-cutting the daisy-chain of opamps imputs is absolutely reasonable, but also difficult.
By the way, the shortcut of the main guitar imput doesn't have any influence on the noise. I found some schematics on line but the resolution is quite bad a some parts are not readable at all. Do you know from where I can source a nice and complete schematics for the Champion 40 ?
Thank you for your suggestions... I'll try my best. It can't be any worse.


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Post subject: Re: Champion 40 Hiss
Posted: Mon Mar 09, 2020 1:30 am
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Google ; fender champion 40 schematic

Champion 100/40/20

You can zoom in for easy work

http://www.fmicassets.com/Damroot/Origi ... ematic.pdf


Last edited by stratele52 on Mon Mar 09, 2020 1:35 am, edited 1 time in total.

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Post subject: Re: Champion 40 Hiss
Posted: Mon Mar 09, 2020 1:34 am
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Zeno69 wrote:
THANK YOU TimsAudio ! The same "hissss" I have from the main speaker is also on the headphones output. Does it means automatically that the problem is located somewhere in the preamp board and therefore the power stage pcb should be OK ?

There are issue in preamp

The procedure of short-cutting the daisy-chain of opamps imputs is absolutely reasonable, but also difficult.

It is not a amp easy to work on for a newbie


I'll try my best. It can't be any worse.


Yes it can be worst, you can damage amp if you don't know what you are doing


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Post subject: Re: Champion 40 Hiss
Posted: Thu Mar 12, 2020 12:39 am
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Sorry, one more question before I start checking the signal path (with all the risks).
Can I use the amp completely disconnecting the preamp pcb and using only the power amp section with any external preamp ? Looking at the schematics it seams easily feasible, just disconnecting the 8 wires harness connecting the two module (pre and power amp) and just plugging the pre signal into the 2 wire connection to the power amp stage. m'I wrong ? any specific warning about impedance matching or signal level or others...? That could be a final solution to use it at least as an "active" cab with external pre or external processors....


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